Evaluation of the necessity for routine terminal subcultures of previously negative blood cultures.

It has been recommended that blood cultures be routinely subcultured aerobically on the day after the specimen is received, anaerobically after 48 h, and, finally, after 5 to 7 days if the cultures appear negative (Bartlett et al., Cumitech 1, American Society for Microbiology, Washington, D.C., 1974). To evaluate the necessity for the final routine subculture, 2,780 previously negative blood culture bottles were subcultured after 7 days of incubation. Of four bottles positive by subculture, three yielded the same organism as previously isolated from the companion bottle, and one yielded an organism considered to represent a contaminant. Since the routine 7-day subculture did not significantly increase the yield from previously negative blood cultures, the time and expense of the terminal subculture appears not to be warranted. Whereas a total of 7 days of incubation of blood cultures is probably adequate for general hospitals, a second week of incubation appears indicated in selected cases of suspected endocarditis and persistent or recurrent infection, as well as in any referral center. Candida and fastidious gram-negative bacilli, such as Haemophilus, Cardiobacterium, and Actinobacillus, usually require extended incubation for their detection.