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University of Kentucky Medical Center, Lexington, KY
Correspondence to: Rolando Berger, MD, FCCP, Division of Pulmonary & Critical Care Medicine, University of Kentucky Medical Center, 800 Rose St, MN614, Lexington, KY 40536-0298
To the Editor:
I have read with great interest the excellent review by Shafazand and Weinacker on the use of blood cultures taken from patients in the critical care unit (November 2002).1 Indeed, problems with false-positive and even false-negative results have long been a bane for all of us who work in the ICU environment. While agreeing with almost everything that the authors said, I do want to suggest two modifications to their recommendations, listed on Table 2 (page 1729).
First, published data2 3 4 strongly suggest that the diagnostic yield of routine anaerobic blood cultures is virtually nil (< 1%); in fact, plating a second aerobic bottle may be a better diagnostic option.3 In two large series (almost 30,000 blood cultures analyzed), all patients with positive anaerobic culture results had very strong pretest probability (ie, clinical suspicion) that such an etiology of bacteremia was likely.2 4 Thus, it would appear that anaerobic bottles do not need to be routinely included in ICU blood culture sets; rather, they should be used only in cases where an anaerobic culture is deemed to be clinically indicated.
Second, arguably the major problem with blood cultures is the relatively high proportion of false-positive results. The authors discussed this issue quite nicely in their article. In our ICU, to better deal with this pervasive problem, we have opted to define a "blood culture set" not as two blood culture bottles from one venipuncture site (as proposed by Shafazand and Weinacker1 ), but as two anatomically separate venipuncture sites obtained at one point in time, with one or two blood culture bottles being filled from each site. In other words, in our ICU, a blood culture set refers to a point in time when blood cultures were obtained, and each set includes a minimum of two separate venipuncture sites. For example, if a patient is to have three blood culture sets drawn, this means that he or she will be sampled at three different points in time over a 24-h period, each time having blood drawn from two separate venipuncture sites (a total of six venipunctures for all three sets). Because anaerobic bottles are not routinely needed, for most patients the number of blood culture bottles to be processed would remain the same, but by this proposed definition each "set" would have its own built-in quality control: ie, a true-positive blood culture result requires that the culture from both sites sampled at the same point in time be positive for the same organism(s).
Finally, I wish to reemphasize the authors’ recommendation that blood cultures from IV catheters should never be used by themselves to diagnose bacteremia—for this purpose, catheter cultures must be coupled with a set of standard peripheral blood cultures.
References
Stanford University, Stanford, CA The George Washington University, Washington, DC
Correspondence to: Ann Weinacker, MD, FCCP, Pulmonary and Critical Care Medicine, Stanford University, 300 Pasteur Dr, Room H3142, Stanford CA, 94305-5236; e-mail: annw{at}stanford.edu
To the Editor:
We appreciate Dr. Berger’s comments regarding our review (November 2002)1 of blood cultures in the critical care unit. We agree with him that the incidence of anaerobic bacteremia has decreased in the past few decades. Nonetheless, some studies2 3 4 5 have demonstrated that the number of positive anaerobic cultures is not trivial and that the mortality rate associated with anaerobic bacteremia remains high. In a study of 281,797 blood cultures at the Mayo Clinic performed between 1984 and 1992, 920 of the organisms (3%) recovered were obligate anaerobes.2 Thus, many experts continue to recommend the routine use of anaerobic blood cultures.3 6 7 In fact, one such recommendation was made in the editorial7 accompanying the study by Ortiz and Sande,8 which was cited by Dr. Berger.
Although some experts agree with Dr. Berger’s suggestion that anaerobic cultures be used only when the clinical suspicion for anaerobic infection is high, this approach would obviously require consistent communication between clinicians and the microbiology laboratory. At this time, the most cost-effective approach to the diagnosis of anaerobic bacteremia is unclear. In our institution and others, the cost of an anaerobic culture is the same as the cost of an aerobic culture, and many institutions continue to routinely perform anaerobic cultures. At the Mayo Clinic, a blood culture set consists of blood inoculated into two aerobic bottles and one anaerobic bottle (Franklin R. Cockerill III, MD; personal communication; February 27, 2003). In our own institution, when blood cultures are indicated, we typically order two sets simultaneously (ie, blood is drawn from two separate sites), and our laboratory inoculates blood from the first set into both an aerobic bottle and an anaerobic bottle. The second set is inoculated into two aerobic bottles. Both of these approaches increase the yield of blood cultures by culturing a large volume of blood in multiple bottles, without taking the risk of missing an anaerobic infection. Both approaches differ slightly from the recommendation we made in Table 2,1 but in principle they support our recommendation for the routine anaerobic culturing of blood. An added benefit of routinely performing anaerobic blood cultures is that some facultative aerobes grow faster in anaerobic media than in aerobic media.
Regarding Dr. Berger’s second point, we completely agree that to minimize the number of false-positive cultures, blood should be drawn from more than one puncture site. The optimal timing of these separate vascular punctures is unclear, although drawing blood from two or three sites within a 24-h period appears to be adequate to detect bacteremia in most patients. Although we have chosen to define blood culture sets differently from Dr. Berger, in principle it seems we are in complete agreement.
References
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