Impact of tuberculosis control measures and crowding on the incidence of tuberculous infection in Maryland prisons

Abstract

Our aim was to determine the incidence of tuberculin skin test (TST) conversion in the Maryland state correctional system. We conducted a historical longitudinal cohort study. A sample of 1,289 inmates, incarcerated in 16 of 23 prisons, who had a negative TST and a second test within 24 months was selected. The incidence of recent conversion was 6.3 per 100 person-years. Risk factors for conversion included high prison-population density (relative risk [RR] = 2.4; 95% confidence interval [CI], 1.5-3.8) and incarceration in a higher-security institution (RR = 2.4; 95% CI, 1.4-4.3). Incarceration in an institution with higher levels of isoniazid prophylaxis (>65% of TST positives) reduced the risk of infection by 50% (RR = 0.5; 95% CI, 0.3-0.7). Crowding was strongly correlated with risk of conversion (r = 0.83; P < .001), while rates of isoniazid prophylaxis initiation were inversely correlated with risk of infection (r = -0.82; P < .001). In stepwise regression, higher prison-population density was the strongest predictor of increased infection. In a final model, inclusion of the rate of isoniazid prophylaxis initiation reduced the risk associated with crowding (RR = 1.4; P = .4). Annual screening programs for prisons can identify recent conversions that may not otherwise be detected.