DISCREPANCIES IN SUSCEPTIBILITY TEST-RESULTS FOR IMIPENEM EMPLOYING DIFFERENT IN-VITRO TEST METHODS AND DIN-58-940 BREAKPOINTS

During the first half of 1993, bacteria that were isolated from clinic al materials and found to have intermediate susceptibility by an agar dilution breakpoint method were collected in a large service laborator y in Germany. All of these isolates were gram-negative bacteria. They were re-tested employing full-scale agar dilution, broth microdilution , E-test and agar diffusion procedures. The results obtained indicated that 76.9 % of the isolates were actually susceptible upon re-testing with a reference agar dilution technique. The reason for the discrepa nt results remained largely unclear. There was a high correlation betw een agar dilution and E-test results while the agreement with broth mi crodilution and agar diffusion was less satisfactory. It is suggested that the breakpoint between susceptible and intermediate categories cu rrently recommended by DIN 58 940 (standard set by Deutsches Institut fur Normung e.V.) be raised to reduce erroneous interpretations of min imum inhibitory concentrations.