Abstract & Commentary

No juice for the squeeze: Lab labors yield few bugs

Little bang for buck in elaborate blood cultures

Synopsis: Extensive processing and prolonged incubation of blood cultures in patients with fever of unknown origin or of endocarditis were not effective in the detection of etiologic pathogens.

Source: Baron EJ, Scott JD, Tompkins LS. Prolonged incubation and extensive subculturing do not increase recovery clinically significant microorganisms from standard automated blood cultures. Clin Infect Dis 2005; 41:1,677-1,680.

Beginning in 1995, a remarkably extensive blood culture protocol was established at Stanford University Hospital for use in patients with fever of unknown origin or suspected endocarditis. This included the use of an average of almost 90 mL of blood from patients obtained by several venipunctures. Blood inoculated into BACTEC bottles was incubated for 21 days, but also was subcultured after three and 10 days onto a variety of enrichment media, including buffered charcoal yeast agar, chocolate and rabbit agars, Sabouraud dextrose agar, and Lowenstein-Jensen agar, each with prolonged incubation. Aliquots from anaerobic bottles were subcultured to hemin-supplemented Brucella blood agar and anaerobically incubated. Acridine orange stains were performed on media at days three and 10. For patients with suspected endocarditis, blood treated by lysis-centrifugation was inoculated onto the same media mentioned above and incubated for six days to six weeks, depending upon the target organism and medium.

Blood from 215 patients was managed this way over a three-year period, during which approximately 42,000 blood cultures were treated in the standard manner with five days of incubation on the BACTEC 9240 instrument. During that time, 24 HACEK organisms were recovered from the standard five-day-incubation cultures, and 98% of all positive blood cultures were recovered by the fourth day of incubation. [Editor’s note: The acronym HACEK refers to a grouping of gram-negative bacilli; Haemophilus species (H. parainfluenzae, H. aphrophilus, and H. paraphrophilus), Actinobacillus actinomycetemcomitans, Cardiobacterium hominis, Eikenella corrodens, and Kingella species.) The organisms are prime suspects for endocardial infections.]

Only 15 isolates from the 215 patients that were recovered with the special protocol would not have been detected by the standard method, and 12 of these were apparent contaminant. Of the remaining three, two were Mycobacterium avium complex (MAC) isolates from a patient with AIDS with samples obtained months apart, and one was a Legionella species.

Commentary by Stan Deresinski, MD, Clinical Professor of Medicine, Stanford (CA) University.

I don’t know how many times I’ve heard the term "HACEK" and the phrase "hold the blood cultures" mentioned in the same breath. This is true despite the fact that it has seemed obvious for some time that, with the use of modern microbiologic techniques, prolongation of incubation does not improve the yield of this group of relatively fastidious organisms. In addition to the results discussed here, this conclusion also has recently been confirmed in a report in which HACEK organisms were recovered from less than 0.005% of more than 59,000 blood cultures.1 In addition, none of 407 blood cultures with extended incubation yielded any bacteria after five days. A recommendation against prolonged incubation has been incorporated into the most recently published guidelines of the American Society for Microbiology.2

The experience of Baron and colleagues takes this finding much further by demonstrating that an amazingly complex blood culture protocol, using multiple methods and media, together with prolonged incubation, failed to provide significant benefit in a group of patients with fever of unknown origin or suspected endocarditis. The only clinically relevant organisms exclusively recovered with the extended multifaceted protocol were MAC isolates from an AIDS patient and Legionella from another patient.

Baron et al counsel against a blanket approach to the use of special methods and prolonged incubation in the diagnosis of, e.g., endocarditis. They instead provide a series of recommendations for the detection of agents of septicemia or endocarditis not recovered by routine blood cultures. These apply to a variety of suspected agents, including filamentous fungi, Legionella, Bartonella, and others. In addition, Stanford now requires a mandatory infectious disease consultation when special approaches are needed for the detection of suspected pathogens.


  1. Petti CA, et al. Utility of extended blood cultures for isolation of Haemophilus, Actinobacillus, Cardiobacterium, Eikenella, and Kingella organisms: A retrospective multicenter evaluation. J Clin Microbiol 2006; 44:257-259.
  2. Baron EJ, et al. Cumitech 1C: Blood Cultures IV. Washington, DC: American Society for Microbiology Press; 2005.