College of American Pathologists
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CAP Today




June 2012

Contaminated blood cultures Contaminated blood cultures

I wish to commend Anne Paxton for an excellent and comprehensive discussion on how to reduce the rate of contaminated blood cultures (April 2012). I would like to add additional perspective and suggestions.

One should be cautious about how to define a contaminated blood culture if multiple bottles grow the same skin flora bacteria. This designation should be made after an examination of the patient’s chart by a pathologist, microbiology lab director, infection control practitioner, or other qualified person to determine if there are risk factors for skin flora bacteremia. For example, if multiple blood cultures grow Streptoccoccus viridans, then a clinical presentation of fever of unknown origin, an elevated white blood cell count, and response to antibiotics but without an obvious focus of infection might suggest endocarditis. Multiple blood culture bottles positive for Staph epidermidis might result if the patient has a contaminated hemodialysis port or other intravascular device. If skin flora bacteremia risk factors are present, the attending physician should be so informed.

Staph lugdunensis is uncommon but well described in the literature as a cause of endocarditis. In a low-level bacteremia due to this offender, only a single blood culture bottle might be positive for this coagulase-negative staphylococcus. Because these are among the very few coagulase-negative staph that are positive by PYR disk, an easy and inexpensive test to perform, this should be done on all coagulase-negative staph blood culture isolates, regardless of how many bottles are positive. The vast majority are PYR disk negative and require no further speciation. If PYR positive, a Remel four-hour Staph Ident or a tube biochemical test can be done to determine if the isolate is a “Staph lugnuts.”

The need for effective and continuous phlebotomist education on proper blood culture techniques cannot be overemphasized. I would suggest that the lab director ask the most respected and articulate member of the medical staff to prerecord an audiocassette talk on the importance of blood cultures and the management dilemmas that occur if a blood culture is falsely positive. If this talk is played at the beginning of phlebotomist training sessions, trainees will pay closer attention to the instructions on proper collection.

To reduce the rate of contaminated blood cultures from the ER, or other locations where blood cultures are drawn by nonlaboratory personnel, in addition to the recommendations in Paxton’s article, it is important to package blood culture bottles and sterile venipuncture supplies and instructions in the same zipper storage bag to increase the chances they will be used. If these are in separate bins, someone might forget to restock a bin and/or the sterile venipuncture supplies might not be used in a panic situation in which the patient is, or other ER patients are, in crisis.

Dennis L. Wegner, PhD
Clinical Microbiology Consultant
Ottumwa, Iowa

In the April 2012 article on contaminated blood cultures is the following statement: “Let’s say there is one set that is positive and it’s one species known to be a contaminant like Staphylococcus aureus.” Although the article does make a good point concerning the untoward results of reporting a contaminated blood culture, I question the use of S. aureus as an example of a contaminating organism. In the 2003 article “Blood culture contamination: persisting problems and partial progress” (J Clin Microbiol. 41:2275–2278), Weinstein says “Microorganisms that always or nearly always (>90%) represent true bacteremia or fungemia include Staphylococcus aureus, Streptococcus pneumoniae, Escherichia coli, and other members of the Enterobacteriaceae, Pseudomonas aeruginosa, and Candida albicans.” Although there are rare occurrences when S. aureus could be picked up as a blood culture contaminant, most clinicians likely would not consider it to be.

Harold J. Cannon Jr., RM(NRCM), SM(NRCM), M(ASCP)CM, SM(ASCP) CM
Manager, Clinical Microbiology and Immunoserology Laboratories
Department of Laboratory Medicine Nationwide Children’s Hospital
Columbus, Ohio