Effect of distance between sites and region of the body on results of skin prick tests.

BACKGROUND: Two variables in skin prick testing were examined: the effect of a positive reaction to histamine or allergen on adjacent negative test sites and the relative size of reactions to allergen and histamine on the back and forearm.
METHODS: Fifty-two subjects were tested on the back and arm with an allergen extract to which they reacted with a wheal of 8 mm or greater in diameter and with 50% glycerin at sites ranging from 2 to 7.5 cm distant from the allergen test site. A similar study was performed in 50 subjects by using 1.8 mg/ml histamine base to produce the positive reaction. The data were analyzed for the effect of a positive reaction on an adjacent negative control site and also for the relative size of positive reactions on the back and forearm. Data from a second study were also examined for the difference in reactivity on the back and arm. In this study 77 subjects underwent skin prick testing (in duplicate) with seven allergen extracts, 50% glycerin, and 1.8 mg/ml histamine base on the forearm and back.
RESULTS: Positive reactions at the 50% glycerin sites were observed at a rate of 0.52% when defined as wheals of 3 mm in diameter or greater and at a rate of 1.11% when defined as flares of 10 mm in diameter or greater. There was no increase in the rate of false-positive reactions over the range of 2 to 5 cm distance from the sites of positive reactions to allergen or histamine. Fewer reactions were present at the 50% glycerin sites, 7.5 cm from the positive sites. The diameters of the reactions to allergen in the primary study were significantly (p < 0.001) smaller on the forearms than on the back (27% less for the wheal and 14% less for the flare). The mean diameter of the wheals induced by histamine did not vary between back and arm (p = 0.3); however, the mean diameter of the flares on the forearm with histamine were 8% smaller than those on the back (p = 0.003). In the second study the reactions to cat extract (n = 59) were significantly smaller on the forearm than on the back (mean wheal diameter 16% less, mean flare diameter 14% less, p < 0.001 for both). The reactions to histamine (n = 76) in this study were also significantly smaller on the forearm (mean wheal diameter 6% less, p = 0.027, and mean flare diameter 14% less, p < 0.001).
CONCLUSION: If a relatively nontraumatic technique for skin testing is used, adjacent positive test reaction sites of moderate size, which are 2 cm or more from the site of the prick test, are unlikely to cause false-positive reactions on either the back or forearm. Skin test reactivity is less notable on the forearm than on the back for both histamine and allergen. This effect was more pronounced for allergen tests (16% to 27% decrease in mean wheal diameter) than for histamine tests.