Anaerobic bacteremia: incidence, patient characteristics, and clinical significance

In the 1970s, blood culture for obligate anaerobic bacteria became routine in most United States hospitals. Since then, various authorities have reported isolation of obligate anaerobes in 5% to 25% of blood cultures. Our experience suggests a much lower frequency; therefore, we retrospectively assessed the occurrence and significance of these cultures at our institutions.

Sixty-six patients at the University of Michigan Hospitals (UMH) and nine patients at the Ann Arbor Veteran's Administration Medical Center (AAVAMC) had one or more blood cultures positive for an obligate anaerobe between July 1, 1987, and December 31, 1988. Their medical records were reviewed retrospectively.

The proportion of positive blood cultures yielding obligate anaerobes was 3.2% at the UMH and 1.8% at the AAVAMC. The incidences of clinically significant anaerobic bacteremia at the two hospitals were 0.68 and 0.54 cases per 1,000 patient admissions. Among the 40 patients from whom significant isolates were obtained, 15 (38%) had a fatal outcome. Bacteroides and Clostridium species accounted for 90% of the isolates and all of the fatal cases. The source for anaerobic bacteremia was usually obvious; 30 of the 40 patients were given empiric antibiotic therapy for anaerobes. The gastrointestinal tract was the source in two thirds of the cases and was clearly implicated as the source of 80% of the fatal bacteremias.

The frequency of anaerobic bacteremia in our hospitals is much lower than was suggested in several large studies during the 1970s, probably reflecting a real decline in the incidence. The clinical features of our cases are similar to those of previous studies, and the mortality is still high despite the use of antibiotics effective against anaerobes. Since most patients were thought to have anaerobic infections at the time that cultures were obtained, they were usually treated empirically. Subsequent blood cultures positive for anaerobes infrequently influenced clinical management.