# Diagnosis of Infective Endocarditis — AHA 2015

## Modified Duke Criteria

The AHA endorses the **Modified Duke Criteria** (Li et al., 2000) as the diagnostic framework for infective endocarditis (IE). Patients are classified as **Definite**, **Possible**, or **Rejected** based on pathological and clinical criteria.

> **OpenMedicine Calculator:** `calculate_duke_criteria` — available via MCP for automated Modified Duke Criteria evaluation.

### Definite IE

- **Pathological criteria:** Microorganisms demonstrated by culture or histology of a vegetation, embolized vegetation, or intracardiac abscess; OR vegetation or intracardiac abscess confirmed by histology showing active endocarditis
- **Clinical criteria (any of the following):**
  - 2 major criteria
  - 1 major + 3 minor criteria
  - 5 minor criteria

### Possible IE

- 1 major + 1 minor criterion
- 3 minor criteria

### Rejected IE

- Firm alternative diagnosis explaining findings
- Resolution of IE syndrome with antibiotic therapy for 4 days or fewer
- No pathological evidence of IE at surgery or autopsy with antibiotic therapy for 4 days or fewer
- Does not meet criteria for possible IE

## Major Clinical Criteria

### Major Criterion 1: Blood Cultures Positive for IE

- **Typical microorganisms** consistent with IE from **2 separate blood cultures**: viridans group streptococci, *Streptococcus gallolyticus* (bovis), HACEK group organisms, *Staphylococcus aureus*, or community-acquired enterococci in the absence of a primary focus
- **Persistently positive blood cultures**: recovery of a microorganism consistent with IE from blood cultures drawn **> 12 hours apart**; or all of 3, or a majority of 4 or more separate blood cultures (with first and last sample drawn **at least 1 hour apart**)
- **Single positive blood culture** for *Coxiella burnetii* or anti-phase I IgG antibody titer **> 1:800**

### Major Criterion 2: Evidence of Endocardial Involvement

- **Positive echocardiogram:** oscillating intracardiac mass on valve or supporting structures, in the path of regurgitant jets, or on implanted material in the absence of an alternative anatomic explanation; OR abscess; OR new partial dehiscence of prosthetic valve
- **New valvular regurgitation** (worsening or changing of pre-existing murmur not sufficient)

## Minor Clinical Criteria

| Criterion | Definition |
|---|---|
| **Predisposing condition** | Prosthetic valve, previous IE, congenital heart disease, valvular heart disease, or injection drug use |
| **Fever** | Temperature > 38.0 degrees C (100.4 degrees F) |
| **Vascular phenomena** | Major arterial emboli, septic pulmonary infarcts, mycotic aneurysm, intracranial hemorrhage, conjunctival hemorrhages, Janeway lesions |
| **Immunologic phenomena** | Glomerulonephritis, Osler nodes, Roth spots, positive rheumatoid factor |
| **Microbiological evidence** | Positive blood culture that does not meet a major criterion, or serological evidence of active infection with organism consistent with IE |

## Blood Cultures

- At least **3 sets of blood cultures** should be obtained from **different venipuncture sites**, with the first and last drawn at least **1 hour apart** (Class I, Level of Evidence A)
- Blood cultures should be obtained **before initiation of antimicrobial therapy** whenever possible (Class I, Level of Evidence B)
- Each set should include **both aerobic and anaerobic bottles**
- If initial cultures are negative after 48-72 hours in patients with suspected IE, **2-3 additional blood culture sets** should be obtained
- Persistently positive blood cultures serve as both a diagnostic criterion and a marker of treatment response

## Echocardiography

### Transthoracic Echocardiography (TTE)

- TTE is recommended as the **first-line imaging test** in suspected IE (Class I, Level of Evidence B)
- TTE sensitivity for vegetations on native valves is approximately **70%**
- TTE is reasonable at the completion of antibiotic therapy to establish a **new baseline** for future comparison (Class IIa, Level of Evidence C)

### Transesophageal Echocardiography (TEE)

- TEE is recommended when **TTE is nondiagnostic** and clinical suspicion for IE remains (Class I, Level of Evidence B)
- TEE is recommended for patients with **prosthetic heart valves** and suspected IE (Class I, Level of Evidence B)
- TEE is recommended for evaluation of patients with **S. aureus bacteremia** without a known source (Class I, Level of Evidence B)
- TEE sensitivity for vegetations is approximately **90-100%** and is superior to TTE for detecting abscesses, prosthetic valve complications, and small vegetations
- If initial TEE is negative but clinical suspicion remains high, **repeat TEE in 3-5 days** (Class IIa, Level of Evidence B)
- TEE should be repeated if clinical features suggest a **new intracardiac complication** (e.g., new conduction abnormality, persistent fever, heart failure) (Class I, Level of Evidence B)

### Indications for TEE over TTE

- Prosthetic valve or intracardiac device
- Complicated IE (paravalvular abscess, fistula)
- Inadequate TTE images
- S. aureus bacteremia
- Persistent bacteremia of unknown source

## Additional Diagnostic Considerations

### Cardiac CT and MRI

- Cardiac CT can be useful in identifying paravalvular complications (abscesses, pseudoaneurysms, fistulae), particularly when echocardiographic findings are inconclusive (Class IIa, Level of Evidence B)
- Cardiac MRI may provide incremental information when echocardiography is nondiagnostic

### Nuclear Imaging

- FDG-PET/CT may be considered in patients with prosthetic valve endocarditis when Duke criteria yield a "possible" classification and additional diagnostic information is needed

## Limitations

- The Modified Duke Criteria have a sensitivity of approximately 80% for definite IE; clinical judgment remains essential, particularly in prosthetic valve IE and culture-negative cases
- TTE has limited sensitivity (~70%) for vegetations, particularly with prosthetic valves, obesity, or COPD
- Duke criteria were developed and validated primarily for native valve IE caused by common organisms; performance may be reduced in prosthetic valve IE, right-sided IE in injection drug users, or atypical organisms
- Blood culture negativity (5-10% of IE cases) can result from prior antibiotic exposure, fastidious organisms (HACEK, Bartonella, Tropheryma whipplei), or fungal infections
