Histamine control affects the weal produced by the adjacent diluent control in skin prick tests

The effect of histamine chloride control (10 mg/ml) on the weal reaction produced by the adjacent test solution (buffered glycerol-saline diluent control) in skin prick tests was studied among 270 peat workers and 214 lumberjacks. The diluent control was placed on the skin at two test sites: 3-4 cm and 18-19 cm from the site of the histamine control. Prick-test lancets (Dome/Hollister-Stier) were used to prick the skin. Weal areas of 6 mm2 or larger were considered to be positive reactions. In both populations the diluent control caused positive reactions nearly three times more often when the test site was 3-4 cm from the site of the histamine control than when it was 18-19 cm away from the control (P less than 0.001). Similar relationships might be found between histamine control and allergen solutions. This finding should be taken into account in histamine-based standardisation of the results of skin prick tests as well as in biological standardization of allergen solutions.