Susceptibility testing with clavulanic acid: fixed concentration versus fixed ratio

The susceptibility testing methodology for beta -lactam- beta -lactamase inhibitor combinations, in particular, co-amoxiclav, is not universally agreed upon, and this compound is tested both by the use of a fixed concentration of clavulanic acid (2 mu g/ml) and at a 2:1 ratio of amoxicillin to clavulanic acid. In contrast, the testing of piperacillin-tazobactam and ticarcillin-clavulanic acid is universally performed at fixed inhibitor concentrations of 4 and 2 mu g/ml, respectively. One hundred consecutive amoxicillin-resistant Escherichia coli isolates obtained from patients in the Edinburgh Royal Infirmary between January and March 1993 were tested for susceptibility to co-amoxiclav by the two different testing strategies currently recommended. The beta -lactamase profile of each isolate was determined by analytical isoelectric focusing as described previously. Ninety-one of the strains contained a plasmid-mediated class A beta -lactamase, TEM-1, SHV-1, or TRC-1. The MIC of amoxicillin (Sigma) was determined i) in the absence of clavulanic acid (Mast Laboratories, Liverpool, United Kingdom), ii) in the presence of defined ratios of amoxicillin to clavulanic acid (2:1, 4:1, and 8:1), and iii) in the presence of a fixed clavulanic acid concentration (2 and 4 mu g/ml). All MIC testing was performed at the same time with the same inocula as described previously.