Carbapenem Resistance and Carbapenemase Confirmation Testing

Hello. In this lesson, you will work through a distinction that an automated AST result cannot settle on its own: an isolate can resist a carbapenem without producing a carbapenemase, and an isolate can produce a carbapenemase while testing susceptible to a particular carbapenem. You will use that distinction to choose and interpret confirmation tests for Enterobacterales, Pseudomonas, and Acinetobacter.

For your presentation, the central question is not simply “Is meropenem resistant?” It is: What did the susceptibility test establish, what mechanism remains uncertain, and which test can answer the next question? Allow about 40–45 minutes, including the selected readings and video segments.

Two findings that must not be conflated

Carbapenem resistance is a susceptibility finding: an organism–drug pair meets the resistant criterion under the applicable, current CLSI or EUCAST method and breakpoint. It does not name the mechanism. If ertapenem is resistant but meropenem is susceptible, report and investigate those as two distinct drug results rather than calling the isolate resistant to every carbapenem.

A carbapenemase is a bacterial enzyme capable of hydrolyzing carbapenems. In Enterobacterales, clinically important acquired examples include KPC (Ambler class A), NDM, VIM, and IMP (class B metallo-β-lactamases), and OXA-48-like enzymes (class D). Class B enzymes require zinc for activity; that property makes an EDTA-based differentiation test possible. An ESBL or AmpC combined with reduced outer-membrane permeability can also produce carbapenem resistance without a carbapenemase. In Pseudomonas aeruginosa, porin loss and other non-carbapenemase mechanisms are especially important.

The reverse mismatch matters just as much. Some carbapenemase producers, particularly those with OXA-48-like enzymes, have carbapenem MICs at or below a clinical susceptibility breakpoint. A breakpoint classifies the measured drug response for clinical interpretation; it is not a universal screen for enzyme production.

Watch the short opening of “Detection of carbapenemases – Editors in Conversation (JCM Edition)” from ASM. Listen for the separation between reasons to identify an enzyme and reasons to report an MIC.

For the mechanistic foundation, read the indicated parts of the UK Standards for Microbiology Investigations guidance, Detection of bacteria with carbapenem-hydrolysing β-lactamases (carbapenemases). Its examples explain why neither “resistant” nor “susceptible” identifies the underlying mechanism.

This distinction also explains why an automated expert-system alert is a prompt, not a molecular diagnosis. An elevated carbapenem MIC may justify investigation, but the AST instrument measured growth, not carbapenem hydrolysis, enzyme protein, or a gene. Conversely, a low MIC does not exclude every carbapenemase. Before attributing a mechanism, verify identification, culture purity, the AST result, and the organism’s expected intrinsic susceptibility pattern.

Testing enzyme activity: mCIM first, eCIM second

The modified carbapenem inactivation method (mCIM) asks whether the test isolate inactivates meropenem. A meropenem disk is incubated with the isolate, then transferred to an agar lawn of carbapenem-susceptible indicator E. coli. If active drug remains, it inhibits the indicator and creates a substantial zone. If the isolate has inactivated the drug, the indicator grows close to the disk and the zone is small or absent.

The EDTA-modified CIM (eCIM) adds a narrower question: if mCIM shows carbapenemase activity, is that activity consistent with a metallo-β-lactamase (MBL)? EDTA binds the zinc needed by class B enzymes. In a paired eCIM, inhibition of an MBL preserves more meropenem, so the indicator’s zone becomes appreciably larger than in mCIM. A positive mCIM without that EDTA-associated increase supports a non-MBL carbapenemase; it does not distinguish KPC from an OXA enzyme.

The ASM article “Opening the Black Box of Phenotypic Carbapenemase Detection” provides a compact visual explanation. Read its mCIM/eCIM section rather than treating the diagrams as stand-alone interpretation rules.

The accompanying image places several assay readouts side by side:

Interpret the paired disk results in the correct order:

This is a logic table, not a substitute for the current interpretive criteria. Measure zones and apply the current CLSI instructions or validated method instructions for that organism. An intermediate-sized zone, ambiguous growth within a zone, or an unexpected paired result may be indeterminate and require repetition or another method. In particular, an eCIM result should not be interpreted as an enzyme classification when mCIM has not established activity. Co-produced enzymes can also complicate a simple MBL-versus-non-MBL reading.

The modified Hodge test pictured in the image is useful historically, but its sensitivity and specificity limitations make it a poor choice for resolving an important modern discordance. A color-change hydrolysis assay such as Carba NP instead looks for acidification associated with carbapenem hydrolysis; it can be rapid, but a negative result must be judged against the assay’s performance for the organism and enzyme in question.

