2 COMMON LABORATORY TESTS AND PROCEDURES
Common Laboratory Tests and Procedures
Judy Perrigin, Laurance Freier, and Bruce E. Onofrey
Laboratory tests can provide valuable information about the blood and many organ systems. They can be used to
- Screen for certain diseases or conditions (infections, inflammatory processes, neoplasms, and so forth).
- Assess the function of a particular organ or system.
- Help with a differential diagnosis (e.g., in the case of a patient with chest pain, an acute myocardial infarction [MI] would show an elevated erythrocyte sedimentation rate [ESR] test versus angina pectoris, in which the ESR would remain normal).
- Confirm a diagnosis (e.g., if a patient shows a positive venereal disease research laboratory [VDRL] test and a positive fluorescent treponemal antibody absorption test [FTA-ABS] test, a diagnosis of syphilis would be confirmed).
- Rule out diseases (e.g., if a patient shows a negative antinuclear antibody [ANA] test, systemic lupus erythematosus [SLE] can almost certainly be ruled out).
- Follow the course of a disease and help monitor the effects of treatment (e.g., an ESR test will generally increase if a disease worsens and decrease as the disease improves).
- Determine compliance with medications.
- Determine whether a disease is in its active or dormant state.
- Check for drug toxicities and side effects of medications.
- Determine if a patient is in a risk group for a particular disease.
- Help identify systemic disorders that have ophthalmic manifestations.
- Determine whether a disease has spread systemically.
- Detect a disease that is otherwise not expected.
Knowledge of laboratory tests helps ensure appropriate patient care, facilitates communication with other health care professionals, and assists in good patient education.
Every laboratory test has reference (or normal) values. Reference values are used as a general guide to help clinicians identify diseased patients. They can also be used to monitor the effects and course of therapy. Reference values, however, should not be considered as rigid dividers of normal from abnormal. Many factors can cause variations in laboratory test results, including the effects of medication; the presence of concurrent diseases; the effects of diet, age, sex, race, and emotional status; how the patient was prepared; patient posture; and intrapersonal differences (in a particular patient, the concentration of some substances may vary from day to day or even during the course of a day). Therefore, a definite diagnosis should not be made on the basis of a single test result. All laboratory tests should be evaluated in light of the patient’s other clinical findings. Tests may have to be repeated, or a combination of test panels may be needed before a diagnosis can be confirmed. Reference values also vary from lab to lab because of differences in instrumentation and methodology. It is important, therefore, that the clinician only uses reference values standardized by the laboratory that actually performed a particular test. Errors in interpretation can result if a clinician attempts to apply the reference values of one laboratory to the test results received from another laboratory. One common practice is to compare the patient’s present laboratory results with the patient’s previous values (these act as the patient’s own reference values).
Clinicians should be aware of the possible effects of any medications a patient is taking on the results of a test. If a patient is taking a drug that is known to affect a particular test, the patient’s physician should be consulted and the drug, if possible, should be discontinued or its initiation postponed until after the completion of the test. If this is not possible, any drugs the patient is taking should be noted on the laboratory request form.
The following sections describe some of the many laboratory tests that are available.
ANCA (ANTINEUTROPHIL CYTOPLASMIC ANTIBODY) TESTING
ANCA (perinuclear antineutrophil cytoplasmic antibody)
ANCA (cytoplasmic antineutrophil cytoplasmic antibody)
The antineutrophil cytoplasmic antibodies (ANCAs) represent a group of antibodies directed against cytoplasmic components of neutrophil granulocytes and monocytes. Testing is primarily used for diagnosis and monitoring of granulomatous vascular diseases such as Wegener’s granulomatosis.
The gold standard of ANCA determination remains indirect immunofluorescence on ethanol fixed neutrophils. Two main patterns can be distinguished: (a) a cytoplasmic pattern of coarse granular fluorescence with accentuation in the area of the nuclear lobes (cytoplasmic or ANCA; formerly termed “ACPA” for anticytoplasmic antibodies) and (b) a perinuclear pattern (p-ANCA). The specificity of this test depends on the ability of the laboratory personnel to distinguish these two patterns from a more diffuse cytoplasmic staining pattern and granulocyte-specific ANA staining. In addition, without additional testing, determination of the p-ANCA pattern is not possible in most antinuclear antibody–positive sera. The p-ANCA staining pattern represents an artifact of ethanol fixation, which allows cationic proteins to move to the negatively charged nuclear membrane. This staining pattern reverts to a cytoplasmic fluorescence pattern if cross-linking fixatives such as formalin are used to prepare the neutrophil substrate. These formalin-fixed neutrophils can be used in special circumstances to allow the distinction of a true p-ANCA from an ANA (Table 2-1).
Reactive versus nonreactive
The anergy panel is used to verify a patient’s ability to exhibit a delayed hypersensitivity response. It is commonly used in combination with the tuberculin purified protein derivative (PPD) skin test as a control. This serves to validate a negative PPD test.
A common antigen such as mumps, Candida, or trichophytin is injected intradermally at the same time as the PPD tuberculin test. An individual with a normal immune response would produce a localized reaction to the injection within 48 to 72 hours. A lack of reaction suggests that the patient’s immune system is compromised, and therefore the results of a negative PPD test would be less conclusive. Such an individual should undergo a chest x-ray and sputum testing.
ANGIOTENSIN-CONVERTING ENZYME (ACE) TEST SERUM ANGIOTENSIN-CONVERTING ENZYME (SACE)
A 23 to 57 U/mL (U = nmol/min)
The angiotensin-converting enzyme (ACE) test is used to help diagnose and manage patients with sarcoidosis. Serum levels of ACE are increased in 85% of patients with active sarcoidosis. Normal levels may be found in dormant/inactive sarcoidosis. The ACE test is primarily used to evaluate the severity of the sarcoidosis and its response to therapy. ACE is found in pulmonary epithelial cells. It converts angiotensin I to angiotensin II, a potent vasoconstrictor. The ACE test is therefore sometimes used to evaluate special cases of hypertension. Clinical disorders that may increase ACE levels include active multiple sclerosis, histoplasmosis, amyloidosis, alcoholic cirrhosis, diabetes mellitus, Gaucher’s disease (a familial disorder of fat metabolism), Hodgkin’s disease, hyperthyroidism, leprosy, pulmonary embolism, idiopathic pulmonary fibrosis, scleroderma, and tuberculosis. ACE levels may be decreased in patients with sarcoidosis who are treated with prednisone. The ACE test is not done on patients less than 20 years old, because this age group normally has very high levels. Patients on ACE inhibitor medications will show low values. Smokers may have lower ACE levels than nonsmokers.
- Five milliliters of venous blood is collected in a serum tube.
- The sample is taken to the lab.
