What are the diagnostic criteria for infective endocarditis, specifically the Duke criteria, in a patient with a suspected Methicillin-resistant Staphylococcus aureus (MRSA) infection?

Duke Criteria for Infective Endocarditis

The Modified Duke Criteria classify infective endocarditis into three diagnostic categories—definite, possible, or rejected—using a combination of major criteria (blood cultures with typical organisms and echocardiographic findings) and minor criteria (predisposition, fever, vascular/immunologic phenomena, and microbiologic evidence). 1, 2

Diagnostic Categories

Definite Infective Endocarditis is diagnosed when any of the following are met: 1, 3, 4

• Pathological criteria:

  • Microorganisms demonstrated by culture or histology from vegetation, embolized vegetation, or intracardiac abscess 1, 3
  • Pathological lesions showing active endocarditis on histology 1, 3

• Clinical criteria:

  • 2 major criteria, OR 1, 2, 4
  • 1 major criterion AND 3 minor criteria, OR 1, 2, 4
  • 5 minor criteria 1, 2, 4

Possible Infective Endocarditis is diagnosed when: 1, 3

• 1 major criterion AND 1 minor criterion, OR 1, 3
• 3 minor criteria 1, 3

Rejected Infective Endocarditis applies when: 1

• Firm alternative diagnosis explains the clinical picture, OR 1
• Resolution of syndrome with ≤4 days of antibiotic therapy, OR 1
• No pathological evidence at surgery/autopsy after ≤4 days of antibiotics, OR 1
• Does not meet criteria for possible IE 1

Major Criteria

Blood Culture Criteria

Typical microorganisms from 2 separate blood cultures: 1, 3, 4

• Viridans streptococci 1, 3
• Streptococcus gallolyticus (S. bovis) 3, 4
• HACEK group organisms (fastidious gram-negative rods) 1, 3, 4
• Staphylococcus aureus (community-acquired, without primary focus) 1, 3
• Community-acquired enterococci (without primary focus) 1, 3

Persistently positive blood cultures: 1, 3, 4

• ≥2 positive cultures drawn >12 hours apart, OR 3, 4
• All of 3 separate blood cultures, OR 3
• Majority of ≥4 separate blood cultures (first and last drawn ≥1 hour apart) 3

Special organisms: 1, 3

• Single positive blood culture for Coxiella burnetii, OR 1, 3
• Anti-phase I IgG antibody titer >1:800 for C. burnetii 1, 3

Echocardiographic Criteria

Positive echocardiogram showing: 1, 3, 4

• Oscillating intracardiac mass on valve or supporting structures 1, 3, 4
• Abscess (periannular) 1, 3, 4
• New partial dehiscence of prosthetic valve 1, 3
• New valvular regurgitation 3

Enhanced imaging (2015 ESC additions): 2

• Paravalvular lesions detected by cardiac CT 2
• Abnormal activity around prosthetic valve on 18F-FDG PET/CT 2

Minor Criteria

Predisposition: 1, 3, 4

• Predisposing heart condition (valvular disease, congenital heart disease) 1, 3, 4
• Injection drug use 1, 3, 4
• Prosthetic valve or prior IE 4

Fever: Temperature ≥38°C 2, 3, 4

Vascular phenomena: 1, 3, 4

• Major arterial emboli 3, 4
• Septic pulmonary infarcts 3, 4
• Mycotic aneurysm 3
• Intracranial hemorrhage 3
• Conjunctival hemorrhages 3
• Janeway lesions 3

Immunologic phenomena: 1, 3

• Glomerulonephritis 3
• Osler's nodes 3
• Roth spots 3
• Rheumatoid factor 3

Microbiological evidence: 1, 3

• Positive blood culture not meeting major criterion 3
• Serological evidence of active infection with organism consistent with IE 3

Special Considerations for MRSA

For patients with suspected MRSA endocarditis, the diagnostic approach follows the same Modified Duke Criteria framework. 1 S. aureus bacteremia (including MRSA) qualifies as a major criterion only when it is community-acquired and without an apparent primary focus, as S. aureus commonly causes both IE and non-IE bacteremias. 1 The distinction is critical because nosocomial S. aureus bacteremia or bacteremia with an identifiable primary source (e.g., IV catheter infection) does not automatically meet major criteria and requires additional supporting evidence. 1

Diagnostic Performance and Clinical Application

The Modified Duke Criteria demonstrate approximately 80% sensitivity with high specificity across diverse patient populations, including those with prosthetic valves, native valves, and injection drug users. 3, 4, 5, 6 However, sensitivity is lower for early diagnosis, particularly in prosthetic valve endocarditis and pacemaker/defibrillator lead IE. 2

Critical pitfall: The Duke Criteria should guide but not replace clinical judgment—clinicians may appropriately decide to treat patients who do not meet formal criteria if clinical suspicion remains high. 1, 2 This is especially important in culture-negative IE, where prior antibiotic administration (occurring in up to 63% of cases) significantly reduces diagnostic sensitivity. 2, 7

Diagnostic Workup Algorithm

When IE is suspected: 1, 2

1. Obtain ≥3 sets of blood cultures from separate venipuncture sites before antibiotics, with first and last samples drawn ≥1 hour apart 2
2. Perform transthoracic echocardiography (TTE) first in all cases 2
3. Proceed immediately to transesophageal echocardiography (TEE) if: 2
   • TTE is negative but clinical suspicion remains high 2
   • Prosthetic valve is present 2
   • Intracardiac device leads are present 2
   • Optimal TTE windows cannot be obtained 2
4. Repeat echocardiography 7-10 days later if initially negative but suspicion persists, or earlier if S. aureus infection suspected 2
5. For culture-negative cases at 48 hours: Initiate specialized testing including serological testing for Coxiella burnetii, Bartonella spp., Brucella spp., and molecular diagnostics 2

TEE detects vegetations in >95% of cases versus 60-75% for TTE, and may identify vegetations missed by TTE in up to 41% of definite cases. 1, 5 Both modalities can produce false-negatives if vegetations are small, have embolized, or if imaging is performed too early before abscess cavitation occurs. 1