Febrile Infants: How Long Until a Positive Blood Culture?

William T. Basco, Jr., MD, MS

November 14, 2014

Blood Culture Time to Positivity in Febrile Infants With Bacteremia

Biondi EA, Mischler M, Jerardi KE, et al; Pediatric Research in Inpatient Settings (PRIS) Network

JAMA Pediatr. 2014;168:844-849

Study Summary

This was a multicenter, retrospective evaluation of blood culture results in 17 pediatric hospitals. The study included febrile infants (aged 0 to 90 days) who experienced bacteremia. Children who were admitted to an intensive care unit, had central lines, or had undergone surgery were excluded. Other clinical parameters were used to classify the children as either "low risk" or "high risk" for serious bacterial infection to allow for grouping of children for the analyses. Each site provided at least 2 years' worth of data for the study, and they all used the same microbiology blood culture system. The time to culture positivity was calculated in minutes. Typical contaminants and cultures positive with nonpathogenic organisms were excluded from the analyses.

Positive blood cultures from a cohort of 392 children (51% boys, 40% younger than 30 days of age) were included in the analysis. Only 25% of the children would have been classified as "low risk." Among all children, the mean time to positive blood culture was 15.4 hours (median, 13 hours). The average time to positive culture (13.9 hours) was shorter among the youngest infants compared with 15.6 hours for children who were 31-60 days old and 17.9 hours for children who were 61-90 days old. The time to positive culture was not significantly altered by inclusion of contaminant organisms. By 24 hours, 91% of the cultures that would become positive were positive (95% confidence interval, 88%-93%). By 36 hours, 96% of the cultures were positive; and by 48 hours, 99% of the cultures were positive.

Approximately 30% of the bacteria that grew after 24 hours were Escherichia coli. In fact, E coli was the most commonly isolated bacterium (41%), followed by group B streptococci (22%). All other bacterial species found were cultured from less than 10% of the children. When comparing time to positivity among the species, the median time to positivity for E coli was 13 hours compared with 10.5 hours for group B streptococci and 18.5 hours for Staphylococcus aureus. Significantly longer median times to positivity occurred with coagulase-negative streptococci (27.2 hours), Moraxella species (39.8 hours), and Neisseria species (23.5 hours). Biondi and colleagues concluded that most children with a positive bacterial blood culture will be identified within the first 24 hours of obtaining the culture. Keeping children longer than 24 hours would identify one child with bacteremia for every 556 children treated.

Viewpoint

To anyone who has seen some of the previous studies evaluating time to positive blood cultures, this study demonstrates results that are very much in line with previous studies. By far, most pathogenic blood cultures will turn positive within the first 24 hours. These data support 24 hours as a reasonable duration of observation if the infant appears well enough for discharge, and proper follow-up (ideally the next day) can be ensured. These data also reinforce the fact that E coli is the predominant blood pathogen in the newborn, likely owing to more prevalent screening for group B streptococci in pregnant women.

Finally, I was struck by the fact that the time to positive culture varied little among the infants classified as "low risk" or "high risk" on the basis of screening criteria, but the decision to admit the child from the emergency department or outside facility is already a marker of "high risk" in children. Therefore, additional laboratory parameters may be less helpful among the subset of children for whom the decision to admit has already been made.

 

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