Prevalence of isoniazid resistance in cases of rifampicin resistance detected on GeneXpert MTB/RIF assay.

Background: The fight against tuberculosis in our country has taken a new shape with the inclusion of rapid nucleic acid amplification tests like GeneXpert MTB/RIF assay which rapidly detects and rifampicin resistance. Rifampicin resistance detected on GeneXpert has been considered as a sine qua non for the presence of isoniazid resistance and hence classified as multidrug-resistant tuberculosis (MDR-TB). However treatment of rifampicin-resistant, isoniazid-monoresistance, and MDR-TB are different. Our study was done with the aim of identification of the prevalence of isoniazid resistance on culture, in cases which had rifampicin resistance on GeneXpert.

Methods: Pulmonary samples of patients of presumptive tuberculosis were subjected to GeneXpert testing and liquid MGIT (mycobacterium growth indicator tube) culture. On detection of rifampicin resistance on MTB/RIF assay, the patients were included in our study and cultures were followed-up for sensitivity to isoniazid. A total of 76 patients were included.

Results: 76 patients of rifampicin resistance on GeneXpert MTB/RIF assay were followed-up for the sensitivity of isoniazid on culture media. Out of the 76 cases, 62 (81.57%) were found to have isoniazid resistance. Out of the 14 patients, the cultures showed no growth in 6, and in the rest, isoniazid was found to be sensitive.

Conclusion: GeneXpert MTB/RIF assay is an excellent modality for the detection of and rifampicin resistance. The decision to exclude isoniazid from the treatment regimen in patients with rifampicin resistance should be made only after conducting further molecular/phenotypic tests.