Culture Positivity and Predictors in Persistent Smear-Positive TB at Two Months

Introduction: In pulmonary tuberculosis (TB), smear positivity and infectivity typically decrease quickly with effective treatment, but the time required to become non-infectious varies, affecting the duration of respiratory isolation. The CDC recommends isolation until patients produce three consecutive negative sputum smears. However, it’s suggested that patients who remain smear-positive despite treatment may be shedding only non-viable bacilli, questioning the need for prolonged isolation until smear negativity is achieved. In patients remaining smear-positive after two months of therapy, we aim to evaluate this hypothesis by conducting sputum cultures to assess the viability of bacilli in their smears. Aim: This study aimed to (1) determine the proportion of culture-positive cases among pulmonary tuberculosis patients remaining smear-positive at two months of treatment and (2) 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. Results: We 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. Conclusions: Over half of persistent smear-positive patients remain potentially infectious at two months. However, those without cavitary lesions and with low AFB counts could be candidates for earlier isolation discontinuation, optimizing resources and reducing patient burden. This study suggests refining isolation protocols using clinical and microbiological predictors of infectivity.