Criteria for Performance of ESBL Test (per CLSI)
Disk Diffusion:
- For K. pneumonia, K. oxytoca, and E. coli by disk diffusion, CLSI recommends the use of cefpodoxime (10 µg), ceftazidime (30 µg), aztreonam (30 µg), cefotaxime (30 µg), or ceftriaxone (30 µg) discs. For P. mirabilis isolates, the use of cefpodoxime (10 µg), ceftazidime (30 µg), or cefotaxime (30 µg) is proposed. For screening purposes in all of these organisms, the use of more than one of these agents improves the sensitivity of detection.
- If the isolates show the following zone sizes, it may indicate ESBL production and phenotypic confirmation should be performed. For K. pneumonia, K. oxytoca, and E. coli: cefpodoxime (≤ 17 mm), ceftazidime (≤ 22 mm), aztreonam (≤ 27 mm), cefotaxime (≤ 27 mm), or ceftriaxone (≤ 25 mm) For P. mirabilis: cefpodoxime (≤ 22 mm), ceftazidime (≤ 22 mm), or cefotaxime (≤ 27 mm).
Broth Microdilution:
- For screening of K. pneumonia, K. oxytoca, and E. coli isolates by broth microdilution, the use of cefpodoxime (4 µg/mL), ceftazidime (1 µg/mL), aztreonam (1 µg/mL), cefotaxime (1 µg/mL), or ceftriaxone (1 µg/mL) is recommended. For P. mirabilis isolates, the use of cefpodoxime (1 µg/mL), ceftazidime (1 µg/mL), or cefotaxime (1 µg/mL) is recommended. Again, for screening purposes in all of these organisms, the use of more than one of these agents improves the sensitivity of detection.
- For K. pneumonia, K. oxytoca, and E. coli, MIC growth ≥ 2 µg/mL ceftazidime, aztreonam, cefotaxime, or ceftriaxone or MIC ≥ 8 µg/mL for cefpodoxime may indicate ESBL production and phenotypic confirmation should be performed. For P. mirabilis, an MIC ≥ 2 µg/mL for cefpodoxime, ceftazidime, or cefotaxime may indicate ESBL production and phenotypic confirmation should be performed.
