Effect of agitation and terminal subcultures on yield and speed of detection of the Oxoid Signal blood culture system versus the BACTEC radiometric system.

In an initial evaluation, we found the Oxoid Signal blood culture system inferior to the BACTEC radiometric system for detection of some microorganisms causing septicemia (M. P. Weinstein, S. Mirrett, and L. B. Reller, J. Clin. Microbiol. 26:962-964, 1988). To determine whether modified processing of the Oxoid Signal blood culture system could improve its yield and speed of detecting positive cultures relative to the BACTEC radiometric system, we agitated all Oxoid bottles during the first 24 to 48 h of incubation and performed aerobic and anaerobic subcultures of all Oxoid bottles negative after 7 days of incubation. These modifications improved the overall performance of the Oxoid system, particularly with regard to the yield of streptococci, members of the family Enterobacteriaceae, and Haemophilus, Neisseria, and Acinetobacter spp. The speed of detecting positive cultures also was improved, especially within the first 24 h of incubation. However, the BACTEC system still detected more positive cultures (P less than 0.005), especially of obligate aerobes such as Pseudomonas aeruginosa (P less than 0.05) and yeasts (P less than 0.005). The BACTEC system also detected positive cultures earlier than the Oxoid system (e.g., at 24 h of incubation, 70.5% of BACTEC positive cultures detected versus 62.1% of Oxoid positive cultures detected). Further modifications of the Oxoid system which might include a revised medium, additional processing modifications, altered headspace atmosphere, or a complementary second broth medium should be considered, since the system is attractive in concept and is easy to use in the clinical laboratory.