Evaluation of routine subcultures of macroscopically negative blood cultures for detection of anaerobes.

Routine anaerobic subcultures of macroscopically negative blood culture bottles, performed within 1 day of receipt of the culture and after 5 days of incubation, were evaluated. Anaerobes were recovered from 207 (12.3%) of the total 1,688 positive cultures and, of these, 154 were only detected macroscopically, 11 only by subculture, and 42 by both procedures. In no instance was the anaerobe detected earlier with the subculture, and the time required for a definitive identification was reduced for only 10 isolates. Since the subcultures did not significantly improve the detection or early identification of anaerobes, routine anaerobic subcultures are not recommended.