Use of incubation time to detection in BACTEC 9240 to distinguish coagulase-negative staphylococcal contamination from infection in pediatric blood cultures

Abstract

Growth of coagulase-negative staphylococci (CoNS) in a blood culture indicates infection or contamination. Differentiation may be difficult in pediatric patients from whom only a single blood culture is commonly obtained. We evaluated the use of the incubation time to detection of growth [time to positive (TTP)] of CoNS in a continuously monitored blood culture system (BACTEC Peds/F bottles in BACTEC 9240) to differentiate infection from contamination. Three groups of febrile pediatric patients with blood cultures growing CoNS were identified: Group 1, children with vascular catheter-associated bloodstream infection; Group 2, previously healthy children with a contaminant; Group 3, chronically ill children with a contaminant. The median TTP for blood cultures from the 17 infection episodes, 11 h [mean, 13.2 h; 95% confidence interval (CI), 11.4 to 17.7], was significantly shorter than either the 30 episodes in the previously healthy group with contaminant, median 19 h (mean, 22.7 h; 95% CI 19.5 to 26.2; P < 0.0001) or the 12 episodes in the chronically ill group with contaminant, median 24 h (mean, 35.2 h; 95% CI 17.2 to 53.1; P < 0.0001). A TTP of < or =15 h had a positive predictive value of 84% for diagnosis of infection. A TTP of > or =22 h had a positive predictive value of 87% for diagnosis of contaminant. In our experience the TTP was a useful adjunctive test to determine the clinical significance of isolation of CoNS from a blood culture from a pediatric patient.