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

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Abstract
It was recommended that [human] blood cultures be routinely subcultured aerobically on the day after the specimen is received, anaerobically after 48 h, and, finally, after 5-7 days if the cultures appear negative. To evaluate the necessity for the final routine subculture, 2780 previously negative blood culture bottles were subcultured after 7 days of incubation. Of 4 bottles positive by subculture, 3 yielded the same organism as previously isolated from the companion bottle, and 1 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 7 days of incubation of blood cultures is probably adequate for general hospitals, a 2nd wk of incubation appears indicated in selected cases of suspected endocarditis and persistent or recurrent infection, and in any referral center. Candida and fastidious gram-negative bacilli, such as Haemophilus, Cardiobacterium and Actinobacillus, usually require extended incubation for their detection.