ANTINUCLEAR ANTIBODY TEST
Negative—no ANA detected
ANAs are immunoglobulin protein antibodies (IgG, IgM, IgA) that react against the nuclear material in leukocytes. They act as antibodies against DNA, RNA, and other nucleoproteins. The ANA test was developed to assess tissue-antigen antibodies. It is often used to diagnose SLE, an autoimmune collagen disease. Patients with autoimmune diseases have many abnormal antibodies. Two antinuclear antibodies, anti-DNA and antideoxyribose nucleoprotein, are almost always present in patients with SLE. The ANA test is quite sensitive in detecting SLE (~95% of patients with SLE will show a positive ANA) but is not very specific, because many other disorders can cause a positive ANA. The ANA is therefore often used as a screening test for SLE. A negative ANA would suggest that it is unlikely that the patient has SLE. When a patient has a positive ANA, along with a positive LE (lupus erythematosus) prep, SLE would be strongly suspected.
Other disorders that can cause a positive ANA include systemic sclerosis, scleroderma, rheumatoid arthritis (RA), myasthenia gravis, leukemia, cirrhosis of the liver, ulcerative colitis, and infectious mononucleosis. Many drugs can also cause a positive ANA, including penicillin, tetracycline, acetazolamide, thiazides, and oral contraceptives.
- Five to seven milliliters of venous blood is collected in a red-top tube.
- The serum is then incubated with traumatized rat’s liver to obtain the antinuclear immune complex.
- The mixture is then incubated with fluorescein-labeled antihuman serum (to tag any immune complex).
- The preparation is then examined under an ultraviolet microscope for fluorescent ANAs.
- The patient’s serum is then serially diluted, and the ANA test is performed on each dilution (the most dilute serum in which the ANAs can be detected is called the titer).
- The test is considered positive if ANAs are detected in a titer with a dilution greater than 1:32.
- The pattern of nuclear fluorescence is also documented (this pattern is considered equally important in determining whether or not a patient has SLE or other autoimmune disease).
- A positive ANA should then be compared with other tests for SLE.
ANTICARDIOLIPIN ANTIBODIES (ACL ANTIBODIES, ACA)
Less than 23 GPL (IgG phospholipid units)
Less than 11 MPL (IgM phospholipid units)
Less than 12 APL (IgA phospholipid units)
Cardiolipin antibodies are the most common form of antiphospholipid antibodies. Antiphospholipid antibodies are the most common cause of thrombosis in nonelderly patients. Thrombosis may occur in response to transient APL elevations as well as in antiphospholipid syndrome. Transient increases in antiphospholipid levels occur in viral conditions including Epstein Barr, mumps, and measles. This test is also positive in some patients with SLE and places the patient at higher risk for “antiphospholipid syndrome” (venous or arterial thrombosis, thrombocytopenia, and recurrent spontaneous abortions).
- Interfering Factors
- Patients who have had syphilis can produce a false positive on this test.
- Presence of APL antibodies will produce a false positive on the VDRL test.
- Patients with infections, HIV-positive status, inflammatory disease, or cancer can produce false-negative results.
- The drugs chlorpromazine, procainamide, phenytoin, penicillin, hydralazine, and quinidine can produce false-positive results.
Seven millilters of blood in a serum separator tube (SST).
Send to lab.
Normal range: 1 to 9 minutes
Critical value: more than 15 minutes
The bleeding time (BT) is a test to evaluate the vascular and platelet factors associated with hemostasis. When vascular injury occurs, the first hemostatic response is a spastic contraction of the injured microvessels. Next, platelets adhere to the injured area. Failure of either process results in prolonged BT. Since this test is labor intensive and dependent on the operator’s skills, laboratories are beginning to use the platelet closure time test instead.
A small, standardized incision is made in the forearm, and the time required for the bleeding to stop is recorded. Because vessel constriction and platelet adherence are not affected by coagulation (intrinsic and extrinsic), defects in this system will not affect this test. It should be noted that the BT is only an indirect method of platelet function. A complete blood count (CBC) with platelet count should be performed to identify this directly. Prolonged BT with a normal platelet count indicates that a qualitative platelet disorder is present.
BLOOD UREA NITROGEN TEST
Men: 10 to 25 mg/dL
Women: 8 to 20 mg/dL
Children: 5 to 20 mg/dL
Infants: 5 to 15 mg/dL
Renal function studies typically include the blood urea nitrogen (BUN) and creatinine tests. Both tests are included in blood chemistry panels. Renal involvement in diabetics and hypertensives can increase risk of retinopathy, making these tests of great interest to eye care practitioners. The BUN measures the amount of urea nitrogen found in the blood. Urea is formed in the liver as an end product of protein metabolism. It circulates in the blood and is excreted by the kidneys. The concentration of urea in the blood is directly related to how well the kidneys are excreting it. The BUN, therefore, is used as an indicator of liver and kidney function.
Most renal diseases decrease the kidney’s ability to excrete urea. The result is an increased BUN level. Other problems can also increase the BUN level. Dehydration from diarrhea, vomiting, or inadequate fluid intake can cause the BUN to elevate up to 50 mg/dL. The BUN should return to normal once the patient is rehydrated; if it does not, renal or prerenal failure should be suspected. Excessive protein intake causes the body to make large quantities of urea. The kidneys may become overloaded and be unable to excrete the excess urea. The BUN level will then rise. Gastrointestinal bleeding, diabetes, renal insufficiency because of shock and sepsis can all cause increased BUN levels. Many drugs can also elevate the BUN, including diuretics; antibiotics such as bacitracin, gentamicin, and neomycin; antihypertensive agents; sulfonamides; and salicylates. Older patients may also show an increased BUN, because the amount of nephrons tends to decrease with age. Because urea is formed in the liver, decreased BUN levels are seen in patients with severe liver damage. Low BUN levels may also indicate overhydration, malnutrition, or a low-protein diet. Pregnancy, which causes an increase in fluid volumes, also reduces the BUN. Phenothiazines can also reduce the BUN.
- Five to seven milliliters of venous blood is collected in a serum separator or red-top tube.
- The sample is sent to the chemistry lab. A multifunctional analysis machine determines the BUN.
- Some labs prefer that the patient has not eaten for 8 h before the test.
CPTs may vary slightly depending on the exact instrument used.
CPT for basal metabolic panel (BMP) 80048QW
CPT for comprehensive metabolic panel 80053QW
Chemistry panels/profiles are groups of related chemistry tests. The number of tests included depends on which instrument the lab uses but generally ranges from 7 to 22 tests. A newer but abbreviated panel of seven tests now commonly available is the BMP. The comprehensive panels include the seven in the BMP plus others. Chem panels typically include analyses of blood electrolytes and chemical elements involved in renal, liver, glucose, heart, and bone function. Panels provide greater diagnostic comparative value than single tests and are significantly less expensive than ordering the tests individually. A typical comprehensive chemistry panel includes
- Electrolytes: NA, K, Cl, CO2 or HCO3
- Kidney function: BUN, creatinine, uric acid, albumin, total protein, glucose
- Liver function: transaminases (AST/SGOT, ALT/SGPT), albumin, total protein, bilirubin, alkaline phosphatase
- Acute MI enzymes: SGOT, LD (also CK-MB or CPK but not part of chem panel)
- Metabolic bone disease: CA, P, alkaline phosphatase
- Depending on the reason for the testing, the patient may or may not be fasting. If planned in advance, it is best to fast for 10 to 12 h, but ingest plenty of water.
