Real-time PCR followed by high-resolution melting curve analysis: A rapid and pragmatic approach for screening of multidrug-resistant extrapulmonary tuberculosis.

INTRODUCTION

Multidrug resistance (MDR) in extrapulmonary tuberculosis (EPTB) is a diagnostic challenge in an endemic country like India. Timely detection of MDR-TB can contribute to a better patient outcome.

OBJECTIVE

To perform real-time PCR (qPCR) using rpoB, mpb64 and IS6110 gene on a variety of EPTB samples and to compare the performance of different gene targets. All qPCR positive samples were subjected to high resolution melt-curve analysis (HRM analysis) for rpoB and katG gene to evaluate its potential for MDR screening among different sample types.

METHODS

Real-time PCR using rpoB, mpb64 and IS6110 genes was carried out on 200 cases of study group and 100 cases of non-TB control group. The study group consisted of 100 culture-confirmed and 100 clinically suspected cases of EPTB. Phenotypic drug susceptibility testing (DST) for culture isolates was performed by the 1% indirect agar proportion method. DNA extracted from all qPCR positive samples was subjected to rpoB and katG HRM analysis for screening of MDR. Sequencing was used to confirm the results of HRM analysis and the results were also compared with phenotypic DST in all culture positive cases.

RESULTS

The sensitivity of qPCR using rpoB, mpb64 and IS6110 was 86.5%, 86.5% and 76.5%, respectively. All isolates from the control group were negative by all the three targets, giving a specificity of 100%. HRM analysis detected MDR in 22/200 (11%) isolates. 3/200 (1.5%) had mono-rifampicin resistance while 8/200 (4%) had mono-isoniazid resistance. HRM analysis identified an additional 4 MDR cases directly from the samples which were negative by culture. On sequencing, mutations were observed at codon 531 (60%); 533 (16%); 516 (12%) and 526 (12%) of the rpoB gene and at codon 315 (100%) of the katG gene. There was 100% concordance in the results of phenotypic DST, HRM analysis and sequencing.

CONCLUSION

The HRM analysis can play a promising role in the reliable and rapid screening of EPTB samples for detection of MDR.