Which methods are used to screen for extended-spectrum beta-lactamase (ESBL) organisms?

Updated: Jun 10, 2019
  • Author: Shahab Qureshi, MD, FACP; Chief Editor: Michael Stuart Bronze, MD  more...
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The rising importance of ESBL-producing organisms has mandated effective screening methods for their detection. Use of aztreonam or ceftazidime resistance as a marker misses approximately 15-20% of ESBL-producing organisms. Resistance to cefpodoxime as a screening method, with sensitivity breakpoints of ≥2 mcg/mL by minimal inhibitory concentration or < 22 mm by disk diffusion (for a 30-mcg cefpodoxime disk), has a sensitivity of at least 98% for ESBL detection.

Different tests that help confirm ESBL susceptibility are available. One test involves using disks that contain cefotaxime and ceftazidime alone and disks containing a combination of clavulanic acid with these antibiotics. These are placed on Mueller-Hinton agar. A positive test result is defined as a 5-mm or greater increase in the size of the zone diameter for either cefotaxime or ceftazidime tested in combination with clavulanic acid versus the zone for either antibiotic tested alone. Another method is the E-test screen, which evaluates third-generation cephalosporins with and without a beta-lactamase inhibitor. Finally, the Vitek ESBL test, which is an automated broth microdilution test, uses cefotaxime and ceftazidime alone and in combination with clavulanic acid.

A good screening strategy might include a cefpodoxime screen followed by confirmatory disk diffusion for screen-positive isolates. The Vitek test has sensitivity of at least 99.5% and specificity of 100%. It is a reliable single-test alternative.

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