HEP versus PNU standardization of allergen extracts in skin prick testing. A comparative randomized in vivo study

During 1 year, all patients referred to an allergy outpatient clinic for adults were skin prick tested with a panel of standard allergen extracts from two manufacturers using different methods of standardization. One company referred to the histamine equivalent prick (HEP) and the other used the more traditional protein nitrogen units (PNU). Standard extracts and five-fold dilution were tested. The results indicate that the ratio of concentration between two extracts of the same allergen should be measured by the absolute difference of the wheal diameters. We found significant differences between corresponding extracts from the two manufacturers.