Evaluation of Positivity Times of Isolates Growing in Blood Cultures

Introduction: Blood culture is the gold standard method in the diagnosis of bacteremia and fungemia. Rapid identification of the pathogen and determination of its antimicrobial susceptibility have great importance in treatment and prognosis in patients with bacteremia and fungemia. The aim of this study was to evaluate the isolates that have positive signals in blood culture bottles and to compare the positivity times in Central Laboratory of Akdeniz University Hospital.Materials and Methods: A total 6167 of blood culture bottles that came to Central Laboratory of Akdeniz University Hospital between 2020 February and October were included in this study. The blood culture bottles were monitored on the BD BACTECTM FX automated system (Becton Dickinson, USA). All machine signal positive bottles were sub-cultured on aerobic and anaerobic media and the resulted colonies were identified by MALDI-TOF MS (Bruker Daltonics, Germany). The study also compared the positivity times of blood culture bottles in which methicillin-resistant Staphylococcus aureus (MRSA) versus methicillin-susceptible Staphylococcus aureus (MSSA), car- bapenemase-producing versus non-carbapenemase-producing Enterobacterales, extended-spectrum beta-lactamases Enterobacterales (ESBL) producing and not producing; vancomycin-resistant (VRE) versus vancomycin-susceptible enterococci (VSE) isolates and among Candida species.Results: Of the blood cultures 20.6% yielded positive signals and 13.6% of them were accepted as pathogens, while 5.8% were identified as contaminants. The most commonly isolated agents were Gram-positive cocci (51.3%). Then, Enterobacterales members (21.5%), nonfermentative gram-negative bacilli (13.6%) and yeasts (10.6%) were, respectively, isolated. The positivity times ranged from 2-113 hours. The average durations of signaling positive were 15.1 +/- 11.5, 15.9 +/- 15.0, 14.2 +/- 13.0, 12.1 +/- 9.9, 11.7 +/- 7.2, 16.8 +/- 10.2, 32.1 +/- 18.6 hours for Enterococcus species, Staphylococcus aureus, Klebsiella species, Escherichia coli, Acinetobacter species, Pseudomonas species, and Candida albicans, respectively. 40.8% of the causative pathogens gave positive signals in the first 12 hours, 82.3% in the first 24 hours and 96.2 % within 48 hours. The positivity time of causative pathogens (17.9 +/- 13.8 hours) were lower than contaminations (30.4 +/- 19.3 hours). This difference was statistically significant (p< 0.05). Mean positivity times were found as 12.3 +/- 7.4 hours for carbapenemase-producing Enterobacterales, 12.4 +/- 11.6 hours for non-carbapenemase-producing Enterobacterales (p= 0.942); 14.8 +/- 8.6 hours for MRSAs, 16.5 +/- 17.2 hours for MSSAs (p= 0.657); 11.7 +/- 8.9 hours for ESBL-producing Enterobacterales species, 13.7 +/- 11.9 hours (p= 0.443) for non-ESBL-producing species, 18.2 +/- 11.4 hours for VREs, and 14.2 +/- 11.6 hours (p= 0.155) for VSEs.Conclusion: The most commonly isolated factors in positive-signaling blood cultures are gram-positive cocci, Enterobacterales members, nonfermentative gram-negative bacilli, and yeasts. Our results showed that the positive signal duration of blood culture bottles can be an important role in distinguishing between the agents considered as contamination and pathogenic agents, but, the positivity time are not related to the resistance profile of the isolates.