Initial Concentration of Staphylococcus epidermidis in Simulated Pediatric Blood Cultures Correlates with Time to Positive Results with the Automated, Continuously Monitored BACTEC Blood Culture System

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Abstract
The relationship of initial concentration of Staphylococcus epidermidis in blood cultures and time to positivity (TTP) in an automated, continuously monitored blood culture system was assessed. Blood and 1 to 1,000 CFU of S. epidermidis per ml in stationary or exponential phase were inoculated in BACTEC Pediatric Plus F bottles and incubated. The TTP was inversely proportional to the initial inoculated concentration. Blood culture bottles with initial bacterial densities of 20 h (upper limit of 95% prediction interval, 20.7 h) and bottles with initial bacterial densities of ≥50 CFU/ml had a TTP of ≤15 h (lower limit of 95% prediction interval, 13.4 h).