The medical treatment of culture-negative infective endocarditis.

The most common cause for persistently negative blood cultures is the previous administration of antibiotics, but other causes include fastidious organisms (such as Brucella and Legionella), cell-dependent organisms (such as Chlamydia and Coxiella), fungi and a major immune reaction. Fastidious organisms may take up to 3 weeks to grow in optimal media. Abscess formation may take the organisms inaccessible. If the diagnosis is in doubt, echocardiography, and more specifically transoesophageal echocardiography, is invaluable. If the clinical diagnosis is made but cultures are unavailable or negative, treatment should be started without delay. The choice of antibiotic depends on the clinical setting. In general, penicillin and gentamicin are indicated for a subacute onset: flucloxacillin and gentamicin if the onset is acute. Intravenous drug abusers should receive vancomycin; those who have recently had a prosthetic valve inserted should receive vancomycin, together with rifampicin and gentamicin.