Time to Positivity of Blood Cultures in Infants 0 to 90 Days Old Presenting to the Emergency Department: Is 36 Hours Enough?

Background. Continuous monitoring blood culture systems (CMBCS) now allow for more rapid detection of microbial growth. We aimed to determine whether a 36-hour period was sufficient to detect all blood cultures positive for pathogenic bacteria in infants 0 to 90 days old undergoing a septic workup in the emergency department of a tertiary care pediatric center. Methods. We performed a retrospective study of all positive blood cultures collected in these infants over a 5-year time period (from March 13, 2008 to July 29, 2013). Bottles were incubated in a CMBCS. The time to positivity (TTP) was calculated from time of blood culture registration into the laboratory system to time of Gram stain. Medical charts were reviewed for relevant clinical information. Cultures were classified as pathogenic or contaminant using microorganism type and clinical presentation. Results. Three thousand five hundred fifty-nine blood cultures were collected. Of these, 98 (2.8%) were positive. Fifty-two (53.1%) were deemed pathogenic and 46 (46.9%) were deemed contaminant, for a true prevalence of bacteremia of 1.5%. At 24, 36, 48, and 50 hours, 87.8% (86 of 98), 96.9% (95 of 98), 99% (97 of 98), and 100% (98 of 98) of all cultures were positive. Considering only pathogenic organisms, 96.1% (50 of 52) and 100% (52 of 52) were positive at 24 and 36 hours. Mean TTP for pathogens and contaminants was 14.40 and 23.18 hours, respectively (P < .001). Conclusions. An incubation period of 36 hours is sufficient to detect 100% of blood cultures positive for a pathogenic organism in our population.