How do you diagnose an Extended-Spectrum Beta-Lactamase (ESBL) infection?

Diagnosing Extended-Spectrum Beta-Lactamase (ESBL) Infections

ESBL infections are definitively diagnosed through culture and susceptibility testing of appropriate clinical specimens, with specific laboratory tests to detect the presence of ESBL-producing organisms.

Collection of Appropriate Specimens

• Collect representative specimens from the site of infection (at least 1 mL of fluid or tissue, preferably more) and transport to the laboratory in an appropriate transport system 1
• For intra-abdominal infections, peritoneal fluid or pus should be collected in airless sterile syringes or sterile test tubes - peritoneal swabs and fluid from drain tubes are not recommended 1
• For optimal recovery of aerobic bacteria, 1-10 mL of fluid should be inoculated directly into an aerobic blood culture bottle 1
• Send at least 0.5 mL of fluid to the laboratory for Gram stain and fungal cultures if indicated 1
• For anaerobic cultures, at least 0.5 mL of fluid or 0.5 g of tissue should be transported in an anaerobic transport tube, or 1-10 mL of fluid can be inoculated directly into an anaerobic blood culture bottle 1

Laboratory Detection Methods

• ESBL detection is based on the principle that ESBL-producing organisms show enhanced activity against extended-spectrum cephalosporins in the presence of clavulanic acid 2
• Common laboratory methods for ESBL detection include:
  • Double disc diffusion test
  • Automated systems (e.g., Vitek ESBL test)
  • E-tests
  • MIC determination
  • Molecular/genetic methods 3

When to Suspect ESBL Infections

• Consider ESBL infection in patients with specific risk factors:
  • Recent exposure to antibiotics (particularly beta-lactams or fluoroquinolones) within 90 days 1
  • Known colonization with ESBL-producing Enterobacteriaceae 1
  • Healthcare-associated infections 1
  • Recurrent urinary tract infections 4
  • Hospitalization in areas with high ESBL prevalence 5

Antimicrobial Susceptibility Patterns

• ESBL-producing organisms typically show:
  • Resistance to penicillins, broad-spectrum cephalosporins, and monobactams 6
  • Inhibition by beta-lactamase inhibitors such as clavulanic acid 6
  • Susceptibility to carbapenems (imipenem, meropenem, doripenem) 6
  • Frequent co-resistance to other antibiotic classes (aminoglycosides, fluoroquinolones) 6

Rapid Diagnostic Approaches

• Rapid molecular identification of microorganisms and their resistance mechanisms from blood cultures can significantly reduce time to appropriate therapy 1
• In patients colonized or potentially infected with ESBL-producing Enterobacteriaceae, molecular tests should be adopted as they are associated with more rapid administration of appropriate antimicrobial therapy 1

Common Pitfalls and Caveats

• ESBL-producing organisms may appear susceptible to some extended-spectrum cephalosporins in standard testing, but treatment with such antibiotics has been associated with high failure rates 2
• Relying solely on automated susceptibility testing without confirmatory ESBL testing may lead to inappropriate antibiotic selection 2
• According to updated CLSI guidelines, routine ESBL testing is no longer necessary before reporting results when using the new interpretive criteria for cephalosporins 1
• However, ESBL testing remains useful for epidemiological and infection control purposes 1
• Be aware that some ESBL-producing organisms may also express AmpC beta-lactamases, further complicating detection and treatment 6

Clinical Significance

• Identification of ESBL-producing organisms is critical for appropriate antibiotic selection, as these infections are associated with higher mortality, complications, and prolonged hospitalization if not properly treated 1
• Carbapenems are generally considered the drugs of choice for serious infections caused by ESBL-producing organisms 6, 2