Abstract
The performance of the Xpert© MTB/RIF and MTBDRplus assays for the detection of rifampicin resistant Mycobacterium tuberculosis was compared to culture-based drug susceptibility testing in 30 specimens with rifampicin-resistant and rifampicin-indeterminate Xpert MTB/RIF results collected between March 2012 and March 2014. Xpert MTB/RIF and MTBDRplus were 100% sensitive and 100% concordant for rifampicin resistance detection, but 3 of 13 samples (23%) positive for rifampicin resistance on Xpert MTB/RIF and MTBDRplus were negative for rifampicin resistance on mycobacteria growth indicator tube drug susceptibility testing. Specificity was 72% for Xpert MTB/RIF and 80% for MTBDRplus. Positive predictive value for Xpert MTB/RIF for multidrug resistant tuberculosis was 47.8% for new patients and 77.8% for previously treated patients; negative predictive value was 100% for both new and previously treated patients. The discordant rifampicin resistance test results indicate a need to fully characterise circulating rifampicin resistant Mycobacterium tuberculosis strains in Zambia and to inform the development of guidelines for decision-making in relation to diagnosis of drug-resistant tuberculosis.
Highlights
Management and control of multidrug resistant (MDR) tuberculosis, tuberculosis that is resistant to both isoniazid and rifampicin, relies on timely and correct diagnosis
We report comparative observations made in a pilot study on the performance of Xpert MTB/RIF, MTBDRplus and Mycobacteria Growth Indicator Tube (MGIT) drug susceptibility testing methods for detection of rifampicin resistance in Lusaka, Zambia, at the Zambart Central Laboratory (ZCL)
Of the nine patients initially indeterminate for rifampicin resistance, five (56%) tested negative for rifampicin resistance and four (44%) tested negative for M. tuberculosis on repeat Xpert MTB/RIF testing (Table 1)
Summary
Management and control of multidrug resistant (MDR) tuberculosis, tuberculosis that is resistant to both isoniazid and rifampicin, relies on timely and correct diagnosis. The combination of Xpert MTB/RIF and MTBDRplus test results would be a powerful tool to achieve prompt confirmation of diagnosis of MDR tuberculosis. These genotypic methods, do not detect resistance conferred by mutations outside of the targeted gene region.[3,4,5]. We report comparative observations made in a pilot study on the performance of Xpert MTB/RIF, MTBDRplus and Mycobacteria Growth Indicator Tube (MGIT) drug susceptibility testing methods for detection of rifampicin resistance in Lusaka, Zambia, at the Zambart Central Laboratory (ZCL)
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