Blood culture time to positivity corrected for delayed incubation start - a population-based study

Purpose A short blood culture time to positivity (TTP) is associated with an increased risk for death in blood stream infections (BSI). However, TTP is also shortened when the time elapsed from sampling to incubation start (TTI) increases. We aimed to test whether TTP corrected for TTI was more strongly associated with 7-day mortality in BSI. Methods This was a retrospective analysis of positive blood culture bottles in Skåne region, Sweden, 2015 to 2023, with linked clinical data obtained from healthcare databases. Correction factors were derived using linear regression between TTI and TTP. The association between TTP and 7-day mortality was studied in BSI episodes, using non-linear logistic regression. Actual mortality rates and odds ratios (OR) for different TTP cutoffs were calculated. Results In total, 100,829 bottles were included and each hour of TTI was associated with a shortening of TTP by 0.27 h in aerobic bottles and 0.32 h in anaerobic bottles. After deduplication, 31,723 unique BSI episodes were included in mortality analysis, with a 7-day mortality rate of 6.4%. In non-linear regression models, corrected TTP showed a stronger association with 7-day mortality than crude TTP in episodes with TTP < 10 h. Actual mortality rates were higher for corrected TTP at all cutoffs between 6 and 12 h. Conclusion TTP corrected for TTI was more strongly associated with 7-day mortality than crude TTP. We recommend that correction for TTI should be used in future studies on TTP, especially if on-site incubation is unavailable.