Culture result of smear-positive sputum samples after 2 months of antituberculous treatment.

The correlation between sputum smear for acid-fast bacilli (AFB) and culture for Mycobacterium tuberculosis is usually good before antituberculous treatment (ATT), but it is not so good after treatment has started. In previous studies, up to 23.5% of patients had sputum that remained smear positive after 2 months of ATT [1]. Old age, high bacillary load, presence of pulmonary cavitary lesions, treatment interruption and emergence of resistant strains are risk factors of persistent smear positivity. Determining whether these sputum samples contain viable tuberculosis (TB) bacilli is crucial because it indicates higher rates of relapse [2, 3]. Meanwhile, for patients without viable TB bacilli, modifying treatment regimens and isolation appears to be redundant. However, the results of mycobacterial culture are only available after 2 weeks, even when using fluorometric liquid culture technique. Despite having good sensitivity and specificity, commercialised nucleic acid amplification tests are expensive and do not discriminate between viable and dead bacilli and, therefore, are not recommended for TB patients receiving ATT. Thus, we conducted a study to identify clinical factors for predicting the culture results of the persistent smear-positive sputum samples in a 2,100 bed, tertiary-care referral centre in northern Taiwan and an 800 bed local teaching hospital in southern Taiwan.