Booster effect of histoplasmin skin testing in an elderly population.

Ganley, J. P. (Office of Biometry and Epidemiology, National Eye Institute, NIH, Bethesda, Md. 20014), R. E. Smith, D. B. Thomas, G. W. Comstock and P. E. Sartwell. Booster effect of histoplasmin skin testing in an elderly population. Am J Epidemiol 95: 104-110, 1972.—A second histoplasmin skin test was administered to 97 individuals aged 50 years or older who had less than 7 mm of induration on a test given 3 weeks earlier. Of 76 people with 0 to 1 mm of induration on the first test, 35.5% had reactions of 2 to 13 mm on the second test and would be classified as converters. Of 21 people with 2 to 6 mm of induration on the first test, 19.0% had less than 2 mm of induration on the second test and would be classified as reverters. Among those with 2 mm of induration or more on the second test, those with initial reactions of 0 to 1 mm had a mean increase in induration of 7.2 mm, and those with initial reactions of 2 to 6 mm had a mean increase of 6.5 mm. The net conversion rate of 23.7% was attributed to the booster effect of the first test on the cellular immune system. Among this study group, all of whom were over 50 years of age, the booster effect showed no change with sex or age. The estimated agespecific prevalence of histoplasmin reactors, using the combined results of the first and second tests, was higher than that estimated from the results of the first test alone; both estimates declined sharply with age. In an elderly population, the booster effect must be taken into account, both as a cause of skin test conversion, and as a means of obtaining a more complete estimate of past sensitivity to the test antigen.