STABILITY OF POSITIVE TUBERCULIN TESTS - ARE BOOSTED REACTIONS VALID

To determine the stability and presumed significance of tuberculin skin tests, we followed a cohort of 380 tuberculin-positive patients living in chronic care facilities. Each patient had a positive reaction (greater-than-or-equal-to 10 mm induration to 5 tuberculin units of purified protein derivative) to one of three sequential baseline tuberculin tests. One year after the initial series, each patient had a single repeat skin test. Reversion to a negative test occurred in 98 (26%) of the 380 patients. Decreases in induration of 6 mm or more occurred in 88 (90%) of the reverters. Initially positive tests were more likely (p < 0.001) to remain stable than tests that were "boosted" to positive reactions on the second or third initial administration. Stable responses were found in 96% of those whose tests had greater-than-or-equal-to 15 mm induration compared with 61% of those with reactions of 10 to 14 mm induration. Increasing age also was associated with a high rate of reversion. The instability of boosted tuberculin reactions brings into question the clinical significance of these tests. We propose limiting tuberculin testing to two sequential tests.