Allergen skin prick test should be adjusted by the histamine reactivity.

Skin prick test results are mostly reported as mean wheal diameter obtained with one concentration of allergen. Differences in technique between personnel causes variation in wheal size. The research question was whether the influence of differences in skin prick test technique among assistants and centers can be reduced by relating the allergen wheal response to that of histamine. Two methods for estimating skin reactivity, the method of Nordic Guidelines using histamine as a reference and the method of Brighton et al. [Clin Allergy 1979;9:591-596] not using histamine as a reference, were applied to data from two biological standardization trials, using the same batch of freeze-dried timothy pollen preparation. The concentration defining the Nordic biological unit, defined as a concentration of allergen eliciting a wheal of the same size as that of histamine dihydrochloride 10 mg/ml, did not differ between the centers. When not using histamine as a reference, applying the method of Brighton et al., there was a 15-fold difference in the estimate of the biological activity between the trials that was eliminated by adjusting the allergen response to that of the histamine reference. To reduce the influence of differences in test technique among assistants and centers responses to allergen-induced skin prick tests should be compared to that of histamine.