Evaluation of the Epsilometer test (E test) for testing the susceptibility of coagulase-negative staphylococci to...

The antimicrobial susceptibilities of 118 clinical isolates of coagulase-negative staphylococci to teicoplanin were determined by disc diffusion and the Epsilometer test (E test) and the results were compared with the MICs determined by the agar dilution method of the National Committee for Clinical Laboratory Standards (NCCLS). There was a poor correlation of r = 0.5 between the zone diameters of inhibition and agar dilution MICs and 10 and four of the 11 isolates for which the MICs were > or = 32 mg/L were misclassified as susceptible by the disc test after applying the interpretative criteria of the NCCLS and the Comité de l'Antibiogramme de la Société Française de Microbiologie (CASFM), respectively. The E test tended to result in MICs that were lower than those determined by agar dilution and only 66% of MIC were within +/- 1 log2 dilution of each other. Only one of 11 resistant strains was detected by the E test and, although there was no false resistance, six resistant strains were misclassified as susceptible after applying the criteria of the NCCLS as were four such isolates when the criteria of the CASFM were employed, probably as a result of using too light an inoculum. Disc diffusion is not a reliable means of determining the susceptibility of coagulase-negative staphylococci but might be replaced by the E-test provided that discrepant results can be resolved by using a denser inoculum.