Time to Positivity of Blood Culture in an Automated Bact/Alert 3D System and Its Association with Procalcitonin Levels and Organism Type: A Retrospective Observational Study

Abstract Background Bloodstream infections (BSIs) are life-threatening conditions frequently encountered in critical care, where early identification of infection and timely initiation of appropriate antimicrobial therapy are essential for reducing morbidity and mortality. Although blood culture remains the reference method for microbiological conformation, automated blood culture systems provide rapid and standardized detection of microbial growth. Time to positivity (TTP) may provide additional information regarding the infecting organism and microbial burden. While procalcitonin (PCT) is widely used as an adjunctive biomarker in the assessment of suspected bacterial infection and sepsis. In this study to determine the TTP of clinically significant blood cultures processed using the automated Bact/Alert 3D system and evaluate its relationship with serum procalcitonin levels. The study also assessed organism-specific differences in TTP and relevant demographic associations. Methods A retrospective observational study was conducted in the Department of Microbiology, SRM Medical College Hospital and Research Centre, Kattankulathur, Tamil Nadu, from September 2025 to August 2026. All blood culture samples received during the study period were included. Samples were collected under strict aseptic conditions and processed using the automated Bact/Alert 3D system. TTP was recorded in hours. Available serum PCT values were correlated with TTP, and organism-specific differences were statistically analyzed. Results Of the total suspected blood stream infection culture samples analyzed, 8,726(90.2%) showed no growth after five days, 514(5.3%) yielded skin commensals, and 434(4.5%) yielded clinically significant organisms. A valid TTP was obtained for 259 positive cultures. The mean TTP was 14.36 ± 13.67hours, with a median of 9.21 hours (IQR 6.00–16.28). Overall, 62.9% of cultures became positive within 12 hours, 82.6% within 24 hours, and 95.8% within 48% hours. Gram-negative organisms predominated (59.1%), followed by Gram positive organisms (30.9%) and Candida species (8.1%). Candida showed the longest median TTP (32.01 h; p < 0.001). PCT showed a significant inverse correlation with TTP (p = -0.270, p < 0.001). Conclusion TTP varied significantly by organism and showed an inverse correlation with PCT. Combining TTP and PCT may support early risk assessment and clinical management of sepsis in critical care settings.