Although skin prick tests are a widely used tool, the methodology is not well studied. Using standard histamine solution in a highly controlled setting on 40 subjects, a great deal of variation was found in the the size of the wheal from site to site and from subject to subject. This variation suggests that skin prick tests require careful interpretation and that the results may need verification by more sophisticated and reproducible methods. Plasma, serum or urinary histamine testing can be expensive, therefore it is best to have your doctor confirm whether such a test would be useful.