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
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.
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
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