The influence of the severity of community-acquired pneumonia on the usefulness of blood cultures.

The value of blood cultures in community-acquired pneumonia (CAP) has been questioned. At issue is the potential for blood cultures to change management. We prospectively studied the yield and impact of blood cultures in patients admitted with CAP. Two hundred and nine subjects had at least two blood cultures prior to receiving antibiotics. The severity of CAP was graded using the Pneumonia Severity Index (PSI). Twenty-nine patients (13.9%) had a pathogen identified by blood culture. The yield of blood cultures increased with PSI grade (I--5.3%, II--10.2%, III--10.3%, IV--16.1%, V--26.7%), as did the likelihood of blood cultures changing antibiotic therapy (I to III--0%, IV--9.7%, V--20.0%). One hundred and seventy-nine (85.6%) patients received a quinolone, limiting the impact of pathogens resistant to beta-lactams. Four of 16 patients (25.0%) with a culture (blood or sputum)-guided change in antibiotic therapy died, compared to five of 31 patients (16.1%) who had an empiric change. Blood cultures are of minimal value in mild to moderate CAP, and should be limited to patients with PSI grade IV or V CAP unless a specific risk factor for pathogens resistant to the empiric therapy is present.