- Collect 7 to 10 mL of venous blood in a SST and send to lab
CPT: 86631–86632, 87270, 87320, 87110
Negative versus positive
There are several traditional and more advanced methods of testing for chlamydia. The methods are as follows:
Enzyme-linked immunosorbent assay. This common, rapid test detects substances (chlamydia antigens) that trigger the immune system to fight a chlamydia infection. EIA testing is done by taking a sample of secretions from the potentially affected area.
Direct fluorescent antibody (DFA) test. This is a common, inexpensive, rapid, and highly specific test that detects chlamydia antigens. DFA testing is done by taking a sample of secretions from the potentially affected area. This can be done as a slide test where the specimen is applied to a glass microscope slide, fixed, and sent to the lab. The lab applies monoclonal antibodies tagged with fluorescein. If the chlamydia antigen is present, fluorescent antigen–antibody complexes will form. DFA slide tests are available for many other organisms as well. This test is well suited for ocular specimens.
Nucleic acid amplification tests (NAATs). These tests detect the genetic material (DNA) of chlamydia bacteria. Testing can be done on either a urine specimen or a sample of secretions from the potentially affected area. Polymerase chain reaction (PCR) and ligase chain reaction testing are examples of NAATs.
Nucleic acid hybridization tests (DNA probe testing). Probe testing also detects chlamydia DNA. Probe testing is very accurate and can be done by taking a sample of secretions from the potentially affected area. DNA probe testing is not as sensitive as NAATs. Gen-Probe by Pace is an example of a DNA probe test, which is approved for collection and testing of ocular specimens. The Gen-Probe Collection Kit from Pace comes in two different versions: one for cervical specimens with a larger swab and one for conjunctival and urethral specimens with a smaller swab. The collection kits contain an appropriate swab and chlamydial transport fluid. The same kit may be used for gonorrhea. The eye should be cleaned of any mucus and then the infected area swabbed and placed into the transport media. The specimen is then sent to the lab for processing. Other swab types and media cannot be substituted. This is an easy to collect, inexpensive, and sensitive test for chlamydia. The laboratory will typically supply you with the collection kits, but you can also order them independently.
Chlamydia culture. A culture provides the right environment for chlamydia bacteria to grow. This test is expensive and not commonly done. It requires high technical skills and results take 5 to 7 days. The chlamydia culture test is usually done when the number of bacteria is very low, when child sexual abuse is suspected, or when treatment for infection has failed. It is not recommended for ocular specimens.
CHOLESTEROL SERUM (TOTAL)
Adults: Less than 200 mg/dL—desirable; 200 to 239 mg/dL—borderline; ≥240 mg/dL—high
Children: 120 to 240 mg/dL
Infants: 70 to 175 mg/dL
Serum cholesterol levels are used in the diagnosis and management of atherosclerosis and coronary artery disease. They can also be used as an indicator of liver function because cholesterol is synthesized by the liver. Considering only the total cholesterol value can be misleading. Lipid (lipoprotein) profiles/panels that determine values for not only total cholesterol but also HDL, LDL, triglycerides (TGs), and risk factor for coronary disease are preferable.
Cholesterol is a blood lipid. It is found in red blood cells (RBCs), cell membranes, and muscle. Approximately 70% of cholesterol is in an esterified form (i.e., combined with fatty acids) and 30% is in the free form. Plasma cholesterol can be fractionated into high-density lipoprotein cholesterol (HDL-C), which makes up approximately 25% of the total cholesterol and normally ranges from 40 to 60 mg/dL, low-density lipoprotein cholesterol (LDL-C), and very-low-density lipoprotein cholesterol (VLDL-C). The body uses cholesterol to form bile salts for fat digestion and to form hormones made in the adrenal glands, testes, and ovaries. Hypercholesterolemia can lead to the formation of plaques in the coronary and carotid arteries. Elevated cholesterol levels (>250 mg/dL) may be seen in atherosclerosis, acute MI, hypothyroidism, biliary obstruction, uncontrolled diabetes mellitus, familial hypercholesterolemia, type II hyperlipoproteinemia, high-stress periods, high-cholesterol diet (animal fats), and pregnancy. Decreased cholesterol levels may be associated with hyperthyroidism, Cushing’s syndrome (adrenal hormone excess), anemias, acute infections, and malabsorption. Drugs that may decrease the serum cholesterol include the antibiotics neomycin and tetracycline, corticosteroids such as cortisone, estrogens, glucagon, hypoglycemic agents, thyroxine, heparin, nicotinic acid, and aspirin. Drugs that may elevate the serum cholesterol include epinephrine, norepinephrine, steroids, androgens, oral contraceptives, sulfonamides, Thorazine, vitamins A and D, and aspirin.
- Five to ten milliliters of venous blood is collected in a serum separator or red-top tube.
- The blood sample is taken to the lab
CBC WITH DIFFERENTIAL
The CBC with differential is a hematology panel/profile that evaluates the quantity and quality of the formed elements of blood. It can provide a great deal of information about the condition of the blood and many organ systems. It is often used because it is inexpensive, easily performed, and widely available. The test includes six components: (a) a RBC count; (b) hemoglobin (Hb or Hgb); (c) hematocrit (Hct); (d) RBC indices (mean corpuscular volume [MCV], mean corpuscular hemoglobin [MCH], and mean corpuscular hemoglobin concentration [MCHC]); (e) a white blood cell (WBC) count; and (f) a differential peripheral smear. Some labs automatically include the differential as part of the CBC, while others may require that you specifically request the differential. If you want to know if a patient is anemic, look at the RBC, Hb, and Hct. If you want to know what type of anemia, look at the red-cell indices and the morphologic description of the RBCs on the peripheral smear. If you want to know immune status, look at the WBC for elevation or depression and the differential to see which types of cell are increased or decreased.
- Blood may be collected by finger puncture or by peripheral venipuncture.
- Seven milliliters of blood is obtained in an EDTA-containing Vacutainer (lavender top).
- The blood tube is tilted up and down several times to ensure adequate mixing of blood and the EDTA anticoagulant.
- The specimen is then sent to the hematology lab for analysis.
- A differential count may be performed automatically by the instrument or manually by examination of 1 mm3 of a blood smear under a microscope. Each type of leukocyte is identified by morphology and counted and its percentage recorded. A manual count must be specially requested.
