Table 7.

Suggestions for susceptibility testing of M. abscessus, M. chelonae, and the M. fortuitumgroup by broth microdilutiona

DrugSuggestion
TobramycinOnly for testingM. chelonae. If the initial MIC is >4 μg/ml, the test should be repeated. If the repeat result is >4 μg/ml, the MIC should be reported with a commentb.
SulfonamidesMIC is 80% inhibition of growth. Results are predictable by species; therefore, testing may not be necessary.
DoxycyclineProposed breakpoints are ≤1 μg/ml (susceptible), 2–8 μg/ml (intermediate), and ≥16 μg/ml (resistant).
CefoxitinProposed breakpoints are ≤16 μg/ml (susceptible), 32–64 μg/ml (intermediate), and ≥128 μg/ml (resistant).
ImipenemIf MIC for M. fortuitum group is >8 μg/ml, test should be repeated with incubation period of no more than 3 days. If the repeat result is >8 μg/ml, the MIC should be reported with a commentb. For M. chelonae andM. abscessus, MIC results of >8 μg/ml should not be reported until the problem with reproducibility is resolved.
Amikacin M. abscessus for which MIC is ≥64 μg/ml should be retested. If the repeat result is ≥64 μg/ml, the MIC should be reported with a commentb.
  • a For laboratories that infrequently isolate rapidly growing mycobacteria, sending isolates to an experienced reference laboratory is recommended. For laboratories that perform MIC testing, (i) proficiency testing by comparison of test results with those of an experienced reference laboratory is necessary upon initial validation and at regular intervals thereafter and (ii) identification of isolates to the species level or, at a minimum, differentiation of the M. fortuitum group from the M. chelonae-M. abscessus group is recommended.

  • b Comment: (i) the MIC is greater than expected for this species and (ii) if the drug is being considered for therapy, the laboratory should be notified so the isolate can be sent to a reference laboratory for confirmation of resistance.