Assay of the biologic activity of allergen skin test preparations.

We studied the biologic activity of allergen preparations using a method involving skin prick tests in humans and the use of HEP (histamine equivalent prick) units. Results were found to be dependent on the population groups used in assays. If populations are not carefully standardized, results may vary by 1 power of 10. Accuracy can also be improved by the use of suitable allergen standard reference preparations, but such were not available to us. Confidence ranges for the biologic activities were relatively wide and varied with the allergen preparation and the population group. Typically, the 95% confidence range included values from one-fifth to five times the estimated HEP value when the number of subjects in the assay was 30 to 50 persons. When the preparations representing the same source material (e.g., timothy pollen) were assayed simultaneously in one population group of this size, a twofold or larger difference in HEP values generally proved significant. An examination of 43 commercial products showed that allergen preparations with biologic activities declared in HEP units had a more uniform biologic activity than those assayed with traditional methods and units (PNU/ml or weight/volume).