Essential investigation in sepsis, particularly where unusual organisms or deep seated infection eg endocarditis.

The volume of the sample is important. Small volumes have higher false negative rate, and are slower to become positive – 6ml superior to 2ml [j Peds 1996].

Traditionally, thought to be most effective when done at time of pyrexia, but there is little evidence for this.  In a study of 1,436 adult patients with bacteremia and fungemia, the likelihood of documenting bloodstream infections was not significantly enhanced by collecting blood specimens for culture at the time that patients experienced temperature spikes. Nor was there any benefit for any subgroup eg patient age, gender, white blood cell count and specific cause of bacteremia. [J Clin Microbiol. 2008 Apr; 46(4): 1381–1385.  doi:  10.1128/JCM.02033-07]

Also traditionally, considered negative at 48 hours. Canadian study of 98 positive blood cultures in babies up to 90 days of age found 96% of true pathogenic cultures were positive at 24 hours, with 100% positive at 36 hours. Mean time to positivity was 14.4 hours in pathogenic bacteria and 23.2 hours in contaminants.  [DOI: 10.1093/jpids/piv078]

US study of 256 non-critically ill babies up to 60 days of age found median time to positive blood cultures of 16.6 hours for pathogens cf 25.1 for contaminants, for CSF cultures 14 hours for pathogens cf 40 for contaminants. 82% of pathogens positive within 24hrs for both blood and CSF [can’t see figures for 36/48hrs yet, full text embargoed?][Hosp Peds 2020]

Another US study of 392 cases (outside of PICU) found 96% of pathogens positive by 36hrs (95% CI 95-98), and 99% at 48hrs. But not clear how many of these would have been well enough to go home at 36 hours.  Estimated that observation >36hrs would identify 1 bacteraemic infant for 1250-2778 infants [Biondi, JAMAped2014]. Note that there were significant differences between organisms (E coli faster, staph slower), and between sites (sample volumes? time to inoculation?). No correlation with degree of fever, interestingly.

Current Western Australian guideline, FeBRILe3, appears to be safe.

Canadians have published a new position statement too.

In a tertiary neonatal unit, 72 hours was considered necessary. [ADC Fetal & Neonatal 2001]