RATIONALE FOR SELECTIVE USE OF ANAEROBIC BLOOD CULTURES

Because of the declining frequency of anaerobic bacteremia, routinely using half the collected blood volume for anaerobic culture has been c hallenged. There is no data indicating whether more clinically relevan t isolates would be recovered if all or most of the given blood sample were cultured aerobically. In this two-part study, we reviewed cases of anaerobic bacteremia to determine what proportion occurred in situa tions when anaerobes would be expected and then estimated the yield of different culture approaches by reanalyzing the data from a large pro spective clinical blood culture study. The records of 61 patients who had an anaerobic isolate (excluding Propionibacterium species) recover ed only from an anaerobic bottle were examined to define clinical sett ings in which such isolates occur. Fifty-six (92%) patients had clinic ally important isolates, and the source of infection was obvious at th e time of culture in 47 of the 56 (84%). Of 56 patients, 36 (64%) had abdominal signs and symptoms, including 12 with recent abdominal surge ry. Of nine patients without an obvious source of infection, six were on high-dose steroids. Relative yields were compared for (i) one aerob ic bottle and one anaerobic bottle (5 ml to each) for all blood cultur es, (ii) two aerobic bottles (5 ml to each), or (iii) two aerobic bott les plus an extra anaerobic bottle (only for clinically suspected anae robic sepsis) (5 ml to each). The third approach had the highest yield (475 isolates), because the routine use of two aerobic bottles recove red more Candida spp., members of the family Enterobacteriaceae, and n onfermenters than did the first approach (448 isolates) (P < 0.02), an d clinically directed culturing for anaerobes would recover anaerobes missed with the second approach (458 isolates). Our data suggest that the use of two aerobic bottles with selective culturing for anaerobes could increase the number of clinically relevant isolates by at least 6% compared with the current practice of inoculating an aerobic bottle and an anaerobic bottle with equal volumes of blood.