Relative versus absolute noncontagiousness of respiratory tuberculosis on treatment

OBJECTIVE: To assess the validity of current estimates of the noncontagiousness of sputum smear-positive respiratory tuberculosis (TB) on treatment. DESIGN: A descriptive analysis of the mycobacteriologic response to treatment. SETTING: A TB inpatient unit of a Canadian hospital. PATIENTS: Thirty-two HIV-seronegative patients with moderate to advanced sputum smear-positive respiratory TB were treated with uninterrupted, directly observed, weight-adjusted isoniazid, rifampin, and pyrazinamide. Each patient's initial isolate was drug susceptible and each patient's sputum mycobacteriology was systematically followed until 3 consecutive sputum smears were negative on 3 separate days. RESULTS: The time to smear conversion varied remarkably (range, 8 to 115 days; average, 46 days) and was influenced by sputum sampling frequency. Only 3 patients (9.4%) had smear conversions by 14 days and only 8 (25%) had smear conversions by 21 days, the average time it took for drug susceptibility test results to become available. During the first 21 days of treatment, the semiquantitative sputum smear score decreased rapidly and the time to detection of positive cultures doubled. Within the time to smear conversion, virtually all smear-positive specimens (98%) were culture positive and only 34% of the patients had culture conversions (ie, 3 consecutive negative cultures). CONCLUSION: Current estimates of the noncontagiousness of sputum smear-positive respiratory TB on treatment (for 14 days, for 21 days, or until smear conversion) are estimates of relative noncontagiousness. They do not signal absolute noncontagiousness (culture conversion). Semiquantitative smear and time-to-detection data suggest that respiratory isolation beyond 21 days of optimal treatment should be selective.