Infectious endocarditis with negative blood cultures

Blood culture is a key investigation for the diagnosis of infectious endocarditis (IE). When negative, there are diagnostic and therapeutic problems. The aim of this study was to determine the frequency, the clinical features and the aetiological factors of IE with negative blood cultures compared with IE with positive blood cultures compared with IE with positive blood cultures. The authors undertook a retrospective review of 98 cases of patients admitted for IE from 1991 to 2000 to the Department of Infectious Diseases and Cardiology of Sousse (Tunisia). Of the 98 patients, 48 (48.9%), 29 men and 19 women with an average age of 34.3 years, had negative blood cultures. An infectious agent was identified in 7 cases (14.5%) by serology, valve culture or cerebrospinal fluid including Brucella (2), Coxiella (1) and Candida (1). Therefore, in 41 cases (42%), the cause of IE was not determined. Transthoracic echocardiography was of diagnostic value in 96% of cases and transoesophageal echocardiography showed disease not observed on transthoracic echocardiography in 5 cases. The main complication was cardiac failure (27 cases). The mortality was 14.5%. Comparison of the two groups showed that negative blood cultures were associated with a higher incidence of previous antibiotic therapy, extracardiac signs of IE and cardiac failure. Early surgical indications and mortality were the same in both groups. This report confirmed the high frequency of IE with negative blood cultures. Previous antibiotic therapy seems to be an important aetiological factor but cannot explain this high frequency. Methodological problems of blood cultures and the absence of systematic investigation for rare infectious agents are other possible factors.