A 35-year-old IV drug user presents with fever, bacteraemia, and multiple septic pulmonary emboli on CT chest. Echocardiography shows a tricuspid valve vegetation. Blood cultures grow Staphylococcus aureus. Which modified Duke criterion is fulfilled by the positive blood culture and echocardiogram finding?

• A One major criterion + predisposing condition (IV drug use as minor criterion)
• B Two minor criteria only — fever and IV drug use
• C One minor criterion (blood culture positive for non-typical organism)
• D One major criterion (typical organism in ≥2 blood cultures) + one major criterion (echocardographic evidence of endocardial involvement) ✓

Explanation

The modified Duke criteria classify IE. S. aureus is a typical IE organism (first major criterion). Echocardiographic evidence of IE (oscillating intracardiac mass/vegetation, abscess, or new partial prosthetic valve dehiscence) is the second major criterion. Having two major criteria fulfils the 'Definite IE' classification. IV drug use is a minor criterion (predisposing cardiac condition or IV drug use). S. aureus isolated from multiple blood cultures fulfils the major criterion even if only one blood culture is positive for S. aureus (as it is a 'typical pathogen' under modified Duke criteria). Two major criteria = definite IE.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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