Improvement of positive blood culture detection by agitation

To evaluate the advantages of agitation in reducing the detection time and increasing the recovery rate of positive blood cultures, 1,000 three-bottle sets of tryptic soy broth on adult inpatients were analyzed. Two bottles were transiently vented, one of which was agitated (250 rpm) for 7-19 hr at 35 degrees C. The other vented bottle and the anaerobic bottle were incubated stationary at 35 degrees C. Smears and subcultures were performed 7-19 hr after collection on both agitated and nonagitated vented bottles. Subcultures were done on all bottles at 72 hr and smears were performed on the anaerobic bottle. There were 137 of 1000 (13.7%) positive cultures from 90 patients. The agitated bottle detected 112 of 137 (81.8%) positive cultures, was the first or only means of detection in 57 of 137 cultures (41.6%), and was the only positive bottle in 30 of 137 (21.9%) cultures. The nonagitated vented bottle detected 89 of 137 (65.0%) of positive cultures and was the only means of detection in 13 of 137 (9.5%), but was never the first means of detection. The anaerobic bottle detected 76 of 137 (55.5%) of positive cultures, was the first or only means of detection in 11 of 137 (8.0%), and was the first means of detection in one of 137 (0.7%) cultures. When both the agitated and nonagitated bottle were positive, the agitated bottle was positive on the average 35 hr earlier. We conclude that agitation of the vented bottle in a conventional blood culture system significantly decreases the detection time of positive blood cultures and increases the number of positive blood cultures detected.