Comparison between Xpert MTB/RIF threshold cycle value and smear microscopy of bronchial lavage for diagnosis of pulmonary tuberculosis in bronchoscoped patients

Abstract

Context GeneXpert MTB/RIF (GX) has an important role in the diagnosis and treatment of pulmonary tuberculosis (TB) and predicts the infectiousness of the patient through the threshold cycle (Ct) value. Aims To compare the performance of the Xpert MTB/RIF Ct-generated values with microscopy on bronchoalveolar lavage (BAL) fluid to determine the added value of GX Ct for TB diagnosis. Settings and design This was a prospective cohort study that was conducted in Kafr Elsheikh Chest Hospital from September 1, 2018 to December 31, 2019. Patients and methods The study was conducted on 111 BAL samples of highly suspected patients of MTB clinically (+\-) radiologically, who cannot give sputum or with hemoptysis. These samples were examined by direct microscopy, solid culture media, and GX, and the results were compared. An optimum Ct value was identified to determine infectious patients. Results Negative GX was 78 (69.3%) and the positive GX was 33 (29.7%). Culture-positive cases were 30 (27.0%). Smear result showed only seven positive cases. The correlation between smear grade and Ct value yielded a converse relation (r=−0.61). Ct cutoff value for ruling in smear-positive patients was established at 22.3 cycles (sensitivity was 90.4% and specificity of 63.2%). Conclusions The study confirmed that GX examination of BAL provides a good diagnostic rapid test that is superior to smear microscopy in detection of TB. Ct values demonstrate quantifying mycobacterial load and so can predict the infectiousness of the patients. Ct value less than 22.3 provides a good diagnostic test for smear-positive status.