Time to positivity of neonatal blood cultures

AIM: To determine how long it takes neonatal blood cultures to become positive. METHODS: Data were collected retrospectively on 451 positive blood cultures from babies on a tertiary neonatal unit between January 1997 and December 1998. During the study period, the laboratory used the BacT/Alert microbial detection system. RESULTS: Complete information was available on 416 blood cultures. Twelve became positive after 72 hours, none of which were considered to be clinically significant. Of the 404 remaining cultures, 86% were positive at 36 hours, 96% at 48 hours, and 98.5% by 60 hours. If definite bacterial pathogens are considered alone, the time to positivity was 90% by 36 hours, 93% by 48 hours, and 98% by 60 hours. If definite and possible bacterial pathogens are considered (coagulase negative staphylococci taken as possible bacterial pathogens), the time to positivity was 89% at 36 hours and 97% at 48 hours. The negative predictive value, for isolation of any organism before 72 hours, of a negative blood culture was 97% at 36 hours and 99% at 48 hours. The negative predictive value for the isolation of definite bacterial pathogens only was 99.7% at 36 hours and 99.8% at 48 hours. CONCLUSIONS: A period of 36 hours is enough to rule out sepsis in the asymptomatic neonate, and a three day incubation period is sufficient to detect all clinically important infections using the BacT/Alert microbial detection system.