In the modified carbapenem inactivation method (mCIM) assay, which result indicates carbapenemase production by the test organism?

• A. Growth of the control organism up to a carbapenem disk indicates carbapenemase production by the test organism.
• B. No growth of the control organism up to a carbapenem disk indicates carbapenemase production by the test organism.
• C. No growth of the control organism up to a carbapenem disk indicates carbapenemase production by the control organism.
• D. Growth of the control organism up to a carbapenem disk indicates a problem with the assay, and it should be repeated.

Answer: (A)

In this assay, you’re testing whether the test organism can inactivate a carbapenem. If the organism produces carbapenemase, it hydrolyzes the meropenem in the disk during incubation, so the antibiotic loses activity. When that disk is placed onto a plate with a susceptible indicator strain, the indicator can grow right up to the disk because the drug no longer inhibits it. That growth of the indicator up to the disk indicates carbapenemase production by the test organism. If the carbapenem remains active (no carbapenemase), you’d see a zone of inhibition around the disk instead. So the observed growth of the control/indicator organism up to the disk is the sign of carbapenemase production.