Study objectives: Identification of acid-fast bacilli (AFB) in the sputum smear at the completion of tuberculosis therapy is in some settings considered evidence of treatment failure. However, some patients with pulmonary tuberculosis (TB) will have positive smear results with negative sputum culture results at the end of therapy. The objectives of this study were to estimate the prevalence of persisting positive sputum smear results in patients with TB and to identify characteristics that distinguish patients with persistently positive sputum smear results who also had negative sputum culture results from patients identified as treatment failures.
Design: A population-based, historical cohort study with nested case control study.
Setting: British Columbia Division of Tuberculosis Control central case registry.
Patients: All 428 patients with culture-proven pulmonary TB in British Columbia over 7 years with sputum that was positive for AFB.
Methods: Review of laboratory data of all 428 patients, as well as clinical data of a subset of 30 patients with persistently positive smear results beyond 20 weeks.
Results: Sputum smears were positive for AFB in 205 patients (48%) at 4 weeks, in 30 patients (7%) at 20 weeks, and in 12 patients (3%) at 36 weeks. Of the patients with smear results that were persistently positive at 20 weeks, 23 (77%) had negative sputum culture results and 7 (23%) had positive sputum culture results (ie, they were treatment failures). Patients identified as treatment failures had more localized disease as shown on chest radiographs, had less radiographic improvement at follow-up, had a higher prevalence of drug resistance, and were less compliant with medications than patients with persistently positive smear results and negative culture results. No subject with a negative culture result relapsed over the 6- to 48-month observation period.
Conclusion: Sputum that is persistently positive for AFB in patients in developed countries is more likely to be associated with negative culture results than with treatment failure.