A standardized quantitative skin-test assay of allergen potency and stability: studies on the allergen dose-response curve and effect of wheal, erythema, and patient selection on assay results

A quantitative skin-test assay of allergenic potency was developed and tested on 28 allergic patients. The best-fit linear regression line was calculated from the sum of erythema or wheal diameters obtained from four intradermal threefold serial dilutions near the endpoint. Each line must have a correlation coefficient ≥0.85 and the slopes of reference and test extract lines must not be significantly different. The relative potency can be calculated from the horizontal distance between the reference and test lines. The between-assay reproducibility was determined by comparing references against themselves. The 95% confidence limits were 54% to 186% when erythema was used and 27% to 367% when wheal was used. The narrower limits for the assay using erythema may be due to the 5.5 times steeper slope of erythema lines than wheal lines. Potency results on five commercial extracts were highly correlated with potency results by RAST, AgE assay, or rye I assay. Extracts with isoelectric focusing patterns dissimilar from the references were of significantly lower potency by in vivo and in vitro assays. The measured potency of two low-potency short ragweed extracts varied significantly with patient sensitivity to heat-stable and heat-labile ragweed allergens, indicating that patient selection may significantly affect assay results. Dose-response lines using either histamine or short ragweed by puncture (P) and intradermal (ID) techniques in the same patient showed that the P ID dose for equal erythema response was 909 for histamine and 31,250 for ragweed. This quantitative skin-text assay is highly reproducible, yields potency data comparable to those of in vitro tests, can be applied to studies of extract stability, and is suitable as a primary bioassay of allergenic activity.