Reproducibility of histamine skin prick test. Inter- and intravariation using histamine dihydrochloride 1, 5, and 10 mg/ml.

The reproducibility of skin prick test using histamine dihydrochloride 1, 5, and 10 mg/ml was tested by three nurses in five non-atopics in a double-blind trial. The variations day-to-day, within-day, between and for the same tester were calculated. Seventy-five percent of wheal reactions obtained by histamine 1 mg/ml were less than 15 mm2. With histamine 5 mg/ml there were only a few wheals less than 15 mm2 and none at all with histamine 10 mg/ml. The mean coefficient of variation of wheals greater than 15 mm2 was between 20-30%, in contrast to figures between 30-60% with wheals less than 15 mm2. No significant day-to-day or within-day variation was shown concerning histamine wheal areas. It is suggested that histamine dihydrochloride 10 mg/ml should replace histamine dihydrochloride 1 mg/ml as the positive reference in routine skin prick tests and biological standardization.