THE BOOSTER EFFECT IN 2-STEP TUBERCULIN TESTING AMONG YOUNG-ADULTS INMONTREAL

Objectives: No consensus exists regarding the definition and interpret ation of a significant boosting reaction after sequential tuberculin t esting. The booster phenomenon is thought to represent remote tubercul ous infection where tuberculin reactivity has waned, but it has also b een described among persons with previous exposure to other mycobacter ia or Bacille Calmette-Guerin (BCG) vaccine. We studied the factors af fecting the booster phenomenon among Canadian-born young adults to det ermine the definition that would maximize sensitivity and specificity of a positive booster reaction in these persons. Design: Point-prevale nce survey of initial tuberculin reactions and response to repeated tu berculin testing after 1 to 4 weeks. Setting: Community-based study of all students entering health professional training programs at six po st-secondary institutions. Measurements: In 1989, 1990, and 1991, stud ents completed self-administered questionnaires, underwent two-step tu berculin testing with purified protein derivative-tuberculin (PPD-T), and had their childhood BCG vaccination status verified. In 1991, stud ents were also tested with purified protein derivative-Battey (PPD-B) (for Mycobacterium intracellulare). Results: Overall, 74 students (5.2 %) had positive booster reactions, which were significantly associated with older age (P < 0.001), larger initial tuberculin reactions (P < 0.001), previous BCG vaccination (P < 0.001), older age when vaccinate d (P < 0.02), longer interval from vaccination to testing (P < 0.01), and sensitivity to PPD-B (P < 0.001). Boosting was not associated with the number of BCG vaccinations, sex, or risk factors for tuberculous infection. The pattern, mean, and mode of the frequency distributions of booster reactions among those with BCG vaccination and sensitivity to PPD-B were similar to those with assumed tuberculous infection. Con clusions: In young adults, booster reactions due to previous tuberculo us infection are uncommon and cannot be distinguished from false-posit ive reactions due to past exposure to other mycobacteria.