Reproducibility and use of low-concentration skin prick test

We aimed to evaluate the reproducibility of the skin prick test performed with serial 1:4 dilutions of commercial standardized extracts in comparison with serum-specific IgE and the undiluted commercial extract. Twenty-four subjects sensitized to one (17 cases) or two (seven cases) inhalant allergens were selected and submitted to duplicate skin prick tests with concentrated commercial allergenic extracts or with serial 1:4 dilutions of the same extracts in two different examinations 7 days apart. Blood samples were obtained from 17 of the 24 patients for specific IgE determination. No statistically significant within-patient variations in the area of the wheal in skin prick tests done 1 week apart were found up to the eighth dilution (1:256) of the commercial allergen. On a patient-by-patient basis, only some dilutions showed a statistically significant correlation between allergen-specific IgE and the wheal area elicited by the same allergen, and a significant correlation was found between the wheal elicited by 10 mg/ml histamine and both the concentrated and diluted allergens (up to the sixth dilution). In polysensitized patients, the allergen producing the largest wheal when used in concentrated form did not produce the same result when diluted. The skin prick test with low-potency allergens was reproducible in individual patients even after a 7-day interval up to a 1:256 dilution of the commercial extract, although there was no clear correlation with allergen-specific IgE concentration. In polysensitized patients, the use of high-potency or low-potency allergens for skin prick tests can lead to different conclusions regarding the relative importance of each allergen.