Revisiting the time to positivity in blood cultures as a prediagnostic tool: an in vitro experiment and retrospective study.

BACKGROUND

Sepsis is an acute response to infection; rapid detection of pathogens is essential. In this work, the time to positivity (TTP) of blood culture (BC), a key factor for species diagnosis, was studied.

METHODS

In the experimental phase, six bacterial reference strains were tested under varying conditions, and three years of clinical blood culture data were reviewed. A total of 4,924 first positive BC bottles from 3864 patients hospitalized from 2019 to 2021 were analysed.

RESULTS

Experimental results revealed that reducing the inoculum by a factor of 10 increased the TTP by an average of 1.5 hours. A blood sample volume between 1 and 3 mL per vial significantly improved bacterial detection. Each hour of delay before incubation reduced the TTP by 21 minutes; however, the overall TTP was still prolonged. Bacterial growth and viability were preserved at room temperature for up to 24 h. TTP could be exploited as a first diagnostic tool, as it significantly varied between species. Staphylococcus aureus can be differentiated from coagulase-negative staphylococci when the TTP is <12 hours (Specificity (Sp) = 0.95, positive predictive value (PPV) = 62 %, negative predictive value (NPV) = 76 %), and Enterobacteriaceae can be differentiated from Pseudomonas aeruginosa under the same threshold (Sensibility (Se) = 0.70, Sp = 0.93, PPV = 99 %). A TTP >15 hours exclude clinically relevant pathogens such as S. pneumoniae, S. agalactiae, and S. pyogenes.

CONCLUSION

This work shows how preanalytical factors play a major role in the time required for diagnosis, and highlights the potential of the TTP as a prediagnostic tool to improve sepsis management.