European allergen extract units and potency: review of available information

Background: There is considerable variability in how allergen extract potency is measured and reported worldwide. In Europe, where many sublingual immunotherapy studies have been conducted, manufacturers report allergen extract potency as units based on an in-house reference, making it difficult to understand the exact doses used and to compare studies. Objectives: To describe the various methods of expressing extract potency that European allergen extract manufacturers use and to gather reports on the micrograms of major allergen of the in-house units of European allergen extract manufacturers. Methods: Information was derived from 3 sources: data on extract potency in micrograms of major allergen in articles on sublingual immunotherapy found by PubMed (references through October 2005) and in reference articles, brochures on allergen extracts from the manufacturers, and information provided by structured questionnaires e-mailed to the manufacturers. Results: All but 1 of the European allergen extract manufacturers use in-house reference standards that are based on titrated skin prick testing of allergic patients. Subsequently, in vitro tests compare the potency of commercial batches with the in-house reference and potency is assigned as arbitrary units. Most manufacturers measure major allergen content of their standardized products but do not release this information with the package insert. Diversity in major allergen content was found. Conclusions: Micrograms of major allergens given in articles on sublingual immunotherapy to express the dose administered cannot be used to translate the dose to US extracts. Extract potency can only be compared if uniform test methods and reference extracts are used.