How can I determine from an antimicrobial susceptibility report whether a patient has carbapenem‑resistant Enterobacterales (CRE)?

How to Identify CRE from a Sensitivity Culture Report

Look for resistance to any carbapenem (doripenem, imipenem, or meropenem) on the susceptibility report—this defines CRE. 1

Primary Identification Method

The most straightforward approach is to examine the antimicrobial susceptibility testing results for carbapenem antibiotics:

• CRE is defined as Enterobacterales showing resistance to doripenem, imipenem, OR meropenem 1
• This includes both carbapenemase-producing and non-carbapenemase-producing organisms
• If any of these three carbapenems shows "resistant" or "intermediate" on the report, the organism qualifies as CRE

Critical Pitfall: The "Susceptible" Trap

A major caveat exists: some CRE strains may appear susceptible to carbapenems on standard testing. 2 This occurs when:

• Carbapenemase-producing strains have elevated MICs that remain within the "susceptible" range
• These organisms harbor resistance genes (like blaKPC) but don't meet traditional resistance thresholds
• These hidden CRE pose significant infection control risks despite appearing susceptible 2

To address this challenge:

• Isolates with elevated but susceptible carbapenem MICs or reduced disk diffusion zones should trigger additional testing 2
• The Modified Hodge Test (MHT) can detect carbapenemases in these borderline-susceptible isolates (>90% sensitivity and specificity) 2
• Modern laboratories may add comments to reports when carbapenemases are detected despite susceptible results 2

Confirming Carbapenemase Production

While not required for the CRE definition, identifying the specific carbapenemase type is strongly recommended because it directly impacts treatment selection 3:

Rapid phenotypic tests available:

• Combined disc test (CDT) with meropenem plus 3-aminophenylboronic acid (for KPC) or EDTA (for metallo-β-lactamases): 100% sensitivity for single carbapenemase types 4
• Modified carbapenem inactivation method (mCIM)/EDTA-modified CIM (eCIM): 95% compliance with genetic detection 4
• Immunochromatographic assays (NG-Test CARBA 5, color developing immunoassay): higher sensitivity and specificity than phenotypic methods 4

Molecular methods:

• PCR for carbapenemase genes (KPC, NDM, VIM, IMP, OXA-48-like) provides definitive identification 3
• Loop-mediated isothermal amplification (LAMP): 2-3 hour turnaround time with high sensitivity 4

Why Carbapenemase Type Matters

Treatment strategies differ dramatically based on the carbapenemase class 3:

• KPC producers: Ceftazidime-avibactam or meropenem-vaborbactam first-line 3
• OXA-48-like producers: Ceftazidime-avibactam preferred 3
• Metallo-β-lactamase (NDM, VIM, IMP) producers: Ceftazidime-avibactam PLUS aztreonam, or cefiderocol 3

Practical Algorithm

1. Check carbapenem results (imipenem, meropenem, doripenem, ertapenem)

   • If any shows "resistant" or "intermediate" → CRE confirmed

2. If carbapenems show "susceptible" but MICs are elevated or zones reduced

   • Request carbapenemase testing (MHT, CDT, or molecular methods)
   • These may be "hidden" CRE requiring infection control measures 2

3. For confirmed CRE, determine carbapenemase type

   • Essential for selecting appropriate antimicrobial therapy 3
   • Contact laboratory for rapid carbapenemase detection if not routinely performed
   • Turnaround time: 2-3 hours for rapid methods vs. days for culture-based methods 4

Implementation Considerations

Be aware that outdated breakpoints remain common: Studies show 28% of laboratories still use historical carbapenem breakpoints, which can result in false susceptibility reports and missed CRE identification 5. This misclassification has direct consequences for patient outcomes and infection control 5.

Infectious disease consultation is strongly recommended for all CRE infections to optimize antimicrobial selection based on carbapenemase type and local resistance patterns 1.