Positive predictive value of blood cultures growing coagulase-negative staphylococci

Relevance of the Number of Positive Bottles in Determining Clinical Significance of Coagulase-Negative Staphylococci in Blood Cultures

“…Our data showed that all four bottles were positive in only 37% of sets judged to represent sepsis owing to CNS. Similar results were obtained in a smaller study of 147 blood culture sets reported by Peacock et al (6), in which only 27% of the septic patients had four bottles positive with CNS. However, 10% of our four-bottle sets judged to be contaminants also had all four bottles positive, and 43% of all sets with four positive bottles were contaminants.…”

“…Moreover, other investigators have reported similar observations (6). Thus, we believe that our findings are not unique to one medium or system but rather constitute a fundamental truth regarding interpretation of positive blood cultures.…”

“…Thus, the presence of many CFU per milliliter increases the likelihood that microorganisms will be isolated from more than one blood culture bottle. Conversely, the presence of fewer CFU per milliliter increases the likelihood of contamination being the cause of the positive blood culture (6). Because of this assumption, when reporting positive blood cultures to the clinician, the clinical microbiologist frequently is asked how many bottles are positive.…”

“…Contaminated blood cultures result in increased lengths of stay and higher pharmacy, laboratory, and total costs (1). Determining the clinical significance of CNS is difficult, and some clinicians and microbiologists consider the number of positive bottles within a blood culture set in their assessment (4,(6)(7)(8). Our laboratories have accumulated data on CNS while participating in several multicenter, controlled clinical evaluations of blood culture media and systems (9,10,13).…”

Updated Review of Blood Culture Contamination

“…Those authors concluded that the number of positive bottles in a given set did not reliably predict clinically significant infection. Another study found that increasing numbers of positive bottles did predict the likelihood of true bacteremia as defined by clinical parameters: of 129 patients with coagulase-negative staphylococci isolated from a single venipuncture using sets comprised of four bottles, the positive predictive value for true infection was 2% when one bottle was positive, 9% when two bottles were positive, 13% when three bottles were positive, and 27% when all four bottles were positive (94).…”