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

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.

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.

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.

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.