Predictors of culture status in patients with persistent smear-positive pulmonary tuberculosis at two months of treatment.

introductionIn pulmonary tuberculosis (TB), smear positivity usually declines with effective treatment, but the time to non-infectiousness varies, creating uncertainty about the optimal duration of isolation. The Centers for Disease Control and Prevention (CDC) 2005 guidelines allow discharge before smear conversion to home isolation (restricted to healthcare visits until smear negativity) if no vulnerable household contacts are present, whereas hospitalized patients are advised to remain under airborne precautions until they have three consecutive negative smears. The practice in Qatar is to keep sputum smear positive TB patients in isolation facilities until smear negativity is achieved. Relying solely on smear conversion as a marker of non-infectiousness is problematic, as persistent smear positivity may reflect nonviable bacilli, or in some cases non-tuberculous mycobacteria, rather than ongoing transmission risk. This study evaluates the culture status of patients who remained smear-positive after two months of therapy to determine bacillary viability and reassess the validity of smear-based isolation practices.

aimThis study aimed to determine the proportion of culture-positive cases among pulmonary tuberculosis patients remaining smear-positive at two months of treatment and identify factors predictive of culture-negative status to support earlier isolation discontinuation. METHODOLOGY: A retrospective review of electronic medical records (2016–2024) was conducted at a tertiary TB center in Qatar, targeting patients smear-positive at two months. Data included demographics, disease extent (e.g. cavitary lesions), initial and two-month acid-fast bacilli (AFB) smear counts, two-month AFB cultures, drug resistance, and comorbidities.

resultsWe identified 88 patients who remained smear-positive at two months of treatment. Among them, 61.4% were culture positive. Patients without cavitary lesions on the initial chest X-ray and those with two-month AFB counts < 10/100 fields had a 69% negative predictive value for culture negativity.

conclusionsOver half of persistent smear-positive patients remain potentially infectious at two months. However, those without cavitary lesions and with low AFB counts in the two months smear could be candidates for earlier isolation discontinuation, optimizing resources and reducing patient burden. These findings support individualized isolation protocols. CLINICAL TRIAL NUMBER: Not applicable.