A new method of blood culture, the Signal system (Oxoid), was assessed in paediatric practice. Isolation rates of pathogens, frequency of contamination (false positive results), and time taken to detect positive cultures were analysed. Four hundred and seventy nine organisms were isolated from 457 of 3000 cultures collected, of which 283 organisms were considered to be clinically important. The overall rate of positive cultures was 15.2%, and clinically important organisms were isolated from 9.1%, giving an overall contamination rate of 6.1%. The rate of contamination with Gram positive bacilli was 1.1% and coagulase negative staphylococci 4.2%. Over 51.6% of all isolates were detected within 24 hours, 81% within 48 hours, and 86% within three days: 91.1% of clinically important organisms were isolated within three days. Unimportant organisms tended to give a delayed signal, although this sometimes occurred with Candida spp, Klebsiella spp, Pseudomonas spp and a small number of other such organisms. Four hundred and forty nine of the 457 positive cultures gave a visible signal. Six of seven isolates of Haemophilus influenzae failed to give a signal, as did two coagulase negative staphylococci. The Signal system is a convenient and cost effective method of blood culture.
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