Timing of blood cultures

Medical Laboratory Observer, Dec, 2004 by David Sewell

Q I have read conflicting recommendations on the timing of blood cultures. What time interval is appropriate for obtaining multiple blood cultures?

A Over the years, a number of studies have addressed the issues of timing and volume of blood specimens needed for the optimal detection of bacteremia in patients with acute sepsis, infective endocarditis, or fever of unknown origin. Much of the discussion centered on whether the infective process produced a continuous or transient bacteremia and whether the blood cultures should be collected simultaneously (continuous bacteremia) or at some arbitrary interval (transient bacteremia).

The current consensus is that the volume of blood collected is more important than the timing of blood cultures for the laboratory diagnosis of bacteremia. (1, 2) No advantage is gained with any particular interval of collection. (3) In patients presenting with acute sepsis, the recommendation is to consecutively collect two or three blood cultures (20 mL to 30 mL/culture) from separate sites prior to antimicrobial therapy. In cases of fever of unknown origin and subacute bacterial endocarditis, obtain three blood cultures during the first one to two hours. If these cultures are negative after 24 hours, three additional cultures may be obtained. For patients on antimicrobial therapy prior to admission, obtain two separate blood cultures on each of three successive days. The volume of blood cultured is the most important factor in the recovery of bacteria, but recovery rates do not improve significantly beyond a total volume of 120 mL. (2)

References

1. Dunne WM Jr, Nolte FS, Wilson ML. Blood Cultures III. Hindler JA, coordinating ed. Washington, DC: American Society for Microbiology Press; 1997.

2. Baron EJ. Processing and Interpretation of Blood Cultures. In: Isenberg HD, ed. Essential Procedures for Clinical Microbiology. Washington, DC: American Society for Microbiology Press; 1998:58-62.

3. Li J, Plorde JJ, Carlson LG. Effects of volume and periodicity on blood cultures. J Clin Microbiol. 1994;32:2829-2831.

--David Sewell, PhD, ABMM

Director of Microbiology

Veterans Affairs Medical Center

Portland, OR