Blood Culture Time to Positivity in Adults: Pathogen Type Prediction and Contamination Discrimination

This study aimed to evaluate the time to positivity (TTP) of blood cultures in patients with bacteremia and to assess its utility in predicting pathogen categories, distinguishing true bacteremia from contamination and supporting early clinical decision-making.

We retrospectively analyzed adult-patients with bacterial or fungal growth in blood-cultures (May 1-December 31, 2024). Overall, 513 positive culture bottles from 217 patients were included; multiple bottles per patient were analyzed as independent measurements. Demographic characteristics and laboratory/microbiological-parameters were evaluated for associations with TTP.

The most frequently isolated pathogens were Klebsiella spp., Escherichia coli, Staphylococcus aureus, Enterococcus spp., and coagulase negative staphylococci. TTP was significantly shorter in Gram-negative bacteremia compared with Gram-positive bacteremia and shorter in Gram-positive infections than in fungal infections (p < 0.001 and p = 0.007, respectively). Among Gram-positive organisms, S. aureus and Enterococcus spp. exhibited significantly shorter TTP values than other Gram-positive bacteria (p < 0.001). TTP was also significantly shorter in true bacteremia compared with contaminated cultures (p < 0.001). A procalcitonin cut-off value of 1.74 ng/mL predicted Gram-negative bacteremia with 72% sensitivity and 64% specificity. All clinically significant isolates signaled positivity within 96 h. No significant association was observed between TTP and 30-day mortality.

TTP may contribute to predicting pathogen-types and distinguishing true-bacteremia from contamination, thereby strengthening faster and more accurate clinical decision making processes. Clinically, pathogen-specific TTP patterns may be used alongside Gram stain and clinical assessment to prioritize early empiric therapy optimization and contamination management before species identification is finalized. Early identification of true-infection has important potential in antimicrobial-stewardship by reducing unnecessary antibiotic use and enabling more timely initiation of appropriate therapy.