A 52-year-old dairy farmer presents with 2 months of low-grade fever, weight loss, mild hepatosplenomegaly, and transaminitis. Three sets of blood cultures are negative. Transoesophageal echo shows a small vegetation on a previously normal aortic valve. Which additional finding would satisfy a major Duke criterion?

• A Elevated erythrocyte sedimentation rate above 100 mm/hr
• B Positive Brucella serology in a single sample
• C Blood cultures held for 3 weeks growing HACEK flora
• D Anti-phase I IgG titre against Coxiella burnetii greater than 1:800 ✓

Explanation

In the modified Duke criteria, a single positive blood culture for Coxiella burnetii or an anti-phase I IgG antibody titre greater than 1:800 is classified as a major microbiological criterion. This patient's occupational exposure, subacute course, hepatosplenomegaly, and culture-negative vegetations fit chronic Q fever endocarditis. An elevated ESR is only a minor criterion, and brucellosis serology does not appear among the defined major criteria.

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

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