Are Current Critical Concentrations for Mycobacterium tuberculosis Drug Susceptibility Testing in Egg-Based Media Adequate? Insights from Phenotypic and Genotypic Analyses
Abstract
Abstract Phenotypic antimicrobial susceptibility testing (pAST) remains the reference standard for detecting drug resistance in Mycobacterium tuberculosis (Mtb), particularly for identifying resistance levels that cannot be inferred by molecular methods alone. This study evaluated the performance of the Ogawa-Kudoh (O-K) egg-based medium for minimum inhibitory concentration (MIC) determination and establishment of epidemiological cutoff values (ECOFFs) for first- and second-line anti-tuberculosis drugs. A total of 65 Mtb strains with diverse resistance profiles were tested against isoniazid (INH), rifampicin (RIF), levofloxacin (LFX), moxifloxacin (MFX), linezolid (LZD), bedaquiline (BDQ), and pretomanid (Pa). MICs obtained in O-K medium were compared with the EUCAST broth microdilution reference method and genotypic assays. ECOFFs determined in O-K medium were 0.2 mg/L for INH, 20 mg/L for RIF, 1 mg/L for LFX, MFX, and LZD, and 4 mg/L for Pa. Agreement between O-K and the EUCAST reference method was almost perfect (κ = 0.983; 95% CI: 0.959–1.000), with only two discordant results among 356 MIC determinations. Phenotypic methods showed excellent concordance (κ > 0.9), whereas agreement between phenotypic and genotypic approaches was lower (κ
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