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East Tennessee State University, Johnson City, TN
Correspondence to: Ryland P. Byrd, Jr, MD, FCCP, Veterans Affairs Medical Center 111-B, PO Box 4000, Mountain Home, TN 37684-4000; e-mail: Ryland.Byrd{at}med.va.gov
To the Editor:
We read with interest the article by Shafazand and Weinacker (November 2002)1 concerning the utility and limitations of obtaining blood cultures in the intensive care setting. The article is both well-written and informative. The authors, however, failed to discuss the routine use of anaerobic blood cultures. While this topic is somewhat controversial, it deserves recognition.
As Shafazand and Weinacker indicated,1 blood cultures often are included in the assessment of the febrile hospitalized patient. It is common practice to inoculate both aerobic and anaerobic media when blood is obtained from the patient for a culture. While the frequency of aerobic blood-borne infection has increased, the incidence of anaerobic bloodstream infection has declined, even as microbacterial techniques for detecting these anaerobic bacteria have improved.2 3 4 5
Anaerobic bacteremia that is considered to be clinically significant accounts for < 1% of anaerobic blood cultures.6 7 Since the majority of patients with positive anaerobic blood culture findings have clinical conditions in which anaerobes are known to be the causative pathogens, the patients are typically already being treated for their anaerobic infection.7
The mortality associated with an anaerobic bloodstream infection is high. Lower death rates have been documented in bacteremic patients with anaerobic infections who have undergone surgical interventions.8 9 However, therapy with antibiotics alone, based on the results of anaerobic blood cultures, offers little survival advantage unless surgical intervention can be performed.6 Thus, some investigators2 6 10 have suggested that the identification of an anaerobic pathogen by blood cultures seldom alters the antimicrobial treatment or favorably influences outcome. These observations have led some researchers4 5 11 to propose that the selective performance of anaerobic blood cultures, based on the patient’s risk for anaerobic infection, may be more appropriate than the practice of obtaining routine anaerobic blood cultures.
Despite these observations, there is unwillingness in the medical community to alter the current practice of obtaining anaerobic blood cultures. The most compelling reason for this reluctance is that most clinically significant aerobic bacterial pathogens are facultative anaerobes. Continuing the practice of obtaining both aerobic and anaerobic blood cultures, therefore, in effect doubles the volume of blood cultured and may, thereby, increase the isolation of these facultative organisms. Moreover, there are some pathogenic organisms that are classified as facultative anaerobes that grow more quickly in anaerobic conditions. Thus, blood cultures for these organisms may be positive earlier or only positive in the anaerobic blood culture bottle.12
We hope that this information will compliment the review article by Shafazand and Weinacker, and stimulate further investigation into the utility of anaerobic blood cultures in the critically ill patient.
References
Stanford University, Stanford, CA The George Washington University, Washington, DC
Correspondence to: Ann Weinacker, MD, FCCP, Division of Pulmonary and Critical Care Medicine, Stanford University Medical Center, 300 Pasteur Dr, Room H3142, Stanford, CA 94305-5236; e-mail: annw{at}stanford.edu
To the Editor:
We thank Drs. Byrd and Roy for their comments on our review of blood cultures in the critical care unit,1 and we agree with them completely. We did not discuss the use of anaerobic cultures in our review, but in Table 2 we recommended routinely obtaining both aerobic and anaerobic cultures. It is true that this is a controversial area, and that some experts no longer recommend routinely obtaining anaerobic cultures. However, for all of the reasons cited by Drs. Byrd and Roy, we disagree with that practice and routinely obtain anaerobic cultures. In our institution, we typically order two sets of blood cultures to be drawn when blood cultures are indicated. This consists of 20 to 30 mL of blood drawn from each of two separate sites, and then inoculated into one anaerobic bottle and three aerobic bottles. If, for some reason, only one set can be obtained, the blood is inoculated into one aerobic and one anaerobic bottle. We believe this increases both the yield and efficiency of blood cultures. Like Drs. Byrd and Roy, we hope that this discussion will stimulate further research in this area.
References
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