Over eighteen hours to positivity in the BacT/ALERT 3D system with clustered Gram-positive cocci is highly predictive of coagulase negative staphylococci

Abstract

We assessed the predictive value of time to positivity (TTP) for the exclusion of S. aureus in the setting of a positive result for clustered Gram-positive cocci in BacT/ALERT FA aerobic bottles. We retrospectively analyzed all blood cultures collected from patients between January 2005 and February 2006 in a tertiary hospital. A total of 1030 isolates of staphylococci of which 140 (13.6%) were positive for S. aureus and 890 were positive for CoNS (86.4%) (table 1). The PPV for S.aureus was 71% for isolates growing in less than 13.1 hours of incubation (LR of 15.8) and In the same time interval, only 28% of isolates positive were characterized as CoNS (LR of 0.063). For growth over 18 hours, the PPV for S.aureus fell to 5.6% (LR of 0.37) whereas for CoNS the PPV was 94% (LR 2.6). The very high PPV for CoNS permit deferring treatment until knowledge of the second bottle result. Because of the low virulence of CoNs, the delay to start antimicrobial therapy does not jeopardize the clinical outcome in cases of true bacteraemia.