The clinical significance of positive acid-fast bacilli smear in respiratory specimens from patients completing anti-tuberculosis treatment.

PURPOSE: The clinical significance of positive acid-fast bacilli (AFB) by microscopy smear in respiratory specimens remains poorly understood in patients with a history of successfully treated pulmonary tuberculosis (PTB). METHODS: A retrospective study was conducted to screen patients with PTB between 2009 and 2020. They were enrolled if microscopic smears of AFB were positive after they completed successful treatments. The first date of a positive AFB smear after treatment completion was denoted as the index date. According to whether culture results indicated Mycobacterium tuberculosis (Mtb), subjects were categorized into Mtb and non-Mtb groups. RESULTS: A total of 84 patients were included, with 27 (32.1%) patients in the Mtb group. The Mtb group had a higher proportion of active smokers and strong positive smears of AFB than the non-Mtb group. Radiologically, the rate of cavitation on the index date was significantly higher in the Mtb group than in the non-Mtb group (77.8 vs. 22.8%, p < 0.001, respectively). By multivariable logistic regression analysis, cavitation on the index date [adjusted odds ratio (aOR): 6.80, p = 0.003] and active smoker (aOR: 7.14, p = 0.044, vs. non-smoker) were independent risk factors of the Mtb group. There were 52 patients (61.9%) having polymerase chain reaction (PCR) tests. While the sensitivity of PCR was 94.1%, the specificity was only 60.0%. CONCLUSIONS: The detection of AFB by microscopic smears and positive PCR might not be sufficient for diagnosing recurrent PTB. Integrating the clinical factors, including active smoking and cavitation, might enhance the diagnostic power.