Blood culture contamination in a district general hospital in the UK: a 1-year study

Blood culture remains a gold standard to identify patients with bacteraemia. High contamination rates in blood culture are associated with prolonged hospital stay, inappropriate antimicrobial therapy and extra work in the microbiology laboratory. The blood culture contamination rate and the distribution of organisms from patients tested for bloodstream infections were determined for Southend University Hospital, UK. The study was conducted retrospectively over a 1-year period from August 2006 to July 2007 by reviewing patient clinical data collected at the time of positive blood culture and the microbiology records from Southend University Hospital. During the study period, 7203 blood cultures were processed in the microbiology laboratory. Of the total, 1254 (17.4%) were positive. A total of 535 were shown to be contaminated, giving an overall contamination rate of 7.4%. A total of 703 (9.8%) blood cultures were determined as genuinely positive. The highest numbers of false-positive blood cultures were recorded from the Accident and Emergency, Medical and Paediatric units. A total of 1380 organisms including bacteria and fungi were isolated. Coagulase-negative Staphylococci (547 isolates, 39.3%) were the most commonly isolated organisms, followed by Escherichia coli (132 isolates, 9.6%), Staphylococcus aureus (130 isolates, 9.4%), and Streptococcus species (85 isolates, 6.2%). The prevalence of methicillin-resistant Staphylococcus aureus was low, whereas the extended-spectrum β-lactamase producing E. coli and Enterobacter species was high and Klebsiella species was low. The study indicates a high contamination rate of blood cultures and supports the need for regular training and education of healthcare professionals who collect blood samples.