Experience of changing between signal and Bactec 9240 blood culture systems in a children's hospital.

Aim: To compare experience of positive blood cultures in successive years before and after changing from Signal (Unipath) to Bactec 9240 (Becton Dickinson) blood culture systems.

Methods: Analysis of data collected prospectively on 7967 Signal and 7062 Bactec blood culture sets.

Results: Significant growths occurred in 5.7% of Signal and 8.9% of Bactec cultures; 33.0% more significant isolates and 24.0% more episodes of bacteraemia were detected in the second year, following introduction of the Bactec system. Inpatient hospital activity increased by 8.2% between the first and second years, although the numbers of blood cultures received actually fell by 11.4%. There were striking increases in numbers of isolates of coagulase negative staphylococci (47.7%) and Enterobacteriaceae (56.8%) from Bactec cultures. Two anaerobic bacteraemias were detected in Signal blood cultures, whereas none was detected by the Bactec system, despite 12.1% of sets including an anaerobic bottle. Of significant positive cultures, 90.2% were detected within one day with the Bactec 9240, compared with only 50.0% of Signal cultures; 20.7% of significant positive Signal blood cultures were detected only on terminal subculture. Microorganisms that were not significant were isolated from 5.1% Signal and 3.8% Bactec cultures.

Conclusions: Compared with the Signal system, the Bactec 9240 offers markedly more rapid and sensitive detection of bacteraemia, together with a lower rate of non-significant isolates. However, using a single PEDS PLUS/F bottle the few episodes of anaerobic bacteraemia that occur in children are likely to be missed.