Pattern of missing probes in rifampicin resistant TB by Xpert MTB/RIF assay at a tertiary care centre in Mumbai.

Setting: Department of Microbiology.
Objective: To determine the common mutations responsible for rifampicin resistance in TB cases detected by Xpert MTB/RIF assay.
Design: Results of Xpert MTB/RIF assay performed from 2013 to 2017 were analysed for missing probes in different types of specimens containing rifampicin resistant MTB.
Results: Successful results were obtained in14872 of the total 15129 specimens processed by Xpert MTB/RIF assay, of which 9458 (63.6%) were sputum and 5414 (36.4%) were extrapulmonary specimens. MTB was detected in 1624 (17.17%) sputum and 1121 (20.70%) extrapulmonary specimens of which 409 (25.18%) and 277 (24.71%) were rifampicin resistant respectively. Probe E (83.82%) was the commonest probe responsible for rifampicin resistance followed by D (3.93%) and B (3.79%). Mutation in probe C (0.29%) was very rare. Combination of missing probes like AB (0.73%), DE (1.16%) and ADE (0.14%) was observed. 22 (3.2%) specimens showed presence of all five probes.
Conclusion: Xpert MTB/RIF assay uses various combinations of probe to detect MTB along with rifampicin resistance and is a valuable diagnostic tool. It can become a useful epidemiological tool to identify dynamics of transmission of TB by addition of few more probes to identify mutations at specific codons.
(Copyright © 2018 Tuberculosis Association of India. Published by Elsevier B.V. All rights reserved.)