Skin prick test allergen batch differences and duplicate skin prick tests: Which are important?

Background: Skin prick tests (SPT) are widely used both in clinical practice and in research. The standardization of the allergen extracts is well documented to be crucial for the validity of SPT, whereas less emphasis has been on reproducibility and the SPT procedure itself. Objective: To study how the duplicate skin prick test procedure influence the sensitivity and specificity of the test and to analyse the differences in wheal size in skin prick tests with two batches of allergen extracts from same vendor. Methods: The association between rhinitis and SPT was assessed among 1135 subjects from a general population sample. SPT was performed twice with 10 common aeroallergens. In a subsample of 90 persons SPT was performed simultaneously with five of the allergens using two different batches Results: Thirty percent had at least one positive SPT. Only minor disagreements were seen between the sizes of two wheals from the same batch. A second SPT with the same batch did not change the association between rhinitis and sensitization. When performing SPT with two different batches disagreement was observed in 2 % (Birch) to 11% (Dog) of the subjects. Conclusions: Performing SPT twice with the same allergen batch does not enhance the validity of the test, and duplicate testing cannot be recommended. Considerable difference in SPT response was observed when using different batches from the same vendor. Thus, inter batch differences in allergen extracts might be a source of variability.