Effect of repeated delayed hypersensitivity skin tests on skin-test responses.

To determine whether repeated skin tests can augment a previously weak delayed hypersensitivity response or convert previously nonreacting tests and thus yield false-positive data, the authors carried out retrospective analysis of 426 skin tests on 107 patients who had a mean of 4.3 weekly tests with five recall antigens. They also skin tested 10 healthy volunteers weekly for up to 6 weeks. Reactions (induration measured in millimetres) were recorded and a regression and correlation analysis was carried out. Analysis of variance was used to compare reaction means of volunteers for each recall antigen at each time interval. All volunteers were reactive to two or more antigens. In the volunteer group there was no augmentation of initially reactive antigen responses except for Varidase and only after 5 weeks. Of 16 initially negative responses to some antigens in this same group, only 3 were converted to reactions of more than 5 mm, all responses being to purified protein derivative. No conversion occurred in the hospitalized patients. There was no significant correlation between repeated skin tests and the delayed hypersensitivity response to the antigens except for Candida in the retrospective patient group, up to 160 days from the original skin test. The data indicate that there is no augmentation of the delayed hypersensitivity response or conversion of initial nonreacting skin tests with any of the antigens tested except Candida.