17.1 Most Interesting Information

Skin prick testing is fast, reliable, and requires less skill to perform, but some allergists desire more information on the patient’s level of sensitization or wish to determine an “endpoint” for an allergen, so this form of testing is inadequate for their needs. However, Intradermal Dilution Testing (IDT) is much more time-consuming, and requires more skill than skin prick testing. This is where blended testing techniques come in—the marriage of the speed with additional information, for those allergists who want it. The most well-known protocol is termed “modified quantitative testing (MQT),” and it involves combining the results of skin prick testing and single intradermal testing to determine an “endpoint,” a potential starting point for immunotherapy.

17.2 Serious Stuff

Who’s a good candidate for this type of skin testing? Anyone who’s a candidate for skin prick and intradermal testing—patients with symptoms suggestive of immunoglobulin E (IgE)-mediated allergic disease and no medical contraindications listed in Chapters 15 and 16, and who have discontinued any interfering medications.

How does this test differ from skin prick testing or intradermal testing? What is the technique? The test is performed by placing a skin prick test for the antigens that are being tested. Then based on those results, a single intradermal test of two possible dilutions is placed. Based on the results of the skin prick and intradermal test, an endpoint can be assigned for each antigen tested. This technique has been discussed in more detail in this chapter.

What does this test tell? Similar to skin prick and intradermal, it shows whether or not IgE is present to (an) antigen(s). It can also be used to determine a starting point for immunotherapy, if that is desired.

What materials are needed? How are these tests performed? The materials used for prick and intradermal testing and the actual techniques to perform the prick and single intradermal tests are the same—see Chapters 16 and 17. The way in which the results of both are used is discussed further in this chapter.

How long does this test take? How do I read this test? The time is the same as discussed in Chapters 15 and 16 for both types of test: 20 minutes for prick testing, 10 minutes for intradermal testing. The criteria for positive and negative test interpretation are also the same—see Chapters 15 and 16.

What about controls? What controls are used? Do I have to place separate controls for each? The controls are the same as described in Chapter 15: A positive histamine control and negative 50% glycerin control for skin prick testing, and negative phenolated normal saline (PNS) and glycerin control for intradermal testing. However, it is not necessary to place two separate positive histamine tests, so the positive histamine control for intradermal testing can be skipped if the histamine control for prick testing responds appropriately.

What antigens to test for? See Chapters 5 to 9. The test should be tailored according to the patient. Any relevant exposures should be included. Do not use these tests for food or chemical allergy testing.

Where to test the patient (in the office)? Can the same area be used to test for both skin prick and intradermal? Again, most commonly the volar surface of the forearm, arm, and back are used. Whether it’s a prick test or single intradermal test, each test needs to be 2 cm apart to prevent contamination and false-positives from large spreading wheals, so if the testing area can accommodate that then—Yes, that same area can be use. If not, the tests will need to be placed in separate areas; so plan accordingly.

Could anaphylaxis really happen during this type of testing? Yes! It’s rare and unlikely, but patients may experience a variety of adverse events while testing. See Chapters 34 and 35, but the practitioner must be prepared to handle anaphylaxis while performing any form of skin testing.