The result of a skin prick test is usually expressed either as the mean of the longest diameter and the diameter perpendicular to it in millimetres or as a relative size of the allergen-induced wheal, as compared with the wheal caused by the positive control (histamine).
A German study team 1 carried out skin prick testing in 7-year-old school children. The testing was repeated twice with an interval of 12 months. The research team noted that the size of both the allergen-induced wheals as well as those caused by histamines increased year by year. The researchers further noted that a significant discrepancy existed between the interpretations when the results were expressed in millimetres. However, these discrepancies were smaller when the results were expressed as the relative size of the wheal as compared with the histamine-induced wheal.
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