Agar supplemented with clavulanate. Ho et al. (165) de- scribed a method by which Mueller-Hinton agar was supple- mented with 4 µg/ml of clavulanate. Antibiotic disks contain- ing ceftazidime (30 µg), cefotaxime (30 µg), ceftriaxone (30µg), and aztreonam (30 µg) were placed on the clavulanate containing agar and on regular clavulanate free Mueller-Hin- ton agar plates. A difference in þ-lactam zone width of Š10 mm on the two media was considered positive for ESBL pro- duction. The difference in ceftazidime zone sizes was 93 to 96% sensitive and 100% specific in the hands of the originators of the test (165), but somewhat less so according to other investigators (31). A major drawback of the method is the need to freshly prepare clavulanate containing plates. The potency of clavulanic acid begins to decrease after 72 h.
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