Validation of Etest for seven antimicrobial agents using regulatory criteria for the assessment of antimicrobial susceptibility devices.

Etest, a stable-gradient antimicrobial susceptibility test method (AB Biodisk, Solna, Sweden), was compared to the agar dilution method for tests with amikacin, tobramycin, aztreonam, cefdinir, cefprozil, ceftazidime and sparfloxacin. The study design followed recommended guidelines for 510(k) application by the U.S. Food and Drug Administration (FDA). Results demonstrated Etest accuracy (MIC +/- 1.0 log2 dilutions compared to agar dilution MIC) to be 90.4% with only one of 1150 comparisons showing a reproducible variation of 2 log2 dilutions. Categorical equivalency was > or = 92% for all drugs except ceftazidime (82 to 89%), but false-susceptible error was rare (< 0.1%). Etest was very reproducible (100% of results +/- 1.0 log2 dilution) and quality control results conformed to published MIC ranges. The Etest seems to render accurate results for these seven antimicrobial agents evaluated.