A prospective survey of all positive blood cultures was performed duri
ng a 6-month period at a 390-bed, tertiary-care teaching hospital in S
t Louis, Missouri. Data were collected from the clinical microbiology
laboratory, medical records, and physicians caring for patients with p
ositive blood cultures. Of 5,732 blood cultures, 261 (4.6%) were posit
ive, and approximately half of these (51.3%) were categorized as false
positive. Positive cultures were significantly more likely to be true
positives if obtained from patients with malignancies or if they beca
me positive within the first 48 hours of incubation.