Evaluation of Rapid ATB Staph for 5-hour antimicrobial susceptibility testing of Staphylococcus aureus. Groupement pour le Dépistage, L'Etude et la Prévention des Infections Hospitalières-Groep ter Opsporing, Studie en Preventie van Infecties in de Ziekenhuizen.

The accuracy of Rapid ATB Staph (bioMérieux, La Balme-Les Grottes, France) for detection of oxacillin resistance and for detection susceptibility to 11 other antimicrobial agents in 553 and 519 Staphylococcus aureus isolates, respectively, was evaluated by comparing results with those produced by oxacillin agar screen and agar dilution methods, respectively. Further characterization of isolates with discrepant results for oxacillin testing was done by PCR detection of the nuc and mecA genes. By oxacillin agar screening, there were 307 oxacillin-resistant and 246 oxacillin-susceptible isolates. Rapid ATB results were obtained in 5 h for 515 (93.2%) of the isolates tested. Rapid ATB showed 97.0% sensitivity for detection of oxacillin resistance, confirmed by the presence of the mecA gene. After repeat testing of isolates flagged by the ATB software as possible errors, sensitivity increased to 99% for oxacillin-resistant isolates. Essential agreement with agar dilution testing for susceptibility to amoxicillin-clavulanic acid, gentamicin, erythromycin, clindamycin, and ciprofloxacin, as estimated by Youden's J statistic, was > 0.90. Subpopulations of isolates with significantly increased MICs of amikacin, rifampin, and minocycline, indicating borderline susceptibility, were detected by Rapid ATB and categorized as resistant. Rapid ATB Staph showed adequate accuracy for detection within 5 h of the oxacillin- and multiple-drug-resistant S. aureus isolates currently prevalent in Belgium.