- RBC, Hb, Hct, indices (MCV, MCH, MCHC), and WBC are measured automatically by the machine. Most instruments also measure and report the platelet count at the same time.
RED BLOOD CELL COUNT
Men: 4.7 to 6.1 million/mm3
Women: 4.2 to 5.4 million/mm3
Children: 3.8 to 5.5 million/mm3
Newborns: 4.8 to 7.1 million/mm3
The RBC count represents the number of circulating RBCs in 1 mm3 of blood. Reference values vary according to age and sex. A patient is said to be anemic if his or her RBC count is decreased by more than 10% of this expected reference value. There are many causes for low counts, including hemorrhage, anemias, chronic infections, blood dysplasias, chronic renal failure, dietary deficiencies, leukemia, overhydration, and pregnancy. An RBC count can also be greater than normal. Causes include cardiovascular disease, high altitude, and polycythemia vera. Increased RBCs increase the blood viscosity and could put the patient at risk for thrombosis.
HEMOGLOBIN (Hb, Hgb)
Men: 14 to 18 g/dL
Women: 12 to 16 g/dL
Children: 11 to 16 g/dL
Infants: 10 to 15 g/dL
Newborns: 14 to 24 g/dL
The hemoglobin test measures the total amount of Hb found in the blood. Hemoglobin is a pigment found in RBCs that carries oxygen. Most factors that affect the RBC count also affect the Hb concentration.
The Hb concentration, however, is more sensitive to fluid (plasma) volume changes. Abnormally high hemoglobin levels may result from dehydration. Overhydration may cause a low hemoglobin concentration. Clinical problems that may cause a decreased Hb level include anemias, cancers, sarcoidosis, and pregnancy. An elevated Hb may be caused by chronic obstructive pulmonary disease (COPD), congestive heart failure (CHF), polycythemia, and dehydration. Large elevations increase blood viscosity and therefore the risk of occlusion.
GLYCOSYLATED HEMOGLOBIN (Hb A1C)
Nicky R. Holdeman
Glycosylated hemoglobin is produced by nonenzymatic condensation of glucose molecules on the globin component of hemoglobin. The major form of glycohemoglobin is termed hemoglobin A1c, which normally comprises only 4% to 6% of the total hemoglobin. However, glucose passes freely on to erythrocytes, and the rate of HbA1c formation is directly proportional to the concentration of free glucose (Table 2-2). Since the normal erythrocyte (RBC) has a life span of about 8 to 12 weeks, the HbA1c measurement provides an excellent index of the average blood glucose for approximately the preceding 2 to 3 months.
It should be noted (see section on Diabetes Mellitus) that HbA1c measurements are now accepted for diagnosing diabetes and also serve as a critical supplement to home blood glucose testing. HbA1c measurements should be obtained in patients with type 1 or type 2 diabetes at appropriate intervals, so that adjustments to therapy can be made if the glycohemoglobin is either subnormal or if it is more than 1% to 2% above the upper limits of normal.
Several large clinical trials have demonstrated that both type 1 (DCCT) and type 2 (UKPDS) diabetics had less risk of developing long-term complications if the HbA1c was kept close to normal. The UKDPS data showed that for every percentage point decrease in HbA1c, there was approximately a 35% overall reduction in risk of most diabetic complications. In addition, there was a 25% reduction in diabetes related deaths; a 7% reduction in all causes of mortality; and an 18% reduction in combined fatal and nonfatal MI. An A1c kit, A1c Now+ is FDA approved and CLIA waived for home use by patients (Fig. 2-1). The kit is user friendly and suitable for in-office use as well.
Figure 2-1. A1c Now+ Kit by Bayer approved for home and office use. (Photo courtesy of Dr. David Perrigin, University of Houston, College of Optometry.)
The contributions made by fasting plasma glucose (FPG) and postprandial glucose (PPG), to overall glucose control, are different as glucose levels approach the normal range. FPG contributes most to hyperglycemia in patients with high levels of A1c, whereas the PPG contributes most to hyperglycemia in patients who are closer to normal A1c levels. As patients approach their A1c goal, the need to manage PPG increases in importance.
Normal HbA1c for people without DM less than 6%.
Good HbA1c for people with DM ≈ 7%, without significant episodes of hypoglycemia.
Additional action suggested if the HbA1c is greater than 8%.
NOTE: An estimate of the patient’s average blood glucose can be calculated by using the equation: (HbA1c value × 20) + 10. While this equation provides reasonable accuracy for lower levels of HbA1c, it becomes increasingly less accurate when the HbA1c level exceeds 8% to 9%.
Men: 42% to 52%
Women: 37% to 47%
Children: 31% to 43%
Infants: 30% to 40%
Newborns: 44% to 64%
This is a measure of the percentage of RBCs in the total blood volume. The Hct is the volume (in milliliters) of RBCs found in 100 mL of blood, expressed as a percentage. If 45 mL of RBCs were found in 100 mL of blood, the Hct would be 45%. The Hct value should be about 3× the Hb value.
Factors that affect the RBC count and Hb concentration also affect the Hct. Clinical problems that may cause a decreased Hct include anemias, blood loss, leukemias, neoplasms, pregnancy, RA (especially juvenile), and vitamin deficiencies. An elevated Hct may be caused by dehydration, polycythemia, diabetic acidosis, and transient cerebral ischemia.
RBC indices consist of the MCV, MCH, and MCHC. Indices, along with the peripheral smear RBC description, are used to determine the type of anemia present. Indices are an integral part of the CBC and performed simultaneously by the same instrument from the same specimen.
MEAN CORPUSCULAR VOLUME
Adults: 80 to 98 Cu μm
Children: 82 to 92 Cu μm
Newborns: 96 to 108 Cu μm
The MCV is a measure of the average volume or size of a single RBC. The MCV and the other RBC indices (MCH and MCHC) are used to help identify types of anemias. Reference values vary according to age and sex. If the MCV becomes abnormally large, the RBCs are termed macrocytic. Macrocytic RBCs are most frequently associated with megaloblastic anemias (vitamin B12 or folic acid deficiency) or pernicious anemia. Macrocytic anemias are the type most likely to be associated with oculars signs. Microcytic RBCs (i.e., when the MCV value is abnormally small) are associated with iron deficiency anemia, malignancies, RA, sickle cell anemia, and thalassemia. If the MCV value is within the reference value range, the RBCs are termed normocytic.
