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

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 <10 CFU/ml had a TTP of >20 h (upper limit of 95% prediction interval, 20.7 h) and bottles with initial bacterial densities of > or =50 CFU/ml had a TTP of < or =15 h (lower limit of 95% prediction interval, 13.4 h).

FIG. 1.

TTP for blood culture bottles inoculated with 2 ml of blood containing various initial concentrations of exponential-phase (open circles) or stationary-phase (solid circles) S. epidermidis. Each circle represents the result of a single culture. The dashed lines indicate the 95% confidence intervals for the regression line of the data points. The solid lines represent the 95% prediction intervals. The combined r = 0.84.