Evaluation of the Xpert MTB/RIF test accuracy for rapid diagnosis of pulmonary tuberculosis in a tuberculosis endemic area

Abstract

Aim and objectives: Mycobacterium tuberculosis (MTB) is a causative agent of tuberculosis (TB) which remains as an endemic disease in Iran. One of the most significant steps for control of TB is the fast and accurate diagnosis of patients. Examination of sputum smears is the primary method for TB diagnosis, and the culture method remains as the gold standard. Recently, a molecular assay that named Xpert MTB/RIF assay (Xpert), was introduced for rapid diagnosis of TB. The aim of this study was to evaluate the performance of the Xpert assay for diagnosis of TB in Golestan province. Methods: The TB suspected cases which referred to Tuberculosis Reference laboratory, Gorgan, Iran, from March 2018 to February 2019 were included to this study. The sputum specimens were examined using smear microscopy after Ziehl–Neelsen staining and graded according to the International Union Against Tuberculosis and Lung Disease scale (scanty, 1+, 2+, 3+ and 4+). The Xpert analysis was performed according to the manufacturer's instructions and the provided semi-quantitative results for positive specimens (very low, low, medium and high) were documented. The specimens were cultured into Lowenstein-Jensen (LJ) culture media after decontamination by Petroff's method. Sensitivity and specificity of Xpert and smear microscopy were calculated using a culture as reference standard. Results: Of 102 presumptive TB cases, 17 (16.7%) had culture proven TB. The set of 16 (94·1%) culture-confirmed TB cases were equivalently detected using smear microscopy and Xpert. Compared to the culture method, specificity of Xpert and smear microscopy was 100% (85/85) and 97.6% (83/85), respectively. Both tests had a sensitivity of 94.1% (16/17). When the semi-quantitative results of Xpert were compared with smear microscopy, there was 92.3% agreement for specimens with medium/high results, with those that had 1+ to 3+ scales. Conclusion: The rapid and accurate laboratory diagnosis of TB is one of the most important steps for TB control programs. In this study, the Xpert was effectively used to early and accurate diagnosis of TB in the study area. In the other hand, correlation between the Xpert semi-quantitative results and smear microscopy showed that Xpert could help in evaluation of patient's transmission potential.