MEAN CORPUSCULAR HEMOGLOBIN
Adults: 27 to 31 μg/dL
Children: 27 to 31 μg/dL
Newborns: 32 to 34 μg/dL
The MCH is a measure of the average amount of Hb (by weight) within an RBC. The value is derived by dividing ten times the total Hb concentration by the number of RBCs:
MCH = Hb × 10/RBC count
Mean Corpuscular Hemoglobin Concentration
The MCHC is a measure of the average concentration (weight per volume) or the percentage of Hb within a single RBC. It can be calculated from the MCV and MCH:
MCHC = MCH/MCV × 100 or MCHC = Hb × 100/HCt
It can also be derived by dividing the total Hb concentration times 100 by the Hct. If the MCHC value becomes abnormally low, RBCs are deficient in their concentration of Hb and are termed hypochromic. This condition is frequently seen with iron deficiency anemia and thalassemia. Elevated MCHC values do not occur because RBCs can only hold a physiologically limited amount of Hb.
WHITE BLOOD CELL COUNT (TOTAL LEUKOCYTES)
CPT: 85032, 85048, 89055
Reference Values (Total WBCs)
Adults and children more than 2 years old: 5,000 to 10,000/mm3
Children 2 years old and younger: 6,000 to 17,000/mm3
Newborns: 9,000 to 30,000/mm3
The WBC count measures the total number of WBCs (leukocytes) in 1 mm3 of blood. It does not indicate the types of WBCs, only the number. WBCs are an important component of the body’s defense system and respond immediately to foreign invasion. The total WBC count has a wide normal range, but many diseases can significantly decrease or increase the count. An increase in WBCs is called leukocytosis. This condition usually suggests infection or a leukemia. Stress, trauma, and tissue necrosis can also increase the total WBC count. A decrease in WBCs to less than reference values is called leukopenia. Diseases associated with this situation include viral infections, aplastic anemia, pernicious anemia, overwhelming infections, bone marrow failure, autoimmune diseases, and dietary deficiencies. Many drugs can also affect the WBC count. Drugs causing a decreased count include acetaminophen, barbiturates, chloramphenicol, penicillins, sulfonamides, Lasix, Valium, and Librium. Most chemotherapy agents significantly decrease values. Drugs that can cause an increased count include ampicillin, erythromycin, tetracycline, salicylates, and atropine (in children). The degree of increase or decrease in WBC values is directly proportional to the severity of the disease.
DIFFERENTIAL WHITE BLOOD CELL COUNT
CPT: 85004–85007, 85009
Normal, nonmedicated, nonpregnant adults:
The differential WBC count measures each type of leukocyte as a percentage of the total number of leukocytes. The leukocyte types observed on a peripheral blood smear are identified by their morphology. Neutrophils make up the largest percentage of leukocytes in adults. They are the most important leukocyte (because of their ability to use phagocytosis) in the body’s defense against infection and rapidly respond to acute infection, inflammatory disease, and tissue trauma. Eosinophils increase during allergic reactions, parasite infestation, and active sarcoidosis. Basophils prevent blood clotting during an inflammation. They contain granules of heparin and histamine. Lymphocytes make up the second largest group of leukocytes. They are seen to increase in conditions such as chronic bacterial infection, viral infection, infectious mononucleosis, lymphocytic leukemia, and multiple myeloma. Their numbers decrease as a result of immune deficiency diseases, steroid therapy, excess hormone production, or sepsis. Monocytes act as a second line of defense against infection. They respond more slowly than neutrophils to acute infection and inflammation, but once in action, they operate as powerful macrophages that continue into the chronic phase, ingesting dead tissue and debris and clearing the tissue for healing. Diseases in which monocytes increase include collagen diseases, RA, sickle cell anemia, viral diseases, herpes zoster, tuberculosis, toxoplasmosis, and cancers. The elevation, then, of any one type of leukocyte may serve as an important clue in the diagnosis of disease.
COMPUTERIZED TOMOGRAPHY SCAN
CPT: 70450 CT, HEAD OR BRAIN; WITHOUT CONTRAST MATERIAL
CPT: 70460 CT, HEAD OR BRAIN; WITH CONTRAST MATERIAL(S)
CPT: 70480 CT, ORBIT, SELLA, OR POSTERIOR FOSSA WITHOUT CONTRAST MATERIAL
CPT: 70481 CT, ORBIT, SELLA, OR POSTERIOR FOSSA WITH CONTRAST MATERIAL(S)
No evidence of a pathologic condition
The computerized tomography (CT) scan can be used to detect diseases of the brain or orbit. The CT scanner produces a narrow x-ray beam that can examine the body from many different angles. A CT scan of the brain consists of a sequence of tomographic x-ray films taken of the brain tissue at successive layers. The films are subjected to a computerized analysis that builds up the shots into a three-dimensional (3D) picture. The CT x-ray image produced is a view of the head as if one were looking down through its top. Each type of tissue has its own density, and each permits the x-ray beam to penetrate only so far. An attached computer calculates the amount of x-ray penetration of each tissue. It displays this as varying shades of gray. The x-ray image appears on a television screen and is photographed. The denser the tissue, the lighter it appears in the image. The densest tissue appears white and less-dense tissue appears in progressively darker shades of gray. The result is an actual anatomic picture of a coronal section of the brain. The CT scan can be performed with or without iodine contrast dye. The iodine in the contrast dye causes a greater tissue absorption. This is referred to as contrast enhancement. A small tumor may not be observed if contrast enhancement is not used.
CT scans can be used to detect intracranial neoplasms, cerebral aneurysms, intracranial hemorrhages or hematomas, cerebral infarctions, cortical atrophy, arteriovenous malformations, or ventricular enlargement or displacement. CT scans can be repeated to monitor the progress of a disease or to monitor the effects of treatment. One advantage of the CT is that it has eliminated the need for more invasive procedures such as cerebral arteriography and pneumoencephalography.
- The patient is kept nothing by mouth (NPO) for 3 to 4 hours before the test. If contrast enhancement is not to be performed, the patient need not be restricted from food or fluids. Contrast dye can cause nausea and vomiting.
- Jewelry, hairpins, clips, and so forth are removed from the patient’s head.
- Steroids or antihistamines may be ordered several days prior to the test if the patient has a known allergy to iodine or contrast dye. Emergency equipment should be available to treat any severe allergic reaction, such as anaphylactic shock.
- The patient lies supine on the examining table with the head resting in a snug-fitting rubber cap within a water-filled box. A rubberized strap is wrapped around the head to keep it immobilized during the test.
- The patient’s head is moved into a circular scanner. The scanner passes a small x-ray beam through the brain from one side to the other. The machine then rotates 1 degree. The process is repeated at each degree through a 180 degree arc. The procedure takes approximately 45 to 60 minutes (filming 3 to 7 planes).
- If contrast enhancement is needed, an iodine-contrast dye is injected intravenously over a period of 2 minutes. The patient may feel warm and flushed and experience a salty or metallic taste. Nausea may occur. These symptoms usually last only about a minute.
- A mild sedative may be ordered for a restless patient or an analgesic for a patient with neck or back pains.
