A 55-year-old man with a prosthetic aortic valve develops intermittent fever. Transthoracic echocardiography is inconclusive. How many blood culture sets should be collected before starting empiric antibiotics?
- A One set immediately, then start antibiotics without delay
- B Three sets drawn from separate venepunctures over at least 1 hour ✓
- C Two sets drawn simultaneously from one site
- D Six sets drawn hourly over 24 hours before any therapy
Correct answer: B. Three sets drawn from separate venepunctures over at least 1 hour
Explanation
In suspected endocarditis with stable haemodynamics, three separate blood culture sets obtained from different venepunctures over at least an hour maximise yield and help distinguish continuous bacteraemia from transient contamination. In subacute cases some texts advise spacing further apart. Two sets suffice for ordinary sepsis, making C tempting, but endocarditis demands three because organism load can be intermittently low. Six sets over a day unnecessarily delays treatment in a febrile prosthetic valve patient.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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