# Blood culture diagnostics - a comparative and experimental study on the impact of delayed incubation

## TL;DR

This study shows that delaying the incubation of blood cultures increases detection times and reduces the ability to detect certain bacteria, highlighting the importance of timely processing.

## Contribution

The study experimentally demonstrates the negative impact of delayed incubation on blood culture diagnostics using both retrospective and simulation data.

## Key findings

- Delayed incubation significantly increased detection times for Escherichia coli, Staphylococcus aureus, and Streptococcus pneumoniae.
- An 18-hour delay notably reduced the recovery of Streptococcus pneumoniae, with growth detected in only one bottle.
- Minimizing incubation delays reduces overall detection time and improves the detection of fragile bacteria.

## Abstract

This study investigates the impact of transitioning from restricted to 24/7 access to blood culture cabinets on blood culture processing.

A post-hoc retrospective study and a prospective laboratory simulation to evaluate the effects of delayed incubation were conducted. Data analysis evaluated clinical data comparing incubation-to-detection (ITD) and Collection-to-detection (CTD) including Collection-to-incubation (CTI) comparing pre- and post-implementing the new protocol (Cut-point). ITD values were obtained using BD Synapsys™ software, with delays factored into CTD. Of 14,673 blood cultures collected from October 2019 to September 2023 at Rigshospitalet, Copenhagen, 3,323 met inclusion criteria. Escherichia coli, Staphylococcus aureus, Pseudomonas aeruginosa and Streptococcus pneumoniae were selected as indicator organisms. In the simulation, cultures with 0-, 6-, and 18-hour incubation delays were assessed for growth and detection time.

Median ITD increased significantly post-cut-point: for E. coli from 7.8(IQR: 2.28–13.55) to 11.4(IQR: 9.8–14.7) hours, S. aureus from 9.9(IQR: 4.1–15.6) to 14.3 h(IQR: 11.5–19.5), P. aeruginosa from 14.0(IQR 5.3-19.95) to 16.3 h(IQR 11.75–26.1), and S. pneumoniae from 3.3(IQR: 1.55–9.9) to 11.7 h(IQR: 9–12.6) (all p < 0.01). CTD decreased post-cut-point: by 3h03m for E. coli (p < 0.0001), 3h51m for S. aureus (p = 0.0016), and 4h24m for S. pneumoniae (p < 0.0001). The reduction for P. aeruginosa (1h48m) was not statistically significant.

In the simulation-study, delayed incubation increased CTD for all species in aerobic bottles: E. coli (p = 0.0036), S. aureus (p = 0.0036), P. aeruginosa (p = 0.0036), and S. pneumoniae (p = 0.0429); and in anaerobic bottles: E. coli (p = 0.0036), S. aureus (p = 0.0036), and S. pneumoniae (p = 0.0071). No anaerobic growth of P. aeruginosa was observed. An 18-hour delay notably reduced recovery of S. pneumoniae, with growth detected in only one bottle.

Minimizing incubation delays significantly reduces CTD and improves detection of fragile bacteria. These findings potentially have significant implications for clinical practice, emphasizing the importance of protocols that limit pre-analytical delays to optimize blood culture diagnostics.