Blood culture sensitivity tracks the volume of blood in the bottle more closely than almost anything else the laboratory does. In adults, yield rises roughly three per cent for every additional millilitre, because bacteraemia is often at one to ten colony-forming units per millilitre. An under-filled bottle is a test that has been half-ordered.
Two sets from two separate venepunctures, before the first dose of antibiotic, is the standard for a reason: it distinguishes true bacteraemia from skin contamination, which is the single commonest cause of an unnecessary vancomycin course. Drawing both sets from the same site, or from an existing line, defeats that.
In neonates and small infants the constraint is different — one millilitre is often all that is available, and the compensating factor is that neonatal bacteraemia is usually higher density. Even so, document the volume obtained. A negative culture from 0.3 mL means something quite different from a negative culture from 1 mL, and nobody can tell from the report which one it was.
- Fill adult bottles to the marked volume — 8 to 10 mL each — and say so if you could not.
- Two sets from two separate sites, before antibiotics, whenever the clinical situation allows the minutes.
- Do not draw routine cultures through an existing central line unless you are specifically investigating line infection, and then pair it with a peripheral set.
- Record the volume drawn in neonates