Rifampicin Resistance by Xpert MTB/RIF Assay in Pulmonary Tuberculosis- Is there a Need for Confirmation by Retesting?

Abstract

Introduction: Xpert Mycobacterium tuberculosis/ resistance to Rifampicin (MTB/RIF) assay detects MTB complex (MTBC) and rifampicin resistance simultaneously. In high prevalence countries like India, detection of rifampicin resistance in sputum specimen of a newly diagnosed case of pulmonary TB with a low pretest probability needs to be confirmed by retesting. Aim: To evaluate the results of retesting of rifampicin resistant specimens in newly diagnosed pulmonary TB cases. Materials and Methods: A retrospective analysis of the data of Xpert assay was performed on specimens received in Department of Microbiology, Seth G.S. Medical College and KEM Hospital, Mumbai, Maharashtra, India. If rifampicin resistance was detected in a newly diagnosed case of Tuberculosis (TB), a second specimen was retested by Xpert assay for confirmation. Concordance of retesting was seen with results of Line Probe Assay (LPA). Results: Total 27,429 specimens were processed by Xpert assay of which 803 specimens showed rifampicin resistance, 157 sputum specimens fulfilling criteria of Programmatic Management of Drug resistant Tuberculosis (PMDT) guidelines were retested. High, medium, low and very low bacterial load was observed in 30, 51, 34 and 42 specimens’ respectively. All specimens having high or medium bacillary load showed rifampicin resistant result on retesting. On retesting 34 sputum specimens with low bacterial load, rifampicin resistance was confirmed in 30 specimens. LPA done after growing them by liquid culture confirmed rifampicin resistance in remaining four specimens. Conclusion: Xpert assay is recommended when the bacterial load identified by Xpert assay is very low and when there is discordance between Xpert results of rifampicin resistance and the reflex LPA testing.