Diagnostic performance characteristics of the standard Phadebas RAST, modified RAST, and pharmacia CAP system versus skin testing.

Results from three in vitro assays for allergen-specific IgE, the standard Phadebas radioallergosorbent test (PhRAST), modified RAST (mRAST), and the new Pharmacia CAP System (CAP) were compared with skin prick testing (SPT) results in 104 patients with allergic rhinitis and/or asthma and 24 nonatopic controls. Five allergens were evaluated: cat, Dermatophagoides pteronyssinus, Alternaria, June grass, and short ragweed. Using SPT results as the reference standard, the PhRAST had the lowest sensitivity (62%) and highest specificity (99%). The CAP achieved higher sensitivity (74%) with comparable specificity (96%) while the mRAST had even higher sensitivity (90%) but lower specificity (87%). The overall frequency of positive results in controls was 0% for PhRAST, 1.7% for CAP, and 3.3% for mRAST. If the threshold for a positive mRAST was raised to greater than or equal to class 2, this assay achieved performance characteristics similar to the CAP. If the results of these in vitro tests are used as the sole guide to the prescription of environmental control and immunotherapy in unselected patients with rhinitis and asthma, the performance characteristics of the CAP make it the preferred test.