What are the diagnostic criteria for infective endocarditis?

Diagnosis of Infective Endocarditis

The diagnosis of infective endocarditis relies on the Modified Duke Criteria, which requires either 2 major criteria, 1 major plus 3 minor criteria, or 5 minor criteria, with blood cultures and echocardiography forming the diagnostic cornerstone. 1

Blood Culture Strategy

Obtain at least 3 sets of blood cultures from separate venipuncture sites before starting antibiotics, with the first and last samples drawn at least 1 hour apart. 2 Each set should include both aerobic and anaerobic bottles. 1

• Blood cultures are positive in approximately 90% of IE cases when properly obtained. 2
• If blood cultures remain negative at 48 hours despite high clinical suspicion, immediately consult microbiology and initiate specialized testing. 1

Blood Culture-Negative IE Workup

When cultures remain negative at 48 hours, pursue systematic serological testing for: 1, 2

• Coxiella burnetii (Q fever) - anti-phase I IgG titer ≥1:800 is a major criterion 1
• Bartonella species 1
• Brucella species 1
• Aspergillus species 1
• Mycoplasma pneumoniae 1
• Legionella pneumophila 1

Additionally, order PCR assays for Tropheryma whipplei, Bartonella species, and fungi (Candida, Aspergillus) from blood. 1

Echocardiographic Approach

Start with transthoracic echocardiography (TTE) in all suspected cases. 1, 2 However, proceed immediately to transesophageal echocardiography (TEE) if: 2

• TTE is negative but clinical suspicion remains high
• Prosthetic valve is present
• Intracardiac device leads are present
• Optimal TTE windows cannot be obtained
• Complications are suspected (abscess, perforation, dehiscence)

TEE is the most sensitive imaging technique for identifying vegetations and has up to 98.6% negative predictive value. 1 TEE has superior sensitivity compared to TTE for detecting both vegetations and paravalvular abscesses. 1

Critical Echocardiography Pitfall

If initial echocardiography is negative but clinical suspicion remains high, repeat the study in 7-10 days, or earlier if Staphylococcus aureus infection is suspected. 2 Early TEE may miss incipient abscesses that appear only as nonspecific perivalvular thickening, which becomes recognizable as it expands and cavitates over several days. 1

Modified Duke Criteria

Major Criteria 1, 2

Microbiological Evidence:

• Typical organisms from 2 separate blood cultures: viridans streptococci, S. aureus, S. bovis, HACEK group, or community-acquired enterococci
• Persistently positive blood cultures (drawn >12 hours apart, or all of 3, or majority of ≥4 separate cultures)
• Single positive blood culture for Coxiella burnetii OR anti-phase I IgG titer ≥1:800 1

Imaging Evidence:

• Echocardiography showing oscillating intracardiac mass on valve or supporting structures, abscess, new partial dehiscence of prosthetic valve, or new valvular regurgitation 1
• 2015 ESC additions: Paravalvular lesions detected by cardiac CT, or abnormal activity around prosthetic valve on 18F-FDG PET/CT 2

Minor Criteria 1, 2

• Predisposition (predisposing heart condition or injection drug use)
• Fever ≥38°C
• Vascular phenomena (major arterial emboli, septic pulmonary infarcts, mycotic aneurysm, intracranial hemorrhage, conjunctival hemorrhages, Janeway lesions)
• Immunologic phenomena (glomerulonephritis, Osler nodes, Roth spots, rheumatoid factor)
• Microbiological evidence not meeting major criteria

Diagnostic Categories 1

• Definite IE: 2 major criteria, OR 1 major + 3 minor criteria, OR 5 minor criteria
• Possible IE: 1 major + 1 minor criterion, OR 3 minor criteria
• Rejected IE: Firm alternate diagnosis, OR resolution with ≤4 days antibiotics, OR no pathological evidence at surgery/autopsy with ≤4 days antibiotics

Pathological Gold Standard

Pathological examination of resected valvular tissue or embolic fragments remains the gold standard for diagnosis. 1, 2 Collect all surgical specimens in sterile containers without fixative or culture medium and send immediately to microbiology for culture, histological examination, and PCR. 1, 2

Advanced Imaging for Difficult Cases

When echocardiography is inconclusive (particularly in prosthetic valve endocarditis or device-related IE where echocardiography is normal or inconclusive in up to 30% of cases), consider: 1

• Cardiac CT for paravalvular complications 1, 2
• 18F-FDG PET/CT for prosthetic valve infection (abnormal activity is now a major criterion) 1, 2
• Whole-body CT or cerebral MRI for silent embolic events 1

Critical Management Points

The Modified Duke Criteria have approximately 80% sensitivity at end of follow-up but lower accuracy for early diagnosis, especially in prosthetic valve endocarditis. 1, 2 The criteria are meant to guide diagnosis but must not replace clinical judgment - you may appropriately decide to treat regardless of whether criteria are met if clinical suspicion is high. 1, 2

Refer complicated cases to centers with multidisciplinary "Endocarditis Teams" including cardiologist, infectious disease specialist, cardiac surgeon, and microbiologist. 2 Obtain repeat blood cultures 48-72 hours after starting treatment to verify effectiveness. 2