Blood cultures in ambulatory patients who are discharged from emergency with community-acquired pneumonia.

OBJECTIVES: To determine the factors that predict whether or not ambulatory patients with community-acquired pneumonia (CAP) treated in an emergency room (ER) setting will have blood cultures drawn and the factors that predict a positive blood culture.

METHODS: Prospective observational study of all patients with a diagnosis of CAP, as made by an ER physician, who presented to any of seven Edmonton-area ERs over a two-year period.

RESULTS: Seven hundred ninety-three (19.2%) of 4124 patients with CAP had blood cultures drawn. The site-specific blood culture rates ranged from 7.8% to 25% (P<0.001); 41 of 793 (5.1%) were positive. Streptococcus pneumoniae accounted for 58.5% of the isolates while Staphylococcus aureus and Escherichia coli each accounted for 14.6%, or six patients each. Only two of the 24 patients with S pneumoniae bacteremia were subsequently admitted to hospital while all six of the patients with S aureus were admitted. Only one of the six patients with E coli bacteremia was treated at home. No factors were predictive of positive blood cultures on multivariate analysis.

CONCLUSIONS: Physicians are selective in ordering blood cultures on patients with ambulatory pneumonia who present to an ER, and the positivity rate of 5.1% is quite high. No factors are predictive of positive blood cultures on multivariate analysis, thus clinical judgment has to prevail in the decision to perform blood cultures. Breakthrough bacteremia can occur with microorganisms susceptible to the antibiotics that the patient is receiving.