Allergy testing: in vivo versus in vitro.

Table 4 briefly summarizes the relative advantages and disadvantages of skin tests versus in vitro tests for detecting allergen-specific IgE. The skin test remains unexcelled as a sensitive and cost efficient test for specific IgE. The high degree of skin test sensitivity is very important when a patient must be evaluated for potentially life-threatening allergies such as to penicillin or stinging insects. The results of both skin tests and in vitro assays depend very much on the quality of the allergen extracts used for the tests. Although the quality of extracts is improving, there is still little standardization. Both skin tests and in vitro assays are difficult to quality control. Practicing allergists rely on experience, and the correlation between patient histories and skin tests results for quality control of the results. Although this system suffices for common allergens, the results for uncommon allergens may be misleading. Quality control is also difficult for in vitro tests. Participation in quality control programs, such as that being offered by the College of American Pathologists, will increase and lead to better quality and standardization of in vitro test results. At the present time, properly performed skin tests are the best available method for detecting the presence of allergen specific IgE. They are rapid, sensitive, and inexpensive on a per test basis. In vitro tests are acceptable substitutes for skin tests in some circumstances. If the patient does not have normal skin, cannot discontinue interfering medications, or is so sensitive by history that anaphylaxis seems possible, in vitro tests are preferred. In vitro tests are better when it is necessary to test a difficult patient such as a combative, mentally retarded adult. In vitro tests also have been invaluable in many allergy research studies. Physicians must remember that positive tests for allergen-specific IgE do not diagnose allergy. They only indicate the presence of IgE molecules with a particular immunologic specificity. A decision whether the specific IgE molecules are responsible for clinically apparent disease must be made by a well-trained physician. The ultimate standard for the diagnosis of allergic disease remains the combination of: (1) positive double-blind challenge, (2) the presence of specific IgE, and (3) demonstration that the symptoms are the result of IgE-mediated inflammation.