For a CRE (carbapenem-resistant Enterobacteriaceae) isolate, the mCIM test is performed. The test is considered positive (carbapenemase-producing) when the zone of inhibition around the imipenem disk is:

• A ≥28 mm (indicating carbapenemase production)
• B Exactly 22 mm (indeterminate zone)
• C Zero to 11 mm only for Class A carbapenemases
• D ≤19 mm (indicating carbapenemase production) ✓

Explanation

In the mCIM test, the test organism is incubated with an imipenem disk in MHB; if carbapenemase is present, the enzyme degrades imipenem, leaving no activity to inhibit E. coli growth around the disk (zone ≤19 mm = positive). A zone ≥28 mm indicates carbapenemase is absent (negative). A zone of 20–27 mm is indeterminate and warrants repeat testing or supplemental eCIM.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

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