Factors influencing time to smear conversion in patients with smear-positive pulmonary tuberculosis

Background and objective:   Persistent smear-positivity in patients with pulmonary tuberculosis has been shown to predict an unfavourable outcome. This study was conducted to identify the factors influencing time to sputum smear conversion.
Methods:   From July 2003 to June 2007, all patients with smear-positive and culture-confirmed pulmonary tuberculosis, who had attended a medical centre and a local teaching hospital, were identified. Factors that might have influenced time to smear conversion were investigated using time-to-event analysis.
Results:   Altogether 305 patients (mean age: 58.6 years) were studied. Diabetes mellitus was the most common underlying comorbidity. Eight patients (2.6%) had AIDS. After 2 months of treatment, 34 (11.1%) patients remained smear- and culture-positive. Cox proportional hazard regression analysis indicated that the presence of a cavity on CXR, smear grading and the first 2-month treatment regimen were independent factors influencing the time to sputum smear conversion. Among patients who had received isoniazid in the first 2 months of treatment, the time to sputum smear conversion in the 24 patients whose isolate showed isoniazid resistance was not different from that in the 236 patients whose isolate was isoniazid-susceptible (hazard ratio 1.061; 95% CI: 0.697–1.616).
Conclusions:   This analysis revealed that 11.1% of tuberculosis patients remained smear-positive after 2 months of treatment. Patients with cavitation, higher smear grading and those who had not used isoniazid, rifampicin, ethambutol and pyrazinamide continuously in the initial treatment phase had a longer time to sputum smear conversion.