How can I determine from laboratory results whether a bacterial isolate is an extended‑spectrum β‑lactamase (ESBL) producer?

Laboratory Identification of ESBL-Producing Bacteria

ESBL-producing bacteria are identified through antimicrobial susceptibility testing showing resistance to third-generation cephalosporins (ceftriaxone, ceftazidime, cefotaxime) and aztreonam, with synergy demonstrated when tested in combination with clavulanic acid. 1

Key Laboratory Findings That Indicate ESBL Production

Resistance Pattern Recognition

• Look for resistance to all penicillins, all cephalosporins (including third-generation), and aztreonam, while maintaining susceptibility to carbapenems 2
• The organism will show preserved susceptibility to carbapenems (imipenem, meropenem, ertapenem) unless additional resistance mechanisms are present 2
• Resistance to extended-spectrum cephalosporins like ceftriaxone, cefotaxime, ceftazidime, and cefepime is the hallmark finding 2

Confirmatory Testing Methods

• The combination disk method demonstrates enhanced activity when a third-generation cephalosporin is tested alongside clavulanic acid 3, 4
• The double-disk synergy test (DDST) shows a characteristic "keyhole" or enhanced zone of inhibition between a cephalosporin disk and an amoxicillin-clavulanate disk 3
• MIC determination showing reduced susceptibility to extended-spectrum cephalosporins that improves in the presence of clavulanic acid confirms ESBL production 1

Important Changes in Reporting Standards

Current CLSI Guidelines

• The Clinical and Laboratory Standards Institute (CLSI) states that routine ESBL confirmatory testing is no longer required before reporting susceptibility results when using newer interpretive criteria 1, 5
• However, ESBL testing remains valuable for infection control and epidemiological surveillance purposes 1, 5

Critical Limitation to Be Aware Of

• The newer susceptible breakpoints for ceftazidime (≤4 µg/mL) and cefepime (≤8 µg/mL) fail to identify many ESBL-producing organisms, particularly E. coli, K. pneumoniae, and K. oxytoca 1, 5
• This means an isolate may be reported as "susceptible" to these agents but still harbor an ESBL enzyme 5
• Automated susceptibility testing systems may have limitations in detecting ESBL producers using these newer criteria 1

Practical Interpretation for Clinical Use

What to Look for on Your Lab Report

• Resistance to ceftriaxone, ceftazidime, or cefotaxime with susceptibility to carbapenems strongly suggests ESBL production 2, 4
• Many labs will explicitly state "ESBL detected" or "ESBL positive" if confirmatory testing was performed 1
• Look for co-resistance patterns: ESBL producers frequently show resistance to fluoroquinolones, aminoglycosides, and trimethoprim-sulfamethoxazole 2

Common Pitfalls to Avoid

• Do not rely solely on susceptibility to cefepime or ceftazidime as evidence against ESBL production—these may appear falsely susceptible 5
• If cefoxitin resistance is present alongside cephalosporin resistance, the organism may produce both ESBL and AmpC β-lactamase, which complicates detection 2, 3
• Approximately 5-10% of cephalosporin-resistant K. pneumoniae produce AmpC enzymes rather than (or in addition to) ESBLs 2

Special Testing Considerations

When Standard Tests May Be Inadequate

• In organisms that overproduce cephalosporinase (AmpC), the ESBL may be masked and require testing on cloxacillin-containing agar or using cefepime in the synergy test 3
• Reducing the distance between cephalosporin and clavulanate disks in the DDST improves sensitivity 3
• If metallo-β-lactamase is suspected (carbapenem resistance present), EDTA inhibition testing can differentiate this from ESBL 3

Clinical Significance of Detection

• Identification is critical because ESBL infections are associated with higher mortality and complications if treated inappropriately 1
• Carbapenems remain the treatment of choice for serious ESBL infections 1, 2
• For uncomplicated infections, alternatives like fosfomycin or nitrofurantoin may be appropriate based on susceptibility 2