The yield of blood cultures in a department of emergency medicine.

This study sought to determine the yield of blood cultures drawn in the department of emergency medicine. The results of 730 blood cultures taken from 718 patients were retrospectively analysed. The total percentage of positive cultures was 9.7%. Only 3.4% of the blood cultures were classified as true bacteraemia and the rest as contaminants. The commonest type of isolate was coagulase-negative staphylococci (49%), which were considered contaminants in all cases. Other contaminants represented 13.2% of all the positive blood cultures. The following bacteria comprised the group of true bacteraemia: Escherichia coli (12.6%), Streptococcus pneumoniae (9.8%), viridans streptococci (7%), Staphylococcus aureus (2.8%), Bacteroides fragilis (2.8%), Moraxella species (1.4%) and Flavobacterium species (1.4%). Blood cultures were positive in 3.6% of patients with pneumonia and in 10% of patients with urinary tract infections. In patients with fever of unclear source blood cultures were positive in 3.1% of children between 0-36 months of age and in 1.1% of patients older than 16 years. As a whole, patients with positive blood cultures were clinically sicker, a higher percentage of them required admission to the hospital and had higher temperatures or rapidly fatal disease, compared with the group of patients with negative blood cultures. In order to improve the yield of blood cultures in febrile patients, first, better a priori identification of those subjects at high risk for bacteraemia will reduce the number of unnecessary blood cultures and second, sterile venipuncture techniques should be improved in order to reduce the number of contaminants.