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.