The shift from “MIC-Only” back to carbapenemase testing among carbapenem-resistant Enterobacterales: what clinical laboratories need to know about updated CLSI guidance

Carbapenem-resistant organisms (CROs) pose a major threat to global health due to limited therapeutic options and their capacity for rapid dissemination. Among these, carbapenemase-producing (CP) strains are of greatest concern, as they hydrolyze most β-lactams, and carbapenemase genes are readily spread via mobile genetic elements. Historically, clinical laboratories relied solely on minimal inhibitory concentration (MIC) results and interpretive criteria to guide therapy, with carbapenemase testing performed mainly for epidemiologic purposes. However, changing carbapenemase epidemiology, the introduction of enzyme-specific novel β-lactam combination agents for treatment, and updated Clinical and Laboratory Standards Institute (CLSI) guidance have renewed the importance of carbapenemase testing among carbapenem-resistant Enterobacterales. The 2025 CLSI M100 update now recommends carbapenemase testing for most Enterobacterales resistant to at least one carbapenem, emphasizing differentiation of key enzymes, such as KPC, NDM, and OXA-48-like, to inform therapeutic decisions and support antimicrobial stewardship. This minireview summarizes the evolution of CLSI guidance from early breakpoint establishment through the "MIC-only" era to the current antimicrobial resistance mechanism-driven framework. Key issues addressed include the clinical limitations of prior clinical breakpoints, challenges in balancing sensitivity and specificity of screening criteria to guide carbapenemase testing in different settings, and the expanding role of rapid phenotypic and molecular detection methods. Revisions to the EDTA-modified carbapenem inactivation method (eCIM) are discussed in light of increasing co-production of metallo-beta-lactamase and serine-carbapenemases. Reintegration of carbapenemase testing into clinical workflows highlights the role of the clinical microbiology laboratory as a critical component of antimicrobial stewardship.