Clinical Utility of the Time-to-Positivity/ Procalcitonin Ratio to Predict Bloodstream Infection Due to Coagulase-Negative Staphylococci

Science Bacteremia is one of the most common causes of mortality in hospitalized patients. Blood culture is currently the recommended method for diagnosing bacteremia. However, a positive result via blood culture does not always mean that bacteremia is present. For example, common skin flora, particularly coagulase-negative staphylococci (CNS), may contaminate blood cultures. It is important to determine whether a positive blood culture reflects a severe threat of sepsis, or occurs due to contaminant of the specimen by normal skin flora. 1 Incorrect interpretation of CNS-positive blood cultures results in unnecessary administration of antibiotics, increases healthcare costs, prolongs hospitalization, and promotes antibiotic resistance in microorganisms. 2 Certain factors help health care professionals determine whether CNS detected via blood culture represents true bacteremia or merely reveals contamination in the specimen. 1 For example, the time to positivity (TTP), which is mainly influenced by the species of bacteria and the original bacterial load, has been used in the interpretation of blood cultures that test positive for CNS. ABSTRACT Objective: To develop a novel clinical diagnostic method to distinguish bacteremia from blood contamination due to coagulase-negative staphylococci (CNS) by using the ratio of the time to positivity (TTP) of a blood culture with positive results to its serum procalcitonin (PCT) level. Methods: We retrospectively reviewed the clinical and microbiological records of 102 patients who had 1 or more blood cultures that tested positive for CNS from August 2007 through February 2012. Results: Receiver operating characteristic (ROC) curve analysis indicated that, at a cutoff of 1.24 ng per mL, 21.5 hours, and 22.5 (hours × mL)/ng,the TTP to PCT ratio, PCT level, and TTP yielded the most favorable discrimination for bacteremia caused by CNS, with sensitivities of 85.7%, 78.6%, and 61.9%,respectively, and specificities of 80.0%, 78.3%, and 61.7%,respectively. The TTP to PCT ratio was the most accurate of the studied variables in predicting CNS-triggered bacteremia levels. Conclusion: The TTP to PCT ratio is a useful predictor to differentiate the culture samples that test positive, revealing the presence of CNS bacteremia, from those that are merely contaminated