Time to positivity for differentiating blood culture contamination: A 20-hour cutoff for major contaminants

Blood culture remains the gold standard for diagnosing bacteremia; however, contamination inevitably occurs in 2-3% of cases, requiring differentiation between true bacteremia and contamination. Although time to positivity (TTP) aids in this clinical decision, with detection after 24 hours generally indicating contamination, technological advances in blood culture systems may have shortened this threshold interval.

This study retrospectively analyzed blood culture data in our hospital from April 2023 to January 2025 to determine the optimal TTP cutoff. Patients with positive blood cultures for major contaminating bacteria were included. Cases were classified as true bacteremia or contamination based on a comprehensive chart review conducted by the antimicrobial stewardship audit, and TTP was compared between the groups. Sensitivity, specificity, and Youden index at various TTP cutoffs were utilized to determine the optimal threshold using the receiver operating characteristic curve analysis.

Seventy-one patients were enrolled, with 34 cases classified as true bacteremia and 37 as contamination. Identified bacteria included coagulase-negative staphylococci (70.4%), viridans group streptococci (18.3%), and others (11.3%). The median TTP was significantly shorter in the true bacteremia group compared with the contamination group (18.6 vs.25.8 hours, p < 0.001). In the contamination group, 43.2% of the cases demonstrated positive growth within 24 hours. Based on sensitivity, specificity, and Youden index, the optimal threshold was estimated to be 20 hours. A subgroup analysis of the CNS-only cohort yielded concordant results.

This study suggests that a 20-hour TTP threshold could help effectively differentiate true bacteremia from contamination in current clinical settings.