An E-test strip for meropenem placed on an MHA plate with a Klebsiella pneumoniae isolate shows an elliptical inhibition zone touching the strip at an MIC value of 8 µg/mL. What is the interpretation per CLSI 2023 breakpoints for K. pneumoniae, and what does this suggest?
- A Susceptible (MIC ≤8 µg/mL for all Enterobacterales)
- B Intermediate (SIR intermediate zone 2–4 µg/mL)
- C Indeterminate — E-test not valid for carbapenems
- D Resistant to meropenem (CLSI breakpoint: susceptible ≤1, resistant ≥4 µg/mL); suspect carbapenem-resistant Enterobacterales (CRE) ✓
Explanation
Per CLSI 2023 updated breakpoints for meropenem against Enterobacterales (including Klebsiella pneumoniae): susceptible ≤1 µg/mL, intermediate 2 µg/mL, resistant ≥4 µg/mL. An MIC of 8 µg/mL is clearly resistant, categorising this isolate as CRE (carbapenem-resistant Enterobacterales). Further testing for carbapenemase production (modified carbapenem inactivation method, mCIM; CarbaNP test; or multiplex PCR for KPC, NDM, OXA-48, VIM, IMP) is warranted. Treatment options for KPC-producing CRE are limited: ceftazidime-avibactam, meropenem-vaborbactam, or imipenem-cilastatin-relebactam.
Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.
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