Blood Cultures 2004

Abstract & Commentary

Synopsis: A retrospective review has resulted in revision of recommendations for optimal methods of blood culturing in adults.

Source: Cockerill FR III, et al. Optimal Testing Parameters for Blood Cultures. Clin Infect Dis. 2004;38:1724-1730.

Cockerill and colleagues examined the effects of the volume of blood, the number of consecutive blood culture specimens, and the incubation time on the recovery of pathogens from 37,568 blood cultures obtained from adults. The study was performed at the Rochester, MN Mayo Clinic from 1996 to 1997, and used the automated BACTEC 9240 instrument. For each culture, 20 mL of blood was split equally into a BACTEC Plus aerobic/F resin bottle and a BACTEC Lytic/10 Anaerobic/F bottle, which were then automatically examined for a fluorescent marker every 10 minutes. The bottles contain enriched soybean-casein digest broth.

Pathogen recovery increased with increasing volume of blood cultured up to 40 mL in patients who did not have endocarditis. As the number of consecutive blood cultures over 24 hours increased to > 2, the relative yield increased. Among patients without endocarditis, the first positive blood culture, when more than 1 was obtained, came on the first specimen in 77%, on the second in 18%, the third in 3.9%, and the fourth in 1.1%. Among patients with endocarditis, with 2 or more cultures, the first positive was the first sample in 91.2%, the second in 5.9%, the third in none, and the fourth in 2.9%.

Among patients without endocarditis, all bacteremias were detected within 6 days, with 99.5% within 5 days of incubation. All bacteremias in patients with endocarditis were detected within 5 days. Of interest, 100% of Streptococcus pneumoniae were detected within 24 hours and, all enterococci were recovered within 96 hours. Two isolates of Cryptococcus neoformans, and 1 of Candida albicans, required more than 6 days for recovery.

Comment by Stan Deresinski, MD, FACP

As Cockerill et al point out, based on limited data using manual blood culture methods, it has been widely accepted that 2 to 3 consecutive blood specimens be obtained for culture over a 24-hour period, from patients in whom bacteremia is suspected. The greater sensitivity of the automated blood culture system used here, when compared to manual systems, would suggest that fewer cultures would prove necessary, but this did not prove to be the case. It may, however, be this greater sensitivity that paradoxically led to failure of this to prove true, since it may lead to a greater likelihood of detection of low level bacteremia, particularly in patient already receiving antibiotics.

These results may differ with use of other blood culture systems, as well as with other variations in methodology. For instance, some laboratories prefer to inoculate 2 aerobic bottles for cultures after the first in order to increase the recovery of organisms that grow poorly in an anaerobic environment. In addition, there may be indications for more prolonged incubation, as when organisms such as Brucella are suspected. It is of interest to note, that the identification of Bartonella quintana as the cause of bacteremia in homeless individuals was the result of blind acridine orange staining of apparently negative BACTEC blood culture bottles after 8 days of incubation.1

An aside: The use of automated blood culture systems is associated with greater rapidity of detection of bacteremia than the use of manual systems. This is only true, however, if a human being is available in the laboratory to respond appropriately. A recent study investigated blood cultures that first became positive at night when no microbiologists were present and at other times when they were present.2 When none were present, it took 7 hours and 26 minutes to report results to a physician, and only 2 hours and 12 minutes when they were there. This greater than 5 hour difference could, in some cases, prove critical.

Cockerill et al make the following recommendations when using the BACTEC 9240 system for the detection of bacteremia in adults:

  • 20 mL of blood should be obtained per venipuncture and distributed equally between an aerobic and an anaerobic blood culture bottle. This constitutes a single blood culture or blood culture set.
  • At the time of the initial order within a 24-hour period, 2 20 mL blood samples should obtained, each by separate venipuncture.
  • Two additional 20 mL samples should be obtained at separate intervals over the remaining 24-hour perio, as signs and symptoms of septicemia persist.
  • Blood culture bottles should be incubated for 5 days.

Reference

1. Spach DH, et al. Bartonella (Rochalimaea) quintana Bacteremia in Inner-City Patients with Chronic Alcoholism. N Engl J Med. 1995;332:424-428.

2. Savinelli T, et al. What Happens When an Automated Blood Culture Instrument Detects Growth But There Are No Technologists in the Microbiology Laboratory? Diagn Microbiol Infect Dis. 2004;48:173-174.

Stan Deresinski, MD, FACP Clinical Professor of Medicine, Stanford; Associate Chief of Infectious Diseases, Santa Clara Valley Medical Center, is Editor for Infectious Disease Alert.