Minimum inhibitory concentration distributions for Mycobacterium avium complex—towards evidence-based susceptibility breakpoints

Background: Patients with clinical infections caused by the Mycobacterium avium complex (MAC) are treated for at least 1 year following sputum conversion with a regimen that suffers from a suboptimal cure rate. The correlation between clinical outcome and drug susceptibility testing breakpoints other than for the macrolides is regarded to be poor. A systematic evaluation of clinical breakpoints for MAC has not been performed so far; thus, the aim of this study was to initiate the process by establishing minimum inhibitory concentration (MIC) distributions. Methods: The MICs of the major drugs used in the treatment of MAC infections were determined for 229 clinical MAC isolates in cation-adjusted Mueller–Hinton II broth. Results: The MIC50 and MIC ranges were established and compared to suggested susceptibility breakpoints for clarithromycin (2; 0.064–128 mg/l), rifabutin (0.25; ≤0.25–16 mg/l), ethambutol (8; 0.5–32 mg/l), amikacin (16; 1–128 mg/l), moxifloxacin (2; 0.25–16 mg/l), linezolid (32; 1–128 mg/l), rifampicin (8; 0.125–16 mg/l), and trimethoprim–sulfamethoxazole (2/38; 0.125/2–16/304 mg/l). Conclusions: These results, together with those from available studies, indicate that MICs are high for drugs such as rifabutin, rifampicin, ethambutol, linezolid, and moxifloxacin used against MAC at levels unlikely to be associated with clinical efficacy at current dosing. This may partly explain the poor correlation between susceptibility testing and clinical outcomes for drugs other than clarithromycin