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Journal of Clinical Microbiology, September 2001, p. 3279-3281, Vol. 39, No. 9
0095-1137/01/$04.00+0   DOI: 10.1128/JCM.39.9.3279-3281.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

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

Stanley Mirrett,1,2,* Melvin P. Weinstein,3,4,5 Larry G. Reimer,6 Michael L. Wilson,1,2,dagger and L. Barth Reller1,2,7

Clinical Microbiology Laboratory, Duke University Medical Center,1 and Departments of Pathology2 and Medicine,7 Duke University School of Medicine, Durham, North Carolina 27710; Microbiology Laboratory, Robert Wood Johnson University Hospital,3 and Departments of Medicine4 and Pathology,5 University of Medicine and Dentistry of New Jersey---Robert Wood Johnson Medical School, New Brunswick, New Jersey 08901; and Department of Veterans Affairs Medical Center, Salt Lake City, Utah 841486

Received 13 April 2001/Returned for modification 9 May 2001/Accepted 13 June 2001

Coagulase-negative staphylococci (CNS) are the most commonly isolated contaminants from blood cultures, yet they frequently cause true infections. Determining the clinical significance of CNS is difficult, and clinicians often consider the number of positive bottles within a set of blood culture bottles in their assessment. Therefore, in three separate studies, we counted the number of positive bottles within blood culture sets comprising two, three, or four bottles in order to predict whether or not CNS were clinically significant isolates (CSI) in adult patients with suspected sepsis. Each culture was evaluated by independent, published clinical criteria to determine its clinical importance. Of 486 positive sets that included two adequately filled bottles, 127 (26%) CNS were CSI, 329 (67%) were contaminants, and 30 (6%) were indeterminate as a cause of sepsis. Among CSI, 39 and 61% were isolated from one and two bottles, respectively. The positive predictive value for sepsis was 18% when one bottle was positive and 37% when both bottles were positive. Of 235 positive sets that included three adequately filled bottles, 81 (34%) were CSI, 109 (46%) were contaminants, and 45 (19%) were indeterminate as a cause of sepsis. Of CSI, 43, 38, and 19% were found in one, two, and three bottles, respectively. The positive predictive value for sepsis was 28, 52, and 30% when one, two and three bottles were positive. Of 303 positive blood culture sets that included four adequately filled bottles, 64 (21%) were considered CSI, 197 (65%) were contaminants, and 42 (14%) were indeterminate as a cause of sepsis. Of CSI, 27, 28, 19, and 27% were found in one, two, three, and four bottles, respectively. The positive predictive value for sepsis was 11, 30, 34, and 37% when one, two, three, and four bottles were positive. We conclude that the number of culture bottles positive in a given culture set cannot reliably predict the clinical significance of the CNS isolated and, therefore, should not be used as a criterion for determining whether or not an isolate represents true infection or contamination.

* Corresponding author. Mailing address: Clinical Microbiology Laboratory, Duke University Medical Center, Box 2902, Durham, NC 27710. Phone: (919) 684-2562. Fax: (919) 684-8519. E-mail: stanley.mirrett{at}duke.edu.

dagger Present address: Department of Pathology and Laboratory Services, Denver Health Medical Center, Denver, CO 80204.

Journal of Clinical Microbiology, September 2001, p. 3279-3281, Vol. 39, No. 9
0095-1137/01/$04.00+0   DOI: 10.1128/JCM.39.9.3279-3281.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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