Diagnosis of infective endocarditis

Because the symptoms of infective endocarditis are nonspecific, highly variable, and may develop insidiously, a high index of suspicion is required for diagnosis. Endocarditis should be suspected in febrile patients without obvious sources of infection, particularly if a heart murmur is present. The suspicion for endocarditis should be very high if blood cultures are positive in a patient with a history of valvular disease, a recent invasive procedure, or an intravenous drug user. Patients with documented bacteremia should undergo repeated, complete evaluations for new valvular murmurs and signs of embolism.

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Bacteriological diagnostics of infective endocarditis

If endocarditis is suspected, three blood cultures (20 ml for each culture) are obtained over 24 hours (if AIE is suspected, two cultures are obtained during the first 1–2 hours). Unless the study has been preceded by antibiotic therapy, all three blood cultures are usually positive in endocarditis because bacteremia is continuous; at least one culture is positive in 99%. If the study has been preceded by antimicrobial therapy, the blood culture may be either positive or negative.

Other than positive blood cultures, there are no specific laboratory findings. The infectious process often causes normocytic normochromic anemia, increased white blood cell count and ESR, and elevated immunoglobulins, circulating immune complexes, and rheumatoid factor, but these findings are of no diagnostic value. Urinalysis often shows microhematuria, occasionally red blood cell casts, pyuria, or bacteriuria.

Identification of the microorganism and determination of its susceptibility to antimicrobial therapy are vital for proper treatment. Blood culture may take 3-4 weeks to identify certain microorganisms. Some microorganisms (eg, aspergilli) may not give a positive culture. Some pathogens (eg, Coxiella burnetii, Bartonellosis sp., Chlamydia psittaci, Brucella) are identified by serodiagnosis, while others (eg, Legionella pneumophila) require special culture media. Negative blood culture results may indicate weakening of the biological properties of the microorganisms due to previous antimicrobial therapy, infection with microorganisms that do not grow on standard culture media, or another diagnosis (eg, noninfective endocarditis, atrial myxoma with embolism, vasculitis).

Infective endocarditis is diagnosed reliably when organisms are identified histologically (or cultured) in endocardial vegetations obtained during cardiac surgery, embolectomy, or autopsy. Because vegetations are rarely available for examination, clinical criteria for establishing the diagnosis (with sensitivity and specificity >90%) have been developed.

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Instrumental diagnostics of infective endocarditis

An echocardiogram is performed, usually transthoracic (TTE) rather than transesophageal (TEE). Although TEE is somewhat more accurate, it is invasive and more expensive. TEE is used in the following situations:

• suspected endocarditis in a patient with prosthetic valves;
• a situation when TTE has no diagnostic value;
• The diagnosis of infective endocarditis was established clinically.

Revised Duke's clinical diagnostic criteria for infective endocarditis

Major criteria for infective endocarditis

• Two positive blood cultures for organisms typical of endocarditis.
• Three positive blood cultures for organisms compatible with endocarditis.
• Serological detection of Coxiella burnetii.
• Echocardiographic evidence of endocardial involvement: a pulsating mass on a cardiac valve, supporting structures, in the regurgitant flow path, or on implanted material without other anatomical prerequisites.
• Cardiac abscess.
• Newly developed/detected prosthetic valve cleft.
• New valve regurgitation

Minor criteria for infective endocarditis

• Previous heart disease.
• Intravenous drug administration.
• Fever of 38°C or higher.
• Vascular symptoms: arterial embolism, septic pulmonary embolism, mycotic aneurysm, intracranial hemorrhage, conjunctival petechiae, or Janeway's sign.
• Immunological changes: glomerulonephritis, Osler nodes, Roth spots, rheumatoid factor.
• Microbiological evidence of infection compatible with endocarditis but not included in the major criteria.
• Serologic evidence of infection with a microorganism compatible with endocarditis

To make a specific clinical diagnosis, two major criteria, or one major and three minor criteria, or five minor criteria are required.

To establish a possible clinical diagnosis, one major and one minor or three minor criteria must be present. The diagnosis of infective endocarditis is excluded in the following cases:

• a credible alternative diagnosis has been made that explains the investigation results similar to infective endocarditis;
• resolution of symptoms and signs after antimicrobial therapy in 4 days or less; absence of pathological signs of infective endocarditis according to the examination of material obtained during surgery or autopsy; absence of clinical criteria for possible endocarditis.