The yield of blood cultures depends mostly on how much blood goes in. For adults, two sets from separate venepunctures, each with adequately filled aerobic and anaerobic bottles, is the standard, taken before the first antibiotic dose whenever that does not delay treatment in a septic patient.
Contamination is the other main failure: skin preparation with alcoholic chlorhexidine allowed to dry, a fresh venepuncture rather than an existing cannula where possible, and discarding the first cultures drawn through an old line. A single positive bottle with a skin commensal is more likely contamination than infection, but still deserves a look at the patient and any line.
- Take two sets from separate sites, with bottles filled to the recommended volume.
- Draw cultures before the first antibiotic dose if it causes no delay in sepsis.
- Use alcoholic chlorhexidine and let it dry before venepuncture.
- Interpret a single skin-commensal positive bottle as probable contamination, after assessing the patient.
Why it matters
Underfilled or contaminated cultures drive both missed bacteraemia and unnecessary antibiotics.
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