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Culture-Negative Endocarditis — RACP Adult Medicine MCQ

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HardInfectious DiseasesCulture-Negative EndocarditisRACP Adult Medicine

A 72-year-old man with a prosthetic aortic valve presents with fever, night sweats, and weight loss for 6 weeks. Three sets of blood cultures are negative after 5 days of incubation. Transoesophageal echocardiography shows a vegetation on the prosthetic valve. Which organism should be specifically tested for?

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Correct answer: B — Coxiella burnetii (Q fever)

Culture-negative endocarditis with prosthetic valve involvement should raise suspicion for fastidious organisms. Coxiella burnetii (Q fever) is an important cause of culture-negative endocarditis in Australia, particularly in rural/livestock-exposed patients. Phase I IgG titre ≥1:800 is diagnostic of chronic Q fever endocarditis. Other causes include Bartonella, Tropheryma whipplei, and HACEK organisms. Brucella is rare in Australia.

Reference: eTG – 2025 – Antibiotic Guidelines; ESC – 2023 – Infective Endocarditis Guidelines