Epidemiology of tuberculous infection in a chronic care population.

A prospective study of serial tuberculin skin testing was performed on 642 patients from the chronic care wards of a Veterans Administration Hospital. Five hundred eighty-eight patients without a documented history of tuberculosis or a positive tuberculin skin test received an initial 5 TU PPD, and results in 139 were positive (23.6%). Of those initially tuberculin negative, 398 received a second 5 TU PPD 2 to 4 wk later; 45 (11.3%) showed a booster effect. Of the initial 642 patients, 64 had documented negative tuberculin skin tests within the year preceding the survey. Eight (12.5%) of the 64 patients were positive at the initial survey. There were 213 patients with 2 negative tuberculin tests within the first 4 wk of the study who were followed for the year of the study, and 10 converted their tuberculin skin test for a converter rate of 4.7%. The decrease in the converter rate from 12.5 to 4.7% suggests that the initial converter rate was falsely elevated by the booster effect. The high rate of conversion during the year of the study (4.7%) suggests that there may have been inapparent spread of tuberculous infection among patients in this chronic care facility.