Routine use of anaerobic blood cultures: are they still indicated?

To determine the number of patients with bacteremia and fungemia and to evaluate the utility of routine anaerobic blood cultures as part of the work-up for suspected bacteremia.

Retrospective review of microbiology data followed by selective chart review at a university-affiliated Veterans Affairs Medical Center. We determined the number of bacterial blood cultures drawn from January 1, 1994, to December 31, 1996, and the number of anaerobic, aerobic, and fungal isolates. Chart reviews were then performed on all patients with a positive anaerobic result.

There were 6,891 sets of blood cultures processed through the laboratory, yielding 1,626 patients with positive results. Anaerobic isolates were recovered from 36 patients (2.2%) in 48 bottles. Aerobic isolates were recovered from 1550 patients (95.3%), and fungal isolates were recovered from 40 patients (2.5%). Seven patients (0.4%) had true anaerobic bacteremia. All seven patients had an obvious source of anaerobic infection that was known or suspected before the cultures were drawn. Antibiotic changes were made in four of these patients after the positive anaerobic results were known. Antibiotic changes led to clinical improvement in one patient.

Routine use of anaerobic blood cultures rarely results in clinically important diagnostic or therapeutic benefits, based on the low incidence of anaerobic bacteremia in patients who are not at increased risk. Anaerobic blood cultures should be selectively ordered in patients at risk for anaerobic infections.