RESUMEN
BACKGROUND: Isoniazid-resistant tuberculosis (Hr-TB) is often overlooked in diagnostic algorithms due to reliance on first-line molecular assays testing only for rifampicin resistance. OBJECTIVES: To determine the prevalence, outcomes and molecular mechanisms associated with Hr-TB in the Eastern Cape, South Africa. METHODS: Between April 2016 and October 2017, sputum samples were collected from patients with rifampin-susceptible TB at baseline and at weeks 7 and 23 of drug-susceptible TB treatment. We performed isoniazid phenotypic and genotypic drug susceptibility testing, FluorotypeMTBDR, Sanger sequencing, targeted next-generation sequencing (tNGS), and whole genome sequencing. RESULTS: We analysed baseline isolates from 766 patients with rifampin-susceptible TB. Of 89 patients (11.7%) found to have Hr-TB, 39 (44%) had canonical katG or inhA promoter mutations; 35 (39%) had non-canonical katG mutations (including 5 with underlying large deletions); 4 (5%) had mutations in other candidate genes associated with isoniazid resistance. For 11 (12.4%), no cause of resistance was found. CONCLUSIONS: Among patients with rifampin-susceptible TB diagnosed using first-line molecular TB assays, there is a high prevalence of Hr-TB. Phenotypic DST remains the gold standard. To improve performance of genetic-based phenotyping tests, all isoniazid resistance associated regions should be included, and such tests should have the ability to identify underlying mutations.
