Who Needs a Blood Culture in the ED?

Although blood cultures often are obtained for patients in the emergency department, little evidence is available to guide patient selection for such testing. Currently, general indications for testing are poorly defined, and only 4% to 8% of blood cultures yield growth. Such low yield suggests that opportunity exists to save both ED and laboratory resources and considerable costs.

In a prospective, observational cohort study of adult patients who had blood cultures obtained at a single academic ED (annual census, 50,000) during 1 year, researchers developed a decision rule to assess risk for bacteremia. Of 3730 patients, 8.2% had true bacteremia. Two thirds of the 3730 patients were randomly selected as the derivation group and the remaining one third as the validation group. In multiple logistic regression analysis, the researchers identified 13 independent predictors of bacteremia and categorized them as major and minor criteria, based on the strength of the association:

Major criteria

• Temperature >39.5°C

• Indwelling vascular catheter

• Clinical suspicion of endocarditis

Minor criteria

• Temperature 38.3°C–39.4°C

• Age >65

• Chills

• Vomiting

• Hypotension (systolic blood pressure <90 mm Hg)

• Neutrophil percent >80%

• White blood cell count >18,000 cells/mm³

• Bands >5%

• Platelets <150,000 cells/mm³

• Creatinine >2.0 mg/dL

According to the decision rule, blood cultures are indicated if at least one major criterion or two minor criteria are present. Fewer than 1% of patients who did not meet any of the criteria had positive blood cultures. In the validation group, the rule had a sensitivity of 97%, a specificity of 29%, a positive predictive value of 11%, and a negative predictive value of 99%. The authors estimate that use of the rule could reduce use of blood cultures by 27%.