Analysis of Discordance between Genotypic and Phenotypic Assays for Rifampicin Resistant Mycobacterium tuberculosis Isolated from Healthcare Facilities in Mthatha

The study sought to determine the rate of discordant results between genotypic and phe-notypic tests for the diagnosis of drug resistant tuberculosis (DR-TB). Sputum samples and cultured isolates from suspected DR-TB patients were respectively analysed for My-cobacterium tuberculosis by Xpert® MTB/RIF (Cepheid, USA) and Line Probe Assays (LPA) (Hain, Germany). Discrepant Rifampicin (RMP) resistant results were confirmed using BACTEC MGIT960 (BD, USA). Of 224 RMP-resistant by Xpert MTB/RIF, 5.4% were suscep-tible to RMP by LPA. MGIT960 showed 75% agreement with LPA. Discrepancy was at-tributed to either heteroresistance or DNA contamination during LPA testing in 58.3% of cases. In 25% of samples showing agreement in RMP resistance between Xpert MTB/RIF and MGIT960, discrepancy was attributed to laboratory errors causing false RMP suscep-tible results with LPA. Of 16.7% of cases, discrepancy was attributed to false RMP suscep-tible results with Xpert MTB/RIF. Of 224 isolates, susceptibility to isoniazid (INH) by LPA was performed in 73.7% RMP resistant isolates, of which, 80.6% were resistant. All RMP resistant isolates by Xpert MTB/RIF were confirmed in 98.5% by LPA if TB isolates were resistant to INH, but only confirmed in 81.3% if TB isolates were INH susceptible (ρ < 0.001). In conclusion, Laboratory errors should be considered when investigating dis-cordant results.