Value of routine aerobic subculturing of unvented blood culture bottles

The value of performing routine aerobic subcultures of both vented and unvented blood culture bottles has not been evaluated critically. We studied 4,954 pairs of blood culture bottles consisting of one vented biphasic tryptic soy broth bottle (Roche Diagnostics) and one unvented Thiol broth bottle (Difco Laboratories). A total of 736 isolates were detected, of which 124 (17%) were in the Thiol broth bottle only. Some 15 isolates were detected only by subculturing the Thiol broth, and 13 of these isolates either were contaminants or were detected in previous positive cultures. Similar results were obtained when the unvented Thiol broth bottle was paired with a vented Difco tryptic soy broth bottle. Analysis of these pairs revealed a total of 360 isolates detected in 2,669 pairs of bottles, of which 83 isolates (23%) were in the Thiol broth bottle only. There were 11 isolates seen only in subcultures of the Thiol broth bottle, and 8 of these were probable contaminants. Thus, routine subculturing of unvented Thiol broth bottles had limited value. These results may differ with the use of other culture media or subculturing procedures. We recommend that each laboratory evaluate critically its experience with aerobic subcultures from unvented bottles.