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Am Fam Physician. 2000;61(3):865

Antimycobacterial therapy for pulmonary tuberculosis is deemed successful when sputum smears no longer show acid-fast bacilli (AFB), cultures are negative and improvement is evident on chest radiographs. Treatment is considered a failure if culture results have not converted to negative after five to six months of therapy. Studies suggest that sputum smears persistently positive for AFB despite negative culture results may in some patients be due to nonviable or atypical mycobacteria. To examine this issue, Al-Moamary and colleagues analyzed the radiologic and clinical characteristics of patients with positive sputum smears for AFB but negative cultures after treatment for tuberculosis.

The authors reviewed laboratory data on 692 consecutive patients with pulmonary tuberculosis. From this group, the authors found 428 cases in which the initial sputum smears were positive for AFB and in which follow-up continued for at least 36 weeks or until sputum smears and cultures became negative. The medical records were also reviewed for data on associated medical conditions, laboratory findings, medication compliance, clinical outcome and radiographic findings. Follow-up chest radiographs were classified as showing improvement, showing disease progression or showing no change.

Of the 428 patients, 205 (48 percent) had sputum smears positive for AFB beyond four weeks. Sputum smears remained positive for AFB beyond eight weeks in 123 patients (28.7 percent), beyond 12 weeks in 76 patients (17.8 percent) and beyond 20 weeks of therapy in 30 patients (7 percent). Of the 30 patients with positive smears after 20 weeks, 21 (76.7 percent) converted to negative culture results before negative smear results, and seven (23.3 percent) had persistently positive culture results. Thus, in this study population, there were only seven true treatment failures by week 20.

Analysis of the data on patients with positive smears but negative cultures showed that sputum smears remained positive for AFB for a median of 31 weeks. The longest that smears remained positive was 112 weeks. None of these patients was found to have multidrug-resistant Mycobacterium tuberculosis. In addition, none of the patients with persistently positive sputum smears but negative cultures relapsed during follow-up, which ranged from six to 48 months.

Chest radiographs were available for review in 20 of the patients with sputum smears positive for AFB but negative culture results. These patients were found to be more likely to have cavitary findings on the initial radiographs. However, clinical and radiographic improvement occurred during follow-up in most of the patients. In the patients who were considered to represent true treatment failures, pulmonary involvement tended to be more extensive than in the other patients, and follow-up radiographs tended to show no improvement.

The authors conclude that sputum smears positive for AFB at the end of treatment do not necessarily indicate treatment failure. The clinical improvement and resolution of radiographic abnormalities observed in this study suggest that the positive smears are due to nonviable mycobacterium. Such patients may not require a change in medical therapy.

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