In Vitro Allergy Testing
The discovery of the role of IgE in clinical allergy testing resulted in the development of in vitro diagnostic assays to test for allergen sensitivity. The first immunoassays were developed to quantify the serum concentration of total IgE. In normal individuals, IgE is usually present at low levels; 130 ng/ml represents the upper limit of the normal range. However, a significant number of asymptomatic normal individuals, such as those with parasitic diseases or with depressed cell-mediated immunity, exceed this level. Also, some allergic patients may exhibit normal total IgE levels in the presence of elevated levels of specific IgE. Methods were therefore developed to assay allergen-specific IgE. The radioallergosorbent test (RAST) system was developed for in vitro measurement of specific IgE in a patient’s serum. Other in vitro tests for specific IgE have been developed and employ the same principles as the RAST but use an enzymatic (MAST) or fluorogenic (FAST) detection system in place of a radioactive label.
In vitro tests that screen for multiple allergens in a single assay (Phadiatop®, Pharmacia Diagnostics) or that can be used in an automated system (ImmunoCAP®, Pharmacia Diagnostics) have been developed. The ImmunoCAP is designed as a “sandwich” immunoassay. The sensitivity and specificity of the ImmunoCAP compares favorably with those of the modified PhadezymRAST® system. Results from studies have indicated that, when compared to skin prick testing as the gold standard, the ImmunoCAP system has been shown to have a greater sensitivity (80–95%) than RAST and to have similar specificity (85%). Other modified versions of the RAST test include the PRIST, RIST, MRT (modified RAST) and ELISA IgE tests.
The overall sensitivity of in vitro immunoassays compared with prick/puncture skin tests has been reported to range from 50–90%, with an average of about 70–75% from most studies. Skin testing, therefore, continues to be the preferred method for the diagnosis of IgE-mediated sensitivity. Selective use of in vitro tests may be justified for patients in whom skin testing is inappropriate. Situations in which specific IgE immunoassays may be appropriate include:
• testing of patients with severe dermatographism, ichthyosis or generalized eczema
• testing in patients who cannot be withdrawn from medications that interfere with skin testing (patients receiving long-acting antihistamines or tricyclic antidepressants)
• testing in patients who have a clinical history suggesting an unusually greater risk for anaphylaxis or who have had a previous systemic reaction to skin testing
• testing of patients with mental or physical impairments
It should be noted that specific IgE immunoassays do not have sufficient sensitivity for absolute positive prediction of anaphylactic sensitization to venoms, penicillin and other drugs. This method of testing should not be used to provide definitive diagnoses, due to the potential for serious consequences resulting from a false-negative outcome. Allergen-specific IgE immunoassays provide neither diagnostic nor prognostic information when measured in the cord blood of newborn infants.
In vitro allergy testing is not indicated when there are no contraindications to skin testing or in patients who are successfully being treated for allergies, have mild symptoms and a short allergy season, or have had negative skin testing for the allergy in question.