Incubation time required for neonatal blood cultures to become positive

We aimed to determine the laboratory detection time of bacteraemia in neonatal blood cultures, and whether this differed by: organism; samples deemed to represent true bacteraemia versus contaminants; and blood cultures collected from an infant <48 h of age (early) or >or=48 h of age (late).

A retrospective audit of all positive blood cultures collected from neonates in the Grantley Stable Neonatal Unit, Royal Women's Hospital, Brisbane, between 1 January 2000 and 31 December 2004 was undertaken. The bacteraemia detection method used was the BacTAlert system with Peds bottles.

Two hundred and three positive blood cultures were included in the analysis. One hundred and sixteen (57%) were deemed septicaemia, 87 (43%) were deemed contaminants. The median (interquartile range) time to positivity for positive blood cultures deemed septicaemia and contaminants were 15.9 (11.6, 22.2) and 30.2 (20.4, 43.9) h, respectively. Fifty-six (28%) positive blood cultures were collected when infants were <48 h of age and 147 (72%) were collected in infants >or=48 h of age. Post hoc analysis revealed that the time to positivity for early septicaemia was 13.7 (11, 16.7) h; early contaminant was 25.2 (19.2, 33.8) h; late septicaemia was 17.2 (12.2, 23.4) h; and late contaminant was 37.9 (21.7, 51.2) h. The time to positivity for: Group B streptococcus was 9.3 (8.2, 11.0) h; Escherichia coli was 11.3 (10.0, 13.5) h; and coagulase-negative staphylococci was 28.9 (20.5, 41.2) h.

The incubation time for positive blood cultures significantly differs by organism type and whether they are considered early or late septicaemia versus contaminants. We recommend that: infants who are <48 h of age at the time of blood culture collection, who remain clinically well and have negative cultures 36 h after the initial collection can safely have their antibiotic treatment ceased; infants who are >or=48 h of age at the time of collection should continue antibiotic treatment for at least 48 h before cessation is considered.