The differential diagnosis is extensive because so many of the clinical and laboratory findings associated with rheumatic fever are not specific and there is no single laboratory test that can confirm the diagnosis. Juvenile rheumatoid arthritis or other connective tissue diseases often need to be considered. Infective endocarditis is frequently confused with rheumatic fever, especially in patients with recurrences of rheumatic fever. Patients with a previous history of rheumatic fever or rheumatic valvular heart disease should be carefully evaluated for infective endocarditis before the diagnosis of recurrent rheumatic fever is made. This may be difficult because such patients may be taking an antibiotic for secondary rheumatic fever prophylaxis in a dose high enough to prevent blood cultures from becoming positive. The typical rash of Lyme disease may be confused with erythema marginatum.