15 Skin Testing: Prick
15.1 Most Interesting Information
Prick testing is the workhorse of allergy testing. It is commonly done without needles and (relatively) pain-free. It’s also technique-dependent and does require some practice to do correctly. It requires enough pressure to indent the skin and cause micro-tears in the epidermis to allow some penetration of the allergen. However, the pressure shouldn’t be enough to cause any bleeding or any significant pain. Regardless of which type of prick-test device is being used, no red blood cells should be released and no patient should be loudly vocalizing expressions of pain. This is definitely one area where less is more.
15.2 Serious Stuff
15.2.1 Who’s a Good Candidate for Skin Testing?
Anyone with symptoms suggestive of immunoglobulin E (IgE)-mediated allergic disease, who doesn’t have any medical contraindications listed in Chapter 14, and who is not taking any interfering medications.
15.2.2 What Does This Test Tell Me?
This test tells whether or not IgE is present to (an) antigen(s). These tests are not perfect and are technique-dependent. It does not tell you if a patient has allergy (see Chapter 3).
15.2.3 How Long Does This Test Take? When Can I Read This Test?
After applying the test to the patient’s skin, a practitioner should wait for 20 minutes, although there may be some variation in that timing among practitioners. Some might wait for only 15 minutes. The author waits for 20 minutes. Then the test results can be read as either positive or negative.
15.2.4 What Is a Positive Test? What Does It Tell Me?
A positive test is growth of a wheal of 3 mm or greater than the negative control. Generally, a positive-test result confirms the presence of IgE to (an) antigen(s). A positive test by itself only indicates allergen sensitization, NOT allergy.
15.2.5 What Is a Negative Test? What Does It Tell Me?
A negative test shows no growth or growth of a wheal less than 3 mm greater than the negative control. Generally, a negative-test result indicates the absence of IgE to (an) antigen(s).
15.2.6 What’s This Negative Control and What’s a Wheal?
One thing at a time! The negative control is whatever liquid the antigen concentrate is suspended in. For some company’s antigens, that’s 50% glycerin so you would use 50% glycerin as the negative control. For others, it may be human serum albumen (HAS) or another solution. Whatever that solution is will be what your negative control is. In the US, most commonly antigen companies use 50% glycerin in their concentrate vials. In that case, the negative control would be 50% glycerin.
15.2.7 And the Wheal?
It’s the palpable induration in the skin. Its size is measured and recorded. In the past, the size of the wheal was rated between 1 and 4, but that sort of grading scale is not recommended. Record the size of the wheal in millimeters.
15.2.8 Do You Need a Positive and Negative Control? Why?
Yes, because the practitioner needs to make sure the skin reacts as expected. The practitioner needs to confirm that it will react to histamine appropriately and won’t react to other things. See Chapter 14.
15.2.9 What’s the Positive Control?
The positive control is histamine (6 mg/mL) for percutaneous testing. There are different concentrations of histamine made for different types of testing. Make sure to use the correct concentration (▶Fig. 15.1).
15.2.10 What Antigens Do I Test for?
See Chapters 5 to 9. A practitioner should tailor tests according to the patient and include any relevant exposures (i.e., if the patient has a guinea pig or works in a research lab with rats, etc.).
15.2.11 Where Do I Test?
Generally, this test is done in the office. But a practitioner can perform this test at other locations as long as he/she is prepared to handle possible anaphylaxis.
15.2.12 No, I Mean, Where on the Patient Do I Place These Tests?
Most commonly the volar surface of the forearm, arm, and back are used.
15.2.13 Could Anaphylaxis Really Happen During Skin-Prick Testing?
Yes! It’s rare and unlikely to happen, but patients may experience a variety of adverse events while testing. See Chapters 34 and 35 for allergy emergencies or urgencies, but a practitioner must be prepared to handle anaphylaxis while performing any form of skin testing.
15.2.14 Tools of the Trade (What You’ll Need)
▶Fig. 15.2 shows tools used for skin testing.
Skin testing device (single- or multi-prick device).
Alcohol swabs or 70% isopropyl alcohol/gloves/skin marker/record sheet or electronic medical record (EMR) entry.
Histamine for percutaneous skin testing.
Negative control (50% glycerin if glycerinated antigen; use whatever solution the antigen is in).
Antigen with or without Dipwell or Tray.
Emergency supplies (see Chapters 34 and 35).
Wheal measuring device.
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