Objective. To determine the time to detection of positive blood, urine, and cerebrospinal fluid (CSF) cultures among febrile 28- to 90-day-old infants.Study Design. Retrospective cohort of consecutive 28- to 90-day-old infants presenting with a temperature of ≥38°C to an urban pediatric emergency department. Positive cultures and times to detection were noted. Patients were categorized as being at high risk for serious bacterial illness (SBI) based on clinical and laboratory criteria.Results. Of the 3166 febrile infants seen in the emergency department during the study, 2733 had blood (86%), 2517 had urine (80%), and 2361 had CSF (75%) specimens obtained for culture, and 2190 had all 3 cultures (69%) sent. There were 224 positive cultures in 214 patients; of these, 191 had all 3 cultures (89%) sent. Subsequent analyses were confined to those who had all 3 cultures sent. The detected rate of SBI was 8.7% (191/2190). There were 28 positive blood cultures (1.3%), 165 positive urine cultures (7.5%), and 8 positive CSF cultures (.4%). Median time to detection of positive cultures was 16 hours for blood, 16 hours for urine, and 18 hours for CSF. Four blood cultures (.1%), 20 urine cultures (.9%), and 0 CSF cultures were noted to have growth of a pathogen 24 hours after the specimen was obtained. All 4 blood cultures and 17 of 20 urine cultures with growth noted after 24 hours occurred among high-risk patients.Conclusions. The risk of identifying SBI after 24 hours is 1.1% among all 28- to 90-day-old febrile infants and .3% in low-risk infants.
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