The clinical significance of coagulase-negative staphylococci-positive blood cultures

Abstract

Study objectives: Coagulase-negative staphylococci (CNS), the most frequent organisms isolated from blood cultures, are an important cause of bloodstream infections and the most common contaminant of emergency department (ED) blood cultures. Although blood culture contamination leads to additional laboratory tests, unnecessary antibiotic use, and longer hospitalizations, failure to recognize and treat a true bacteremia can lead to increased morbidity and mortality. We determine the incidence of clinically significant CNS-positive blood cultures and determine the optimal algorithm for categorizing CNS-positive cultures as either contaminant or clinically significant. Methods: A single experienced reviewer examined the medical records of 960 consecutive patients with positive blood culture results. According to multiple clinical parameters at the index positive blood culture result, the organism identification, results of other cultures, pathology findings, imaging results, and clinical course, each positive culture result was classified as being clinically significant or not. All indeterminate cases were reviewed with a physician specializing in infectious diseases before classification (κ coefficient of 0.947). Results: Of the 960 positive culture results, the organism eventually identified was CNS in 405. According to our medical record review, 89 (22%) were considered to be significant, whereas 316 were contaminant episodes. The algorithm with the optimum sensitivity and specificity was defined as at least 2 blood cultures positive for CNS or 1 blood culture positive plus clinical evidence of infection. In the 108 patients with community-acquired CNS-positive culture results, the optimum algorithm had a positive predictive value of 76% and a negative predictive value of 90%. Conclusion: A substantial portion of CNS-positive blood cultures are clinically significant. Obtaining multiple cultures during the initial ED evaluation will assist in determining the significance of a CNS-positive result.