Underutilization of nontuberculous mycobacterial drug susceptibility testing in Ontario, Canada, 2010-2015

Drug susceptibility testing (DST) in nontuberculous mycobacterial pulmonary disease (NTM-PD) is useful for some Mycobacterium species. International guidelines recommend routine use of DST for clinically relevant mycobacteria. DST use and results are poorly studied at the population level. We sought to identify the frequency of DST utilization for nontuberculous mycobacteria (NTMs) and describe the potential relevance of these results in Ontario.

Using public health laboratory data, we performed a population-based retrospective analysis of NTM DST utilization in Ontario from May 2010 to June 2015. We determined the proportion of incident NTM-PD infections for which DST was performed and analyzed minimum inhibitory concentration (MIC) distributions from NTM testing overall, using thresholds recommended by the Clinical and Laboratory Standards Institute.

The proportion of incident cases of NTM-PD tested for DST was 6.3% (240/3,806) for Mycobacterium avium complex (MAC), 36.2% (67/185) for M. abscessus, and 1.8% (19/1,057) for M. xenopi. Among specimens from all body sites, MAC resistance to clarithromycin occurred in 8.0% of specimens (21/262) and MAC resistance to amikacin (intravenous, MIC > 64 µg/mL) occurred in 22.6% (19/84). M. abscessus resistance occurred as follows: to amikacin, 3.8% (3/79); cefoxitin, 14.0% (11/79); imipenem, 30.4% (14/46); linezolid, 39.2% (31/79); clarithromycin, 54.2% (13/24); ciprofloxacin, 92.4% (73/79); and moxifloxacin, 91.1% (51/56). M. xenopi analysis was limited by few DST requests and a lack of DST clinical correlation.

We found that NTM DST is underutilized in Ontario and observed a very high frequency of amikacin resistance among MAC isolates.