Adults: 0.6 to 1.2 mg/dL; 53 to 106 pmol/L (SI units)
Infants to 6 years old: 0.3 to 0.6 mg/dL; 27 to 54 pmol/L (SI units)
Older children: 0.4 to 1.2 mg/dL; 36 to 106 pmol/L (SI units)
The serum creatinine test and BUN are the tests most commonly used to assess kidney function. Both are included in standard chemistry panels. Creatinine is a by-product of muscle catabolism. It is derived from creatine phosphate, which is used in skeletal muscle contraction. The amount of creatinine produced daily is proportional to the amount of muscle mass. Creatinine circulates in the blood and is excreted by the kidneys. The concentration of creatinine in the blood is directly related to how well the kidneys are functioning. The serum creatinine level, then, can be used as an indicator of kidney function. It is considered to be a more specific and sensitive indicator of renal disease than the BUN, but the two tests are generally considered together. It is not influenced by diet or fluid intake and rises more slowly than the BUN. A small rise in the BUN may indicate dehydration, GI bleeding, or malnutrition; a serum creatinine of 2.5 mg/dL could indicate renal disease. When the BUN level rises but serum creatinine remains normal, dehydration is suspected. When both rise, the patient should be assessed for kidney disease. Clinical problems that can cause the serum creatinine level to rise include acute and chronic renal failure, hypertension, diabetic nephropathy, cancer, SLE, and a diet high in creatinine (lots of beef, poultry, and fish). Drugs that can elevate serum creatinine include antibiotics such as gentamicin and cephalosporin, barbiturates, and ascorbic acid. High intake of glucose or protein can also elevate serum creatinine. A decreased serum creatinine may be seen during pregnancy.
- Seven to ten milliliters of venous blood is collected in a serum separator tiger-top tube.
- The sample is sent to the chemistry lab with a list of any drugs the patient is taking that could elevate the serum level.
ERYTHROCYTE SEDIMENTATION RATE; SEDIMENTATION RATE
Reference Values (Westergren Method)
Men less than 50 years old: 0 to 15 mm/h
Men more than 50 years old: 0 to 20 mm/h
Women less than 50 years old: 0 to 20 mm/h
Women more than 50 years old: 0 to 30 mm/h
Children 4 to 14 years old: 3 to 13 mm/h
Men: The patient’s age/2
Women: The patient’s (age + 10)/2
The ESR test is used to detect and monitor response to treatment of inflammatory conditions, infections, neoplasms, and necrotic processes. It measures the rate at which the RBCs in a sample of anticoagulated blood settle at the bottom of a narrow-bore tube in a 1-h period. The ESR is a nonspecific test and is not diagnostic for any specific disease or injury. The test can detect inflammatory processes but can also be increased by acute and chronic infections, rheumatoid collagen diseases, neoplasms, pneumonia, syphilis, tissue necrosis, pregnancy, and general physiologic stress. All these conditions can cause an increase in the amount of protein (mainly fibrinogen and globulins) in the plasma. As a result, the repellent forces between adjacent RBCs begin to break down, and RBCs tend to stack on top of one another (rouleaux), increasing their weight and causing them to sediment faster than single cells. The ESR is therefore increased.
Decreased ESR values can also occur. Causes include CHF, polycythemia vera, sickle cell anemia, and hypofibrinogenemia. Some clinicians think the ESR is not that useful because it is so nonspecific and because it is affected by so many physiologic factors, it is best to order a C-reactive protein (CRP) test along with the ESR. CRP tends to increase more rapidly during an acute inflammatory process and return more quickly to normal than an ESR. The ESR, however, is a fairly reliable indicator for following the course of a disease. It is, therefore, mainly used to monitor the effects of therapy. Generally, the ESR will increase if a disease worsens and decrease as the disease improves. The ESR is also sometimes used to help with a differential diagnosis (e.g., RA versus osteoarthritis; or, in the case of a patient with chest pain, acute MI, which would show an increased ESR, versus angina pectoris in which the ESR would remain normal). One of the most important roles for the ESR in eye care is in the diagnosis and management of giant cell arteritis. Both CRP and ESR should be ordered in these patients. Both are indicators of inflammation. In the majority of affected patients, both tests will be significantly raised. Usually, the higher the value, the more active the disease is. If both tests are normal, it is highly unlikely that the patient has temporal arteritis, and imaging for other causes of vision loss should be performed.
- Seven to ten milliliters of venous blood is collected in a lavender-top tube containing EDTA or an oxalate black-top tube (to keep the blood from coagulating).
- The specimen is taken immediately to the hematology lab. The blood should not be allowed to stand, because this may affect the sedimentation (SED) rate. Therefore, the test should be performed within 2 to 3 hours after the specimen has been obtained. If refrigerated, the blood should be allowed to return to room temperature before testing.
- The blood is drawn into the Westergren tube, placed in a vertical rack, and left undisturbed.
At the end of 1 hour, the height of the clear plasma above the red cell column is measured. This height, the distance the RBCs settle in 1 hour, is a measure of the rate of fall of the red cells (Fig. 2-2).
Figure 2-2. Westergren ESR. (Photo courtesy of Dr. Bruce Onofrey.)
FASTING PLASMA GLUCOSE FASTING BLOOD SUGAR (FBS)
2009 Reference Values from the American Diabetes Association
Normoglycemic: 70 to less than 100 mg/dL
Impaired (Prediabetic) ≥100 but less than 126 mg/dL or 2-H OGTT ≥140 mg/dL
Diabetes: greater than 126 FPG or 2-H OGTT ≥ 200 mg/dL. Screening with self monitoring devices: Although performed on capillary blood, which has slightly lower glucose values, most self-monitoring blood glucose instruments incorporate a compensation formula reporting the value as if it were plasma. Therefore, the same normal/abnormal ranges may be used for these devices. Refer for a physical examination and further testing if screening values are in either the diabetic or impaired range.
In most cases, the test of choice for detecting and diagnosing diabetes is the FPG. It is less expensive, less invasive, and more readily available than the oral glucose tolerance test (OGTT). Because most diabetics have the disease for 4 to 7 years prior to diagnosis and because diabetes is the leading cause of new blindness in adults aged of 20 to 74 years, early detection is crucial. Glucose is derived from dietary carbohydrates. It is stored in the liver and in skeletal muscle in the form of glycogen. Blood glucose levels are controlled by two hormones, insulin and glucagon, both produced by the pancreas. Insulin is necessary in order for glucose to enter cells. Glucagon helps the liver convert glycogen back into glucose.
An increased blood glucose level (hyperglycemia) occurs when there is an insufficiency of insulin. This condition indicates diabetes mellitus. However, there are other possible causes of an elevated blood sugar, including adrenal gland hyperfunction (Cushing’s syndrome), an acute stress response, acute MI, infections, renal failure, hyperthyroidism, and drugs such as diuretics and corticosteroids. For a diagnosis of diabetes, the patients must have abnormal tests on two different occasions. Tests could be random (casual glucose) with systemic symptoms, FPG, and/or 2-H OGTT.
