Direct identification and antimicrobial susceptibility testing of microorganisms from positive blood cultures following isolation by lysis-centrifugation

We evaluated a direct from positive blood culture pelleting procedure that utilizes a lysis-centrifugation protocol for the identification of microorganisms by MALDI-TOF MS with subsequent antimicrobial susceptibility testing (AST) and rapid methicillin- and beta-lactam-resistance screening. The identification evaluation was performed on 125 cultures and resulted in the correct genus-level identification in 91.2% of cultures and a species-level concordance of 82.4% compared to routine subcultured growth. For the AST evaluation, susceptibility results from direct pelleting and subcultured growth for 187 cultures were compared; an average ±2-fold dilution agreement of 98.2% (1650/1681) and 98.6% (1375/1394) for Gram-negatives and -positives, respectively, was found. Major errors fell below 5% except for MRSA, which was falsely reported as oxacillin sensitive 17.2% (11/66) of the time. Lastly, the sensitivity and specificity of rapid MRSA screening were 94.7% (36/38) and 90.0% (9/10), respectively, while the ESBL screening results were 90.3% (65/72) and 100.0% (13/13) respectively.