Bronchial Gene Xpert detects TB missed by sputum tests

Sputum tests that look for acid-fast bacilli (AFB) are a common first step when doctors suspect pulmonary tuberculosis, but they can miss cases. To better understand who might be missed and whether an alternative test can help, Le Huu Nhat Minh and colleagues carried out a cross-sectional study at the Respiratory Department of Thong Nhat Hospital between July 2022 and August 2023. The study began with 120 patients suspected of pulmonary tuberculosis who had AFB-negative sputum, and 56 of those patients were ultimately recruited. The group was mostly male (67.9%) with an average age of 57.8 years (±20.1) and a mean body mass index of 21.4 kg/m2 (±1.8). Common health problems alongside suspected tuberculosis included hypertension (41.1%), type 2 diabetes (30.4%), and chronic lung disease (5.4%). Chest X-rays varied: nearly half (48.2%) showed more than one lesion. Lesions appeared in the right lung in 46.4% of patients and the left lung in 28.6%, with 51.8% involving an upper lobe and 17.8% a lower lobe. The most frequent X-ray pattern was infiltrative injury (64.3%), followed by consolidation (21.4%), pleural effusion (10.7%), cavity (8.9%) and nodular changes (5.4%).

To see whether testing bronchial lavage fluid could reveal tuberculosis missed by sputum AFB testing, the team used Gene Xpert MTB/RIF on bronchial lavage fluid and compared results to Mycobacteria growth indicator tube culture (MGIT) of the same fluid. Gene Xpert MTB/RIF detected tuberculosis bacteria in 40.8% of cases among the 56 patients, while MGIT culture was positive in 46.7% of cases. Using MGIT as the comparison, the sensitivity of Gene Xpert MTB/RIF was 85.7%, meaning it identified most of the MGIT-positive cases, and its specificity was 98.4%, meaning it very rarely gave a false-positive result against MGIT. The positive predictive value was 97.9% and the negative predictive value was 88.7%, indicating that a positive Gene Xpert result on bronchial lavage fluid was highly likely to reflect culture positivity and a negative result still left some uncertainty. The study also reported a rifampicin resistance rate of 4.1% among the detected cases.

These findings show that pulmonary tuberculosis can present with a wide range of clinical and X-ray features when sputum AFB testing is negative, and that Gene Xpert MTB/RIF performed on bronchial lavage fluid can pick up many of those cases. With an 85.7% sensitivity and 98.4% specificity compared to MGIT culture, Gene Xpert MTB/RIF appears to be a reliable rapid test in this setting. The high positive predictive value suggests clinicians can have strong confidence in a positive Gene Xpert result from bronchial lavage fluid, while the negative predictive value indicates a negative result does not completely rule out disease. The low rifampicin resistance rate of 4.1% reported in the study provides some reassurance about first-line drug susceptibility in this group, but clinicians would still use diagnostics and clinical judgment to guide treatment. Overall, the work led by Le Huu Nhat Minh supports using bronchial lavage fluid testing with Gene Xpert MTB/RIF as a valuable tool when sputum AFB is negative, potentially reducing missed diagnoses of pulmonary tuberculosis.