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Cumitech Blood Cultures
By J. Peter Donnelly, PhD, Clinical Microbiologist, University Hospital, Nijmegen, The Netherlands, Section Editor Microbiology, is Associate Editor for Infectious Disease Alert.
Dr. Donnelly is a consultant for Ortho Biotech, and does research for Janssen, Merck, Novartis, Numico, Pharmacia, and Pfizer.
Source: Baron EJ, et al. Cumitech 1C, Blood Cultures. Coordinating ed., E.J. Baron. ASM Press, 2005.Washington, D.C.
The American Society for Microbiology Periodically publishes updates, called Cumitechs, to their recommendations regarding specific areas of clinical microbiology. The following is a brief summary, written primarily from a clinician’s point of view, of their recent monograph on blood cultures. The recommendations, with the exception of the recommended volume of blood draws, apply to both children and adults.
The taking of blood cultures is standard practice in the diagnosis of infection in patients ill enough to be admitted to hospital for suspected sepsis. As such, they are usually the most productive of samples, as they allow detection of a variety of microorganisms. This Cumitech is written in such a way as to help the clinician and the laboratory get the most of such cultures.
Definition of a Blood Culture
It may seem incredible but there are several different notions of what constitutes a blood culture. The definition used in this monograph helps clarify this. A blood culture is defined as a volume of blood from a single phlebotomy obtained under aseptic conditions that is inoculated into one or more bottles or vials usually containing broth culture medium. In effect, this means that even if a sample of blood is distributed into several sets of blood cultures, they should be none-the-less considered as a single blood culture. Why is this important? Simply that many laboratories try to assess the significance of a positive blood culture by the number of sets or bottles that yield growth. This is not only fallacious but is also misleading. This definition holds true even when a set contains an aerobic culture and an anaerobic culture as is usually the case.
Sensitivity and Specificity of Blood Cultures
The most important factor that determines the sensitivity of a blood culture is the volume of blood obtained. Hence, the recommended volume of blood for investigating adults is 20-30 mL divided into 2 bottles, typically an anaerobic and an aerobic bottle. However, 2 aerobic bottles may be beneficial in cases in which bacteremia due to anaerobes is unlikely. The clinical status of patients should be the primary guide to the timing of blood cultures—there is no evidence that particular (or any) intervals between sampling is beneficial, and there should be a second, third, or fourth blood culture obtained right after one another if circumstances dictate with 2-4 blood cultures being necessary for optimal detection of bacteremia and fungemia. A laboratory policy mandating a second blood culture (if only one has been ordered) as a reflexive test is warranted to ensure compliance. The table on the next page shows that this generally amounts to 4% or less of the total blood volume.
Acquisition and Initial Handling of Routine Blood Cultures
For skin antisepsis, chlorhexidine products and tincture of iodine appear to be equivalent, and both are superior to povidone-iodine. Ideally, blood should be drawn by percutaneous venipuncture. As this is not always possible, blood can be obtained for culture from vascular access devices, but should always be paired with another sample obtained by venipuncture. After blood is drawn, it should be put into sufficient liquid medium to achieve a 5- to 10-fold dilution to neutralize naturally occurring inhibitory substances (eg, phagocytes, as well as antimicrobial agents). Despite many investigations, there is no commercially available system or culture medium that has been shown to be best suited for the detection of all potential blood pathogens. However, there are supplements which have been shown to improve yield. For instance, the use of antibiotic-binding resins or activated charcoal may improve the yield of microorganisms in patients receiving antibiotics, but at the price of an increased rate of contamination.
In the laboratory:
Detection of Rare and/or Fastidious Organisms
Some organisms (eg, Cryptococcus neoformans, Legionella spp., some Helicobacter spp., moulds, and dimorphic fungi) may not be sufficiently metabolically active to trigger detection in a growth detection system or may not produce visible growth. If cultures are negative, but bacteremia or fungemia is still suspected, alternative methods should be considered to improve the chance of recovery of mycobacteria, fungi, and rare or fastidious microorganisms:
Detection of Bacteremia Related to Intravascular Devices
If the clinical setting dictates catheter removal (eg, shock, local purulence), the catheter should be removed and sent for semiquantitative or quantitative culture together with 2 blood samples should be obtained by separate venipuncture for aerobic culture only. However, there are many circumstances where immediate removal of the catheter is not warranted. In this case, an attempt should be made to determine whether the catheter is the source of the bacteremia in one of 2 ways:
When blood is only available from the central venous catheter, CRI infection can be diagnosed if the culture yields > 100 CFU bacteria/mL or > 25 CFU yeasts/mL.
Laboratory Quality Improvement Measures of Interest
The following should be monitored to indicate the quality of blood cultures:
This summary highlights the aspects of blood cultures that we feel most important for getting the best results out of this important diagnostic tool. The booklet can be ordered online from www.asm.org but it is a pity that there is, as yet, no electronic version available for ready access to the widest possible community. Given the drive towards cost-effective diagnostics, we recommend that every laboratory and infectious diseases department at least have a copy and encourage their staff to read and digest its contents.