Rational approaches to the treatment of culture-negative infective endocarditis.

The microorganism responsible for infective endocarditis may not be grown on blood culture in as many as 25% of cases. While this is to be expected with such relatively uncommon organisms as Coxiella burnetti, in most cases failure to grow the organism is likely to be due to either a low concentration of bacteria in the blood or because antibiotics were given before blood was taken for culture. The antibiotic treatment of culture-negative cases should be based on the assumption that the organisms responsible are the same as those found in cases with positive cultures, covering the most likely possibilities in such different circumstances as spontaneous infections of natural valves, endocarditis following cardiac surgery, early and late prosthetic valve endocarditis and infections associated with intravenous drug abuse.