12th March 2024

Diagnosis

Blood tests may show raised inflammatory markers (CRP and ESR) with leukocytosis. While these tests are not specific for IE, they can reflect the severity of IE-induced sepsis. Blood cultures are essential for identifying the organisms involved, and three sets of blood cultures from different venipuncture sites should be taken before initiating empirical antibiotic therapy. 

Transthoracic echocardiography (TTE) can look for evidence of IE in the form of valvular vegetations, abscess formation or prosthetic valve dehiscence. Due to the high virulence associated with Staphylococcus aureus, positive blood cultures with this organism is an indication for performing a TTE to exclude IE.

Where TTE is negative or inconclusive, but the suspicion for IE remains high, transoesophageal echocardiography  (TOE) offers superior views and is advised. The latter is also recommended in suspected cases of IE in patients with prosthetic valves. 

Other imaging for diagnosis of cardiac involvement includes 18F-FDG PET/CT and Cardiac CT. 

Modified Duke’s Criteria

The modified Duke’s criteria uses echocardiographic findings, blood culture results and clinical signs to risk stratify the likelihood of IE. For diagnosis of IE, the following must be fulfilled:

• 2 major criteria OR
• 1 major and 3 minor criteria OR
• 5 minor criteria

Major criteria: Microbiological:

• Growth of typical IE organisms from two separate blood cultures (Streptococcus viridans, Strep. gallolyticus (formerly S . bovis), Staphylococcus aureus, HACEK group, Community-acquired enterococci
• Microorganisms typical for IE from persistently positive blood cultures (two positive blood cultures drawn 12h apart or all three or majority of four positive blood cultures with first and last samples drawn more than 1 hour apart)
• Single blood culture or serology positive for Coxiella burnetii

Major criteria: Imaging:

• TTE/TOE showing vegetations/abscess/pseudoaneurysm/intracardiac fistula/valve perforation/new dehiscence of prosthetic valve/
• 18F-FDG PET/CT showing abnormal activity around prosthetic valve implantation site (if valve was implanted more than three months ago)
• Paravalvular lesions on Cardiac CT 

Minor criteria:

• Risk factors for IE (IVDU, structural cardiac pathology)
• Fever 
• Immunological phenomena (Osler’s nodes, Roth’s spots, Glomerulonephritis)
• Vascular phenomena
• Positive blood cultures failing to meet major criteria.