Decreased blood glucose (hypoglycemia) also has many causes including hypothyroidism, Addison’s disease, cancer, extensive liver disease, malnutrition, and strenuous exercise. Additionally, there are many otherwise healthy people who tend to be consistently mildly hypoglycemic for no apparent pathological reason. This is generally managed by consuming more frequent smaller meals and nonsugary snacks throughout the day.
- The patient should have had no caloric intake for a minimum of 8 hours but should have ingested water to avoid dehydration. If other tests such as lipid profiles are also ordered, the patient should fast for 12 hours and ingest plenty of water.
- Peripheral venipuncture is performed, and 5 to 10 mm of blood is collected in a gray-top tube containing sodium fluoride (to diminish glycolysis) or in a SST if other tests are also ordered. If the SST is used, the test must be performed within several hours or values will drop. Blood should be collected before insulin or any hypoglycemic agent is administered.
- The blood sample is taken to the chemistry lab. The most frequently used method of analysis involves enzyme reactions (hexokinase and glucose oxidase).
2-HOUR POSTPRANDIAL BLOOD SUGAR; 2-HOUR POSTPRANDIAL GLUCOSE TEST
Serum: less than 140 mg/dL at 2 hours
Whole blood: less than 120 mg/dL at 2 hours
Serum: less than 160 mg/dL at 2 hours
Whole blood: less than 140 mg/dL at 2 hours
This is a blood glucose test performed 2 hours after a meal.
GLUCOSE TOLERANCE TEST; ORAL GLUCOSE TOLERANCE TEST
Adults: See Table 2-3
Children: Varies with child’s age. Infants normally have a lower blood sugar level.
Children age 6 or older have GTT values similar to adults.
The World Health Organization (WHO) recommends the GTT as the preferred way to diagnose diabetes. However, the American Diabetes Association (ADA) no longer recommends the glucose tolerance test (GTT) as the test of choice for diagnosis of most diabetes due to cost, invasiveness, and time required. ADA does recommend that it be performed (using the WHO procedure of 75 g glucose load) when the diagnosis is uncertain, for prediabetics, and in some cases of gestational diabetes. The GTT or OGTT measures a patient’s ability to handle a standard oral glucose load. Blood and urine samples are collected at specific times (before glucose administration, 30 minutes after, 1 hour after, 2 hours after, 3 hours after, and sometimes 4 hours after) and checked for their glucose levels. In most cases, the procedure will be stopped after collection of the 2 or 3 hour specimens. Normal patients handle the glucose dose easily and show only a minimal, transient rise in their blood glucose levels within 1 hour, and there is no glucose spillover into their urine. Diabetics are deficient in insulin and therefore have difficulty tolerating the standard glucose load. Their blood glucose levels will increase significantly between 1 and 5 hours, and glucose will usually be detected in their urine.
The GTT is contraindicated if the fasting blood sugar is over 200 mg/dL or if the patient has a serious concurrent illness, endocrine disorder, or infection (because a glucose intolerance will probably be observed even if the patient is not diabetic).
The peak glucose level for an oral GTT is usually observed within 30 minutes to 1 hour after ingestion of the standard dose. If the patient is normal, blood sugar should return to fasting level in 3 hours. Patients older than age 60 often have a blood glucose level 10 to 30 mg/dL higher than the “normal range.”
There are other causes of glucose intolerance besides diabetes. Elevated GTT values may be seen in patients with hyperthyroidism, Cushing’s syndrome, hyperlipoproteinemia, cancer, duodenal ulcer, stress, infection, acute MI, and alcoholism. Elevations may also be seen in pregnant or obese patients. Certain drugs can also lead to increased GTT values, including corticosteroids, thiazide diuretics, salicylates, and oral contraceptives. Decreased GTT levels (<70 mg/dL at 3 hours) can also occur. Causes include hyperinsulinism, adrenal gland insufficiency, protein malnutrition, and malabsorption.
- The patient remains NPO for 8 to 12 hours before the test, except for water. No food, no smoking, no caffeine, no excessive exercise, and if possible no medications.
- Five milliliters of venous blood is collected in a gray-top tube for an FPG. A fasting urine specimen is also collected.
- The patient is given a 75-g glucose load (WHO criteria) dissolved in water and flavored with lemon juice. Some clinicians give glucose according to body weight (1.75 g/kg, or 1 g/lb for children weighing <100 lb). The patient must ingest the entire glucose load because GTT values are based on a standard 75-g glucose load.
- Blood and urine specimens are obtained at 30 minutes, 1, 2, and 3 hours and after glucose ingestion, and their glucose levels determined.
- During testing, no food, caffeine, or smoking is permitted. The patient is encouraged to drink water to facilitate the obtaining of urine specimens. The patient should also be at rest, because exercise (including walking) can affect glucose levels.
- During testing, the patient is checked for any dizziness, sweating, weakness, or giddiness.
- No insulin or oral hypoglycemics should be taken during testing.
ESTIMATED AVERAGE GLUCOSE
CPT: 83036 (Same as for A1c)
Estimated average glucose (eAG) is a new term recently introduced by the ADA due to the close relationship between A1c and average glucose values. The value is reported in the same units as blood glucose (mg/dL) and so is easy to compare to actual glucose values. In response to the ADA request, laboratories are beginning to include this calculation to their A1c reports.
eAG is a calculation that converts A1c values to an estimate of what the patient’s average glucose has been for the past few months. This value was previously estimated using charts with ranges, but these charts used ranges instead of the exact A1c values for individual patients. The eAG value is more meaningful to doctors and patients than the straight A1c value as it is easier to relate it to the patient’s estimated glucose values over the past 3 to 4 months because it is in mg/dL not in percentage. The ADA has a conversion program on their Web site www.diabetes.org where the patient or doctor can simply enter his A1c value, hit calculate, and the estimated glucose value appears. The conversion program can also be downloaded onto other devices. The formula used is eAG in mg/dL = 28.7 × HA1c (%) -46.7.
FLUORESCENT TREPONEMAL ANTIBODY ABSORPTION TEST (FTA-ABS)
Negative or nonreactive
The FTA-ABS test is used to determine whether a patient has or has not had systemic syphilis. It is the most sensitive and specific test for diagnosing any stage of the disease and is the test of choice for confirming a diagnosis. Once the test shows positive, it tends to remain positive indefinitely (even after treatment) in 95% of patients. Therefore, this test cannot be used to establish the activity of syphilis, how recently it was incurred, or how effective any treatment has been.
