Validation of the automated reading and incubation system with sensititre plates for antimicrobial susceptibility testing

The present study compared the antimicrobial susceptibility testing (AST) results generated by the Automated Incubation and Reading System (ARIS) with custom Sensititre plates (TREK Diagnostic Systems, Cleveland, Ohio) and MicroScan PC10 GP and NUMIC10 GN plates interpreted with the WalkAway-96 system (Dade Behring, West Sacramento, Calif.) for gram-positive (GP) and gram-negative (GN) organisms as part of an in-house validation. A total of 326 isolates (3,689 antimicrobial agent-organism combinations) were evaluated. Sensititre plates were inoculated according to the instructions of the manufacturer with a suspension adjusted to a 0.5 McFarland standard, while the Prompt Inoculation System was used for the MicroScan plates. ARIS and the WalkAway system were used for automated reading of the Sensititre and MicroScan plates, respectively, at 18 to 24 h. The results were analyzed for essential (+/-1 twofold dilution) and categorical (sensitive, intermediate, or resistant) agreements. Plates that resulted in ARIS interpretations with major (falsely resistant) or very major (falsely susceptible) errors compared to the results obtained with the WalkAway system were read manually to corroborate instrument readings. Isolates for which very major or major errors were obtained and for which the results were not resolved by manual reading were retested in parallel. Isolates for which very major or major errors were obtained and for which the results were not resolved upon repeat testing were tested by the National Committee for Clinical Laboratory Standards M7-A5 frozen reference microdilution method. Essential agreement was 95.8% for 246 GN isolates. The following categorical error rates were obtained for the GN isolates: 1.3% minor errors, 0% major errors, and 0.4% very major errors. For 95 GP isolates, there was 93.5% essential agreement. Categorical error rates for GP isolates were 0.9% minor errors, 0.6% major errors, and 0.4% very major errors. ARIS-Sensititre is a diagnostic system feasible for use for automated AST in a clinical laboratory.