1. Modified Duke Criteria

The Modified Duke Criteria represent the cornerstone of diagnosis for infective endocarditis (IE), integrating clinical, microbiologic, and echocardiographic findings into a standardized framework. This system allows for a consistent approach to classifying patients.

Major Criteria

• Blood Cultures: Two positive blood cultures for typical IE organisms (e.g., viridans group streptococci, Streptococcus bovis group, HACEK group, Staphylococcus aureus, or community-acquired enterococci) drawn from separate sites at least one hour apart.
• Evidence of Endocardial Involvement: Echocardiographic findings such as an oscillating intracardiac mass (vegetation), abscess, new partial dehiscence of a prosthetic valve, or new-onset valvular regurgitation.

Minor Criteria

• Predisposition: A known predisposing heart condition or history of injection drug use.
• Fever: Temperature greater than 38°C (100.4°F).
• Vascular Phenomena: Major arterial emboli, septic pulmonary infarcts, mycotic aneurysm, or Janeway lesions.
• Immunologic Phenomena: Glomerulonephritis, Osler nodes, Roth spots, or a positive rheumatoid factor.
• Microbiologic Evidence: A positive blood culture that does not meet the major criterion.

Diagnostic Classification

2. Microbiologic Diagnostics

Prompt and accurate identification of the causative pathogen is critical for directing antimicrobial therapy and improving outcomes. Best practices in sample collection are paramount.

Blood Culture Best Practices

• Obtain at least three sets of blood cultures from separate venipuncture sites.
• The first and last draws should be separated by at least one hour to demonstrate continuous bacteremia.
• Cultures should ideally be drawn before initiating antibiotic therapy, if the patient is hemodynamically stable.
• Utilize rapid identification technologies like MALDI-TOF MS to accelerate organism identification and subsequent susceptibility testing.

Navigating Culture-Negative Endocarditis

When blood cultures are persistently negative despite high clinical suspicion, adjunctive testing is necessary:

• Serologic Assays: Test for fastidious organisms that do not grow in standard culture media, such as Bartonella spp., Brucella spp., and Coxiella burnetii.
• Molecular Testing: Polymerase chain reaction (PCR) performed on excised valve tissue or, in some cases, whole blood can identify pathogen DNA. While not formally part of the Duke criteria, it is a powerful adjunctive tool.

3. Echocardiographic Imaging

Echocardiography is the imaging cornerstone for diagnosing IE and its complications. The choice between transthoracic (TTE) and transesophageal (TEE) approaches depends on patient factors and the specific clinical question.

Transthoracic vs. Transesophageal Echocardiography

Advanced Echocardiographic Techniques

3D Echocardiography, particularly with TEE, provides superior anatomical detail of vegetation size, shape, and mobility, which is invaluable for surgical planning. Cardiac CT and MRI serve as important adjuncts when echo is inconclusive, especially for evaluating perivalvular extension of infection into surrounding tissues.

4. Advanced Imaging Adjuncts

In complex cases, particularly involving prosthetic valves (PVE) or cardiac implantable electronic devices (CIEDs), echocardiography may be inconclusive. Nuclear imaging has emerged as a critical tool to resolve diagnostic uncertainty.

¹⁸F-FDG PET/CT and Leukocyte SPECT/CT

These functional imaging modalities detect metabolic activity (inflammation) associated with infection. Their primary role is in suspected PVE or CIED infections where echo findings are equivocal. Focal radiotracer uptake around a prosthetic valve or device lead is a strong indicator of infection.

5. Classification and Risk Stratification

Beyond diagnosis, classifying IE and stratifying risk are essential for determining the urgency of management, particularly the need for surgical intervention.

Key Classification Schemes

• Native vs. Prosthetic Valve IE (PVE): PVE carries a higher mortality and complication rate, often caused by more virulent organisms like staphylococci. Early PVE (<1 year post-op) is typically nosocomial, while late PVE resembles native valve IE.
• Left-Sided vs. Right-Sided IE: Left-sided (mitral, aortic) IE poses a risk of systemic embolization (e.g., stroke) and severe heart failure. Right-sided (tricuspid) IE is classic in persons who inject drugs and typically presents with septic pulmonary emboli.

Predicting Embolic Risk and Mortality

Certain features significantly increase the risk of embolization and poor outcomes. Formal scoring systems can help quantify this risk and facilitate multidisciplinary discussions about the timing of surgery.