The FTA-ABS is usually ordered in combination with a rapid plasma reagin (RPR) or VDRL test to determine if the disease is active. If a patient has a positive FTA-ABS but shows a nonreactive RPR or VDRL, this indicates that the patient was exposed to Treponema pallidum (the causative agent for syphilis) or had syphilis sometime in the past but does not currently have the active disease. A positive FTA-ABS and a positive RPR or VDRL suggest that the patient has active syphilis. A negative FTA-ABS and a positive RPR or VDRL suggest a biological false-positive RPR meaning that the patient may have one of the many other conditions that result in a positive VDRL such as antiphospholipid antibody syndrome, mumps, measles, and other viral conditions. Since antiphospholipid antibodies may take several weeks to receive results, a positive VDRL could be an early clue of the reason for occlusion if it is known that the patient does not have syphilis.
False-positive FTA-ABS tests occur in about 1% to 2% of normals and may also occur in patients with collagen disease (e.g., SLE), but stained preparations from patients with SLE usually show an atypical beaded appearance not usually seen with syphilis.
- Five to seven milliliters of venous blood is collected in a red-top tube.
- The patient’s serum is placed on a slide that contains fixed T. pallidum organisms. If antibodies are present, the organisms become coated.
- A fluorescent antibody against human globulin is then added to the slide. This combines with any coated organisms, causing fluorescence.
- The slide is rinsed and observed under an ultraviolet microscope. If fluorescence is present, the test is positive. If no fluorescence is seen, the test is negative.
Two newer tests for syphilis are the microhemagglutination assay for antibodies to T. pallidum (MHA-TP) and the hemagglutination treponemal test for syphilis. Both tests compare well with the FTA-ABS in terms of sensitivity and specificity (although the MHA-TP is less sensitive in primary syphilis). Their interpretation and use are also similar to that of the FTA-ABS.
Fructosamine is used to monitor short-term changes in glucose homeostasis. It measures glycated proteins in contrast to A1c that measure glycated hemoglobin. The half-life of protein is 2 to 3 weeks, so this test should not be repeated more than every few weeks. Fructosamine should be considered instead of Hb A1c for patients with hemoglobinopathies such as sickle cell disease or trait, thalassemia, and hemoglobin C disease. Suspect the presence of hemoglobinopathy in diabetic patients with consistently high glucose values but low or normal A1c results as the A1c value is often not valid in these patients. Also consider fructosamine when desiring to evaluate short-term glucose homeostasis changes.
Collect 7 mL of venous blood in a red top or SST.
Send to the lab.
Methodology: Kinetic spectrophotometry
No antigen present
The major histocompatibility locus antigens (HLAs) are extremely important in tissue recognition. Genes that regulate the HLAs are located on chromosome 6 at the HLA region. This region is composed of four closely linked genetic loci: HLA-A, HLAB, HLA-C, and HLA-D. Each person possesses two genes (one maternal gene and one paternal gene) at each of the four loci; this is because both mother and father contribute a complete haplotype of their HLA genes to each child. These eight genes together make up a person’s complete HLA phenotype. Antigens are named by giving them a letter representing their locus and a number that is unique for each antigen.
There are many HLAs, but the one with the most clinical significance is HLA-B27. Approximately 5% to 8% of normal white patients and a smaller percentage of normal black patients possess the HLA-B27 antigen. In whites, HLA-B27 is found in more than 90% of the patients with ankylosing spondylitis, 75% to 85% of patients with Reiter’s syndrome, 60% or fewer in those with psoriatic arthritis, and 20% to 50% of patients with acute uveitis regardless of the cause. In blacks with the same diseases, HLA-B27 is found to be about one-half as prevalent. HLA-B27 has also been associated with inflammatory bowel disease. The HLA-B27 antigen, then, can be used to help detect and diagnose some of these diseases, especially ankylosing spondylitis and Reiter’s syndrome.
- At least 10 mL of venous blood is obtained in a heparinized solution.
- Anti-HLA-B27 cytotoxic antibody is then incubated with lymphocytes taken from the patient. If HLA-B27 antigen is present, a complex will form on the cell surface. Serum complement is then added to the mixture; this kills the lymphocytes and recognizes the titer of HLA-B27.
Less than 10 mmol/L
Elevated homocysteine is associated with an increased risk of heart attack and stroke. It has also been associated with an increased risk of retinal vein occlusion.
Homocysteine is an amino acid that is produced as a by-product of meat consumption. Normally, it is converted to methionine and cysteine with the help of folic acid, B12, and B6. A shortage of these compounds can contribute to elevated levels of homocysteine. Elevated homocysteine levels have been associated with an increased risk of atherosclerotic vessel disease. Treatment includes folic acid and B vitamin supplementation.
INR (INTERNATIONAL NORMALIZED RATIO)
Warfarin patient: 2 to 3
The INR is a system established by the WHO and the International Committee on Thrombosis and Hemostasis for reporting the results of blood coagulation tests. All results are standardized using the international sensitivity index for the particular thromboplastin reagent and instrument combination utilized to perform the test. It is a standardized version of the prothrombin time or PT test.
For example, a person taking the anticoagulant warfarin might optimally maintain a prothrombin time (PT) of 2 to 3 INR. No matter what laboratory checks the test time, the result should be the same even if different thromboplastins and instruments are used. This international standardization permits the patient on warfarin to travel and still obtain comparable test results.
LIPID PANEL (SERUM)
CPT: 80061 (May vary depending on exact instrument used)
Cholesterol: Less than 200 mg/dL
TGs: 10 to 190 mg/dL
HDL: 40 to 60 mg/dL
Greater than 60 mg/dL considered a negative risk factor for heart disease
Less than 40 mg/dL considered a major risk factor for heart disease
Less than 100 mg/dL desired
100 to 129 mg/dL borderline
ADA 2009 goals for diabetics: TGs less than 150 mg/dL, HDL greater than 40 mg/dL, LDL decrease by 40% from baseline
Lipid panels generally consist of total cholesterol, TGs, HDL, LDL, total cholesterol/HDL ratio, and coronary heart disease (CHD) risk factor. VLDL may also be included by some labs. The CHD risk factor takes into account all of the lipid values and the patient’s age and sex. Some labs also factor in whether the patient smokes or not. The normal or expected CHD varies depending on the manner of computation used by the lab.
Serum lipid levels are used to help diagnose and manage atherosclerotic disease. When lipids are transported in the blood, they are combined with proteins and called lipoproteins. There are two major classes of lipoprotein: HDL and LDL. LDLs have a strong association with atherosclerosis and coronary artery disease. HDLs are known as the “good” lipoproteins. They are associated with a decreased risk of coronary artery disease as they transport LDL away from the blood vessels and back to the liver for excretion from the body. Many doctors consider the HDL and total cholesterol/HDL ratio more important than the total cholesterol and/or LDL values. When there are increased levels of lipoproteins in the blood, the condition is called hyperlipidemia or hyperlipoproteinemia. There are six types of hyperlipoproteinemia, classified according to the major lipoprotein elevation and the major lipid elevation (Table 2-4). Types II and IV are the most common and most prevalent in atherosclerosis and coronary artery disease.