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Extended-Spectrum Beta-Lactamase (ESBL) Screening by Disk Diffusion

  • For initial screening of K. pneumonia, K. oxytoca, and E. coli by disk diffusion, CLSI proposes 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).