Evaluation of the AutoSCAN-3, a device for reading microdilution trays

The AutoSCAN-3 (American MicroScan, Mahwah, N.J.) is an instrument capable of automated reading of commercially available microdilution trays for identification and quantitative susceptibility testing of rapidly growing bacteria. This study compared the results of visual and automated reading of microdilution trays for determination and interpretation of minimum inhibitory concentrations of 471 selected gram-negative and gram-positive clinical bacterial isolates. Visual and automated readings were performed in a double-blind fashion, and all discrepancies were examined by a referee. A quantitative comparison of minimum inhibitory concentrations was performed for 201 organisms, yielding 2,472 drug-organism combinations. After exclusion of off-scale values, complete quantitative agreement was obtained in 94% of 959 on-scale combinations, and agreement within +/- 1 well was obtained in 99.3%. Considering the minimum inhibitory concentration interpretations routinely furnished by the instrument, a qualitative comparison was performed for all 471 organisms. Complete agreement in interpretation was obtained in 97.6% of 5,843 drugs-organism combinations, with very major discrepancies accounting for only 0.1% and major discrepancies accounting for 0.2% of all combinations tested.