Assessment of the yield of anaerobic blood cultures

Because of the declining incidence of anaerobic bacteremia, the predictable sites of anaerobic infection and the increasing importance of aerobic isolates (eg; yeasts), the practice of routinely culturing half the volume of blood collected anaerobically has been questioned. We have assessed the yield of routine anaerobic blood cultures in our clinical setting. Blood culture isolates from November 1994 through October 1995 at Auckland (AKH) and Green Lane/National Women's Hospitals (GL/NWH) were recorded. The medical records of patients with anaerobic bacteremia were examined. For the three month period April to June 1996, all positive blood cultures were analysed with respect to which bottle (aerobic or anaerobic or both) was positive. For the period November 1994 to October 1995, 5.6% and 5.3% of blood cultures at AKH and GLH respectively were positive. At AKH and GLH anaerobes constituted 0.16% and 0.19% of all blood cultures and 3.1% and 3.5% of all positive blood cultures respectively. Twenty-one of 25 (84%) significant anaerobes were from patients in whom anaerobic infection was predictable. More isolates were recovered from aerobic than anaerobic bottles, 178 versus 71, p < 0.001. Aerobic culture also recovered more pathogens (76 versus 38, p < 0.001 more yeasts (10 versus 0) and more Pseudomonas spp. (10 versus 1) than did anaerobic culture. Only obligate anaerobes were isolated more frequently in anaerobic bottles (5 versus 0, p = 0.03). Most instances of anaerobic bacteremia occurred in patients where anaerobes could be expected. We conclude that routine use of two aerobic bottles with clinically directed use of anaerobic blood culture bottle is an appropriate and effective approach in our setting.