Reducing Contamination of Blood Cultures: Consider Costs and Clinical Benefits

(See the Major Article by Rupp et al on pages 201–5.) Contaminated blood cultures cause expense and can have adverse effects on patients, including collection of additional laboratory tests, administration of unneeded antibiotics, and hospital admission [1]. An important source of blood culture contaminants is the patient’s skin, as shown by the critical role of careful skin antisepsis before phlebotomy [2]. Unfortunately, antiseptic cleaning reduces the number of bacteria on skin but does not sterilize it. Even when skin has been cleaned with iodine in 70% ethanol, rigorous sampling methods, such as biopsy or repeatedly pressing adhesive tape to the skin, demonstrate that approximately 20% of bacteria remain associated with the skin [3]. These remaining bacteria are found both on the surface of the skin and deep within skin structures such as hair follicles [3]. Based on the idea that these contaminating bacteria are likely to be present in the first blood collected, use of blood collection devices that divert the initial blood collected to be discarded is common practice in collection of blood for preparation of blood products for transfusion [4, 5]. In this issue of Clinical Infectious Diseases, Rupp and coauthors present an investigation into whether blood culture contamination is reduced by the use of a device that diverts the first 1.5–2.0 mL of blood to be discarded prior to collection of blood for culture. They compared the contamination rate of paired blood culture bottles collected by phlebotomists in the emergency department of a single academic medical center. The first blood sample was collected for culture without a diversion device, whereas the second sample, usually from the opposite arm, was collected with the use of a diversion device. The use of the diversion device reduced the rate of blood culture contamination from 1.78% to 0.22%, without reducing the sensitivity for detection of true bacteremia and candidemia. Perhaps the most striking result is that 97% of positive cultures represented true positives with the diversion device, compared to 81% without the diversion device. The authors also performed a post hoc analysis of all blood cultures collected in the emergency department 6 months before the study, during the 1-year study, and 6 months after the study based on whether a phlebotomist or nurse collected the sample. Phlebotomists participated in the study, while nurses did not. There was a modest reduction in the contamination rate in samples collected by phlebotomists during the study, but not in samples collected by nurses. Together, these data are consistent with earlier reports showing that use of a diversion device reduces the rate of blood culture contamination [6, 7]. Can we conclude that the use of a diversion device for collection of blood culture specimens will result in significant clinical benefits or cost savings? Clinical benefits and cost were not analyzed in the study by Rupp et al. Administration of unneeded antibiotics, collection of unneeded laboratory tests, and even unneeded hospital admission have been associated with contaminated blood cultures, each of which can result in patient harm [1]. Contaminated blood cultures have also been associated with increased patient charges of $7502 and $8720 in separate studies [8, 9]. These increased costs were primarily due to increased duration of hospital admission, although costs for antibiotic treatment and diagnostic tests were also increased [8]. Unfortunately, these studies do not include information about the time taken to identify contaminants from blood culture, or about the methods used for identification. The time needed to identify an organism growing from a blood culture as a contaminant is likely to be important in determining the effects of that culture result. If contaminants are identified rapidly, the adverse clinical effects and increased cost of blood culture contamination might not be as significant as in the past. Recent improvements in microbiology testing have reduced the time needed to identify organisms, including contaminants, from blood culture. Available methods include matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectrometry or US Food and Drug Administration–approved multiplex nucleic acid amplification test (NAAT) panels for identification of bacteria and yeast. MALDI-TOF can be performed using organisms concentrated directly from blood culture broths, or using the growth from solid media after an incubation of only 3–5 hours [10, 11]. MALDI-TOF will identify a broad range of organisms, including common contaminants such as coagulase-negative staphylococci, viridans group streptococci, Bacillus species, Corynebacterium species, and Propionibacterium species [12]. NAAT assays can be performed directly on blood culture broth with microbial growth and take only a few hours to be completed, so a laboratory that is staffed to run the assays quickly will report identifications 1–2 days earlier by using multiplex polymerase chain reaction (PCR) [13, 14]. One of these assays was used in the study by Rupp and colleagues. The multiplex PCR assays for identification of organisms in positive blood culture bottles provide identification of coagulase-negative staphylococci or specific identification of Staphylococcus epidermidis, but not of other common contaminants. Coagulase-negative staphylococci make up 70%–80% of blood culture contaminants [2]. We are beginning to understand the clinical and cost benefits of rapid identification of blood culture contaminants and our consideration of taking new steps to avoid contamination should include these data. The use of a multiplex PCR panel that included identification of coagulase-negative staphylococci from blood cultures was shown to be associated with reduced administration of antibiotics to patients who had contaminated blood cultures, whether enhanced antimicrobial stewardship was offered or not [13]. A second study in which the same multiplex PCR panel was used showed that rapid identification of coagulase-negative staphylococci was associated with a total cost reduction of $30000 per 100 patients tested [15]. The study by Rupp et al is valuable, as it provides us with a better understanding of an effective tool for reducing blood culture contamination. But there is a welcome trend toward studying clinical and cost outcomes that result from use of newer diagnostic tests for infectious diseases, perhaps because the cost of these tests is high compared to more conventional methods [16]. The complete process of a laboratory test includes collection of the specimen, and our approach to specimen collection should, to the extent possible, be made in light of what we know or can expect in terms of cost and clinical outcomes. Potential conflicts of interest. Author certifies no potential conflicts of interest. The author has submitted the ICMJE Form for Disclosure of Potential Conflicts of Interest. Conflicts that the editors consider relevant to the content of the manuscript have been disclosed.