Evaluation of Stability of Cefotaxime (30-μg) and Ceftazidime (30-μg) Disks Impregnated with Clavulanic Acid (10 μg) for Detection of Extended-Spectrum β-Lactamases

Extended-spectrum β-lactamase (ESBL) activity inEscherichia coli, Klebsiella pneumoniae, orKlebsiella oxytoca can be detected by using a standard disk diffusion susceptibility test method with cefotaxime (30-μg), ceftriaxone (30-μg), ceftazidime (30-μg), cefpodoxime (10-μg), or aztreonam (30-μg) disks (2, 3). The presence of an ESBL can then be confirmed by a phenotypic confirmatory disk diffusion test recommended by the National Committee for Clinical Laboratory Standards (3) or by the double disk synergy test described by Jarlier et al. (1). The phenotypic confirmatory disk diffusion test is relatively simple to perform and easy to interpret. It requires testing of both cefotaxime (30-μg) and ceftazidime (30-μg) disks each alone and in combination with clavulanic acid (10 μg). An increase in the zone diameter of ≥5 mm for either antimicrobial agent tested in combination with clavulanic acid over that when tested alone indicates that the isolate is an ESBL producer. However, as clavulanic acid is labile, it is recommended that the disks be used immediately after preparation and that they not be stored (3). This may be an inconvenience for many clinical laboratories. Therefore, we evaluated the stability of disks containing clavulanic acid stored at −20°C for 14 consecutive days.