What are the diagnostic criteria and treatment options for a patient with suspected infectious carditis, considering their medical history, age, and socioeconomic status?

Diagnostic Criteria for Infective Endocarditis

The Modified Duke Criteria remain the gold standard for diagnosing infective endocarditis, requiring either 2 major criteria, 1 major plus 3 minor criteria, or 5 minor criteria for a "definite" diagnosis. 1

Major Criteria

Blood Culture Criteria

• At least 3 sets of blood cultures from separate venipuncture sites, with the first and last drawn at least 1 hour apart, obtained before antibiotic initiation 1
• Typical IE organisms (viridans streptococci, Staphylococcus aureus, Streptococcus bovis, HACEK group, or enterococci) in 2 separate blood cultures 1
• Persistently positive blood cultures drawn >12 hours apart, or all of 3 or a majority of ≥4 separate cultures 1

Critical pitfall: Blood cultures may be negative if antibiotics were administered before collection—never start antibiotics before obtaining cultures unless the patient is hemodynamically unstable 1, 2

Imaging Criteria (Echocardiography)

• Oscillating intracardiac mass (vegetation) on valve or supporting structures 1
• Abscess (paravalvular or myocardial) 1
• New partial dehiscence of prosthetic valve 1
• New valvular regurgitation (not pre-existing) 1

Minor Criteria

• Predisposing heart condition (prosthetic valve, congenital heart disease, previous IE) or injection drug use 1, 2
• Fever ≥38.0°C (100.4°F) 1, 2
• Vascular phenomena: septic pulmonary infarcts, mycotic aneurysm, intracranial hemorrhage, conjunctival hemorrhages, Janeway lesions 1, 2
• Immunologic phenomena: glomerulonephritis, Osler nodes, Roth spots, rheumatoid factor 1, 2
• Microbiologic evidence: positive blood culture not meeting major criteria or serologic evidence of active infection with organism consistent with IE 1

Diagnostic Algorithm

Step 1: Initial Evaluation

• Obtain 3 sets of blood cultures from separate sites before antibiotics 1, 2, 3
• Perform transthoracic echocardiography (TTE) immediately in all suspected cases 1

Step 2: Risk Stratification for TEE

Proceed directly to transesophageal echocardiography (TEE) if any of the following are present: 1

• Prosthetic heart valve (TTE inadequate due to structural components) 1
• Intracardiac device leads present 1
• TTE nondiagnostic but clinical suspicion remains high 1
• New atrioventricular block (suggests valve ring abscess) 1, 2
• Staphylococcus aureus bacteremia without known source 1
• Complications suspected (abscess, fistula, pseudoaneurysm) 1

TEE sensitivity is >95% versus 60-75% for TTE in detecting vegetations 2

Step 3: Advanced Imaging (When Standard Echocardiography Inadequate)

• Cardiac CT for paravalvular complications when anatomy cannot be clearly delineated by echocardiography, particularly for abscesses, pseudoaneurysms, or fistulas in prosthetic valve IE 1
• 18F-fluorodeoxyglucose PET/CT for patients classified as "possible IE" by Modified Duke Criteria 1

Clinical Presentation Red Flags

Cardinal Features (Must Actively Seek)

• Fever present in up to 90% of cases—may be absent in elderly, immunocompromised, or those who received antibiotics before evaluation 2, 3
• New or changing heart murmur in up to 85%, most commonly valvular insufficiency 2, 3
• Embolic phenomena in up to 25% at diagnosis—septic pulmonary infarcts in right-sided IE, stroke in left-sided IE 2, 3

High-Risk Populations Requiring Lower Threshold for Diagnosis

• Injection drug users with fever and pulmonary symptoms (suspect right-sided endocarditis) 2
• Persistent S. aureus or viridans streptococci bacteremia 2
• Recent dental extraction without prophylaxis (viridans streptococcal IE) 2
• Indwelling IV catheters or implantable cardiac devices 1

Repeat Imaging Indications

Perform repeat TTE and/or TEE if: 1

• Change in clinical signs or symptoms (new murmur, embolism, persistent fever, heart failure, atrioventricular block) 1
• High risk of complications (extensive infected tissue, large vegetation on initial echo, staphylococcal/enterococcal/fungal infections) 1
• Persistent fever despite appropriate antibiotics (suggests abscess, mycotic aneurysm, or treatment failure) 2

Common Diagnostic Pitfalls

• **CT has poor sensitivity for native aortic valve vegetations <1 cm** (NPV only 55.5%), but 100% sensitivity for vegetations >1 cm 1
• Early TEE may miss initial perivalvular abscesses—repeat imaging if clinical suspicion persists 1
• Both TTE and TEE produce false-negatives if vegetations are small or have embolized 1
• Never rely on Duke Criteria alone—clinical judgment supersedes criteria, and clinicians may appropriately treat regardless of whether criteria are met 1