Comparison of skin testing and three in vitro assays for specific IgE in the clinical evaluation of immediate hypersensitivity

A new assay, Pharmacia CAP System (PCS), for allergen-specific IgE (sIgE) was evaluated in 198 new patients presenting with respiratory symptoms to an urban allergy practice. An experienced allergist examined each patient and clinically assessed sensitivity to timothy, short ragweed, Alternaria tenuis, cat, or D. farinae. Puncture and selected intracutaneous skin tests (ST) with these inhalant extracts were then performed. The physician again rated the likelihood of clinical sensitivity to each inhalant, and serum was obtained for sIgE measurements by Phadebas RAST, modified RAST, and PCS. Results of the three in vitro tests (IVT) correlated well with each other and generally agreed with physician assessments and ST results. Individual differences for extracts and assay methods were identified. A few patients with negative ST had positive IVT, but more patients with positive ST were negative by IVT. Modified RAST had greater sensitivity but less specificity than the other two IVT. Analysis of receiver operating characteristic curves showed that sensitivity of the three assays when compared at the 95% level of specificity, did not differ. This result suggests that the cutoff criterion for a positive modified RAST result is too low and should be reevaluated. Skin tests remain the most sensitive and specific test available. The Pharmacia CAP System is a clinically useful assay for sIgE and appears to be a clear advancement for IVT technology.