P38
Discrepancies between routine phenotypic and genotypic isoniazid susceptibility testing of Mycobacterium tuberculosis: Cooperation from the science community to improve diagnostics
E M Streicher(1) S Nagel(1) M Klopper(1) A Dippenaar(1,5) C L Hayes(3) J E Posey(2) R M Warren(4)
1:Stellenbosch University; 2:Centers for Disease Control and Prevention, USA; 3:National Health Laboratory Services, Port Elizabeth, South Africa; 4:SA MRC Centre for Tuberculosis Research; 5:University of Antwerp
Isoniazid (INH) serves as the backbone of combined anti-tuberculosis therapy. However, in South Africa, the current algorithm dictates that only rifampicin resistant cases are tested for INH resistance. This is done by the MTBDRplus line-probe assay (LPA), which reports INH resistance based on mutations in katG gene and inhA promoter, followed by a phenotypic drug susceptibility test (DST) to confirm INH susceptibility. However, discrepancies between the results of these tests are frequently reported. This may result in incorrect diagnoses and the prescription of improper drug treatment regimens. This study investigates 297 clinical isolates obtained from a routine laboratory in Eastern Cape Province with discrepant LPA and DST results.
The isolates were investigated by minimum inhibitory concentration determination to confirm resistance and Sanger sequencing and whole genome sequencing (WGS) to determine reasons for the discrepancies between the LPA and DST.
WGS revealed several genomic features that may explain INH resistance in the absence of canonical mutations, including large deletions of katG, heteroresistance and several potential novel resistance-causing mutations, however for about 35% the mechanism of resistance remained unknown.
While LPA is conveniently rapid, and accurate in most cases, phenotypic DST is necessary to detect resistance conferred by less common mechanisms. WGS can be applied to inform researchers and clinicians of such alternative resistance markers and contribute to improve current diagnostics for detection of INH resistance, but it is of utmost importance to have well curated and updated catalogs of these mutations (such as the WHO catalog for drug resistance mutations).