Four-stage tuberculin testing in elderly subjects induces age-dependent progressive boosting.

We administered four sequential tuberculin skin tests (5 TU, PPD) with intervals of one week to 223 subjects older than 65 years of age to evaluate whether elderly subjects demonstrated progressive boosting. Indurations of at least 10 mm with increases of at least 6 mm (over the previous test) were considered significant reactions, and these were found in 29 percent of the subjects after test 1, in 43 percent after test 2, in 53 percent after test 3, and in 57 percent after test 4 (p less than 0.05), ie, only about 50 percent of all the positives were detected after the first test. The percentage of positive reactors was inversely related to age (p less than 0.001), yet this age-dependent difference decreased with increasing number of tests. For the 65- to 74-year-old age group, 44 percent reacted positively after the first test and after three tests almost a plateau of 65 to 70 percent positive reactors was reached, suggesting that a minority only of about 30 to 35 percent of these geriatric patients might have outlived their bacilli or were never infected. For the 75- to 84-year-old age group, 24 percent reacted after the first test and 55 percent reacted after the fourth one. For the older than 85-year-old age group, 19 percent positive reactors were found after the first test and 46 percent were found after the fourth test, without clear-cut leveling off toward a plateau value, suggesting that additional tests would induce further boosting. Mean diameters of positive reactions were 15 to 24 mm, and were mostly at least 12 mm larger than in the previous tests. These data support the hypothesis that the negative tuberculin reaction, which is often found in elderly subjects, is mainly due to the failing immune response to tuberculin antigen that can be restored progressively by repeated administrations. These findings, furthermore, emphasize that especially in elderly, care should be taken not to interpret a boosting reaction as a conversion and especially that neither a two-step testing as recommended by the ATS and CDC (Am Rev Respir Dis 1990; 142:723-35) nor even a four-step testing may suffice to detect all positives in this type of population.