Four-day versus five-day blood culture incubation: a prospective study of diagnostic performance using the bioMérieux...

Blood stream infections (BSIs) are associated with sepsis, high morbidity and mortality rates, and substantial financial costs. Blood cultures are the gold standard diagnostic test for BSI, with UK Standards for Microbiology Investigations (SMI) recommending routine incubation for 5 days. However, reducing the incubation time to 4 days has been reported to adequately detect true bacteraemia whilst reducing contaminant recovery. In January 2025, Newcastle upon Tyne Hospitals implemented the bioMérieux BACT/ALERT VIRTUO system. This study compared 5-day versus 4-day blood culture incubation, assessing whether a 4-day incubation period is likely to be sufficient for detecting clinically significant bloodstream infections in a UK tertiary centre.

All blood cultures processed between 22 January 2025 and 31 March 2025, inclusive, were analysed. A case series review assessed the clinical significance of all growth from blood cultures after 4 days' incubation (> 96 h). Two doctors independently appraised the medical records to evaluate the degree to which results influenced clinical management, with double coding undertaken without inter-coder conferral. A resource impact model estimated the local operational cost of extending incubation beyond 4 days.

In total, 6994 blood culture sets, including 13,103 bottles, were processed from 3221 distinct patients. Only twenty-three (0.18%) bottles flagged positive beyond 4 days' incubation, of which sixteen (69.6%) led to the isolation of contaminants and seven (30.4%) led to the isolation of clinically significant organisms. Retrospective review revealed that growth from only a single bottle yielded information that influenced a change in patient management, translating to one additional clinically meaningful result from over 13,000 extended incubations (0.008%). On the basis of the resource impact model, routine incubation of blood culture for 5 days is estimated to consume an additional 337 h of staff time annually.

Routine incubation of blood cultures beyond 4 days is resource intensive but offers negligible clinical benefits. Our results support review of recommendations outlined in the UK SMI on Sepsis and Systemic or Disseminated infections (S 12). For laboratories equipped with automated continuous monitoring blood culture systems, a routine 4-day incubation period is likely to suffice, with extended incubation reserved for specialist-driven clinical indications.