Effectiveness of isoniazid chemoprophylaxis for HIV-infected drug users at high risk for active tuberculosis

Objective: To define the effectiveness of chemoprophylaxis, outside of a clinical trial setting, in preventing tuberculosis among tuberculin-reactive and anergic HIV-infected drug users at high risk of developing active tuberculosis. Design: An observational cohort study. Setting: Methadone maintenance treatment program with on-site primary care. Participants: Current or former drug users enrolled in methadone treatment. Interventions: Annual skin testing for tuberculosis infection and anergy was performed, and eligible patients were offered daily isoniazid for 12 months and followed prospectively. Main outcome measure: The development of active tuberculosis. Results: A total of 155 persons commenced chemoprophylaxis. Among tuberculin reactors, tuberculosis rates were 0.51 and 2.07/100 person-years in those completing 12 months versus those not taking prophylaxis [rate ratio 0.25, 95% confidence interval (CI) 0.06-1.01]. Among anergic individuals, comparable rates were 0 and 1.44/100 person-years. Lower tuberculosis rates among completers were not attributable to differences in immune status between the treated and untreated groups. Conclusion: The completion of isoniazid chemoprophylaxis was associated with a marked reduction in tuberculosis risk among tuberculin reactors and anergic persons in this high-risk population. These data support aggressive efforts to provide a complete course of preventative therapy to HIV-infected tuberculin reactors, and lend weight to the findings of others that isoniazid can reduce the rate of tuberculosis in high-risk anergic HIV-infected persons.