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Modified Duke Criteria

The diagnosis of infective endocarditis (IE) is typically based on the modified Duke criteria, which incorporate clinical, microbiological, and echocardiographic findings.

The criteria consist of:

  • two major criteria (positive blood cultures and evidence of endocardial involvement)
  • five minor criteria (predisposing heart condition or intravenous drug use, fever, vascular phenomena, immunologic phenomena, and microbiologic evidence)

A definitive diagnosis requires the presence of two major criteria, one major criterion plus three minor criteria, or five minor criteria.


Laboratory Tests and Blood Cultures

Blood cultures are essential for the diagnosis of IE and should be obtained before initiating antibiotic therapy.

Multiple sets of blood cultures (typically three sets) should be collected over a 24-hour period to maximize the chances of detecting the causative organism.

Additional laboratory tests, such as complete blood count, erythrocyte sedimentation rate, and C-reactive protein, can provide supportive evidence of infection but are nonspecific for IE.


Serological Markers

In some cases, serological markers may be helpful in identifying the causative organism, particularly when blood cultures are negative or the patient has received prior antibiotic therapy.

Examples of serological markers include antibody testing for Bartonella, Brucella, or Coxiella burnetii, which can cause culture-negative endocarditis.


Imaging Studies: Echocardiography and Others

Echocardiography, including both transthoracic (TTE) and transesophageal (TEE) approaches, is a critical imaging modality for the diagnosis and management of IE.

  • TTE is often performed initially due to its noninvasive nature
  • TEE may be indicated when TTE findings are inconclusive or when better visualization of cardiac structures is required

Other imaging studies, such as computed tomography or magnetic resonance imaging, may be useful in specific situations, such as evaluating for abscesses or other complications.


Challenges in Diagnosis and Special Populations

The diagnosis of IE can be challenging due to its variable clinical presentation and the limitations of diagnostic tests.

Patients with prosthetic valves, intracardiac devices, or other predisposing factors may present with atypical symptoms or have a higher likelihood of culture-negative endocarditis, necessitating a high index of suspicion and a comprehensive diagnostic approach.