Faster answers—and what each answer actually means

When the clinical question is urgent, an overnight growth-based confirmation may not be the best first additional test. Three other approaches target different evidence:

• A rapid biochemical hydrolysis assay detects a chemical consequence of carbapenem breakdown, often a pH-linked color change. It supports enzyme activity but usually does not name the enzyme family.
• A lateral-flow immunochromatographic assay uses antibodies to detect specified carbapenemase proteins. A valid control line and a KPC, NDM, VIM, IMP, or OXA-48-like target line can rapidly identify a covered family. It detects antigen, not hydrolytic activity; available targets and recognized variants depend on the kit.
• A molecular assay, commonly PCR, detects specified carbapenemase genes. It can be useful on a validated isolate or, with an assay approved for that specimen type, directly on a clinical specimen or positive culture. Gene detection does not itself measure expression, enzyme activity, or an MIC.

Watch the lateral-flow and molecular portions of the ASM discussion. The useful comparison is not simply “fast versus slow”; it is protein present versus gene present, and what a negative result cannot exclude.

A negative targeted result means “not detected among this assay’s targets under these conditions,” not “no carbapenemase exists.” It may reflect a carbapenemase outside the panel, a variant the assay detects poorly, or a test problem. Likewise, an activity-positive result and a negative five-target panel are not necessarily contradictory: the activity test asks a broader question. Check assay coverage and quality controls, then consider a suitable alternative assay or reference-laboratory investigation.

A rapid positive protein or gene result can justify prompt communication under local policy. It does not replace susceptibility testing of the relevant treatment drugs. Nor should identifying a carbapenemase cause every carbapenem MIC to be overwritten automatically as “resistant”: report susceptibility according to the current applicable breakpoints and local guidance, while clearly communicating the confirmed mechanism where required.

Let the organism shape the confirmation pathway

For Enterobacterales, reduced carbapenem susceptibility deserves particular attention because non-carbapenemase mechanisms and acquired carbapenemases can produce overlapping AST patterns. After confirming a pure, correctly identified isolate and its AST, choose between an activity assay such as mCIM and a faster target-specific assay according to urgency, local epidemiology, and test availability. If mCIM is positive, eCIM can address MBL activity; a lateral-flow or molecular assay can then identify a covered family. Isolated ertapenem resistance is not proof of a carbapenemase, but neither does meropenem susceptibility safely exclude one.

For P. aeruginosa, carbapenem resistance often reflects permeability changes, sometimes with efflux or other mechanisms. A carbapenem-resistant result alone therefore does not establish an acquired enzyme. A striking, unusual multidrug-resistant pattern, relevant epidemiological information, or an outbreak may justify targeted investigation—especially for an MBL. Ensure that any mCIM, rapid assay, or molecular assay is validated for Pseudomonas and covers the enzyme being sought. Do not transfer Enterobacterales eCIM interpretations automatically to a non-fermenter.

For Acinetobacter, carbapenemases—often OXA-family enzymes—commonly contribute to resistance, but they are not necessarily the OXA-48-like enzymes targeted by an Enterobacterales-focused panel. The intrinsic OXA-51-like gene in A. baumannii also requires careful interpretation: its presence alone does not prove that it is expressed at a level causing resistance. If the question is whether an unusual acquired MBL such as NDM is present, use a validated targeted assay rather than relying on a generic positive activity test. EDTA-based synergy can mislead in non-fermenters, so corroborate consequential findings.

For the organism-specific cautions, read the short non-fermenter section of the UK SMI guidance:

Consider a presentation-ready laboratory narrative. A pure Enterobacterales isolate has an automated ertapenem-resistant, meropenem-susceptible pattern. After the identification and AST are checked, mCIM is positive. Its paired eCIM shows no qualifying increase in zone size. The defensible interim conclusion is carbapenemase activity consistent with a non-MBL enzyme—not “KPC confirmed.” A subsequent valid lateral-flow result with an OXA-48-like target line supplies family-level evidence. The apparently susceptible meropenem result no longer seems impossible: enzyme production and the measured MIC answer different questions. The laboratory verifies the MIC and breakpoint interpretation, communicates the mechanism under its notification policy, and does not infer susceptibility to another drug without that drug’s AST.

Takeaways

Keep three questions separate: Does this isolate meet a carbapenem resistance breakpoint? Is there evidence of carbapenemase production? Which enzyme family, if any, has been demonstrated? AST addresses the first; mCIM or a hydrolysis assay addresses activity; eCIM probes for an MBL contribution; lateral-flow and molecular tests identify covered proteins or genes. Every result is conditional on organism suitability, assay coverage, valid controls, and the applicable interpretive rules.

For your presentation, that sequence will make unusual automated results understandable without overstating what any single test proves. The next related lesson turns to ESBL confirmation, where the same discipline—separating a resistance mechanism from the susceptibility category ultimately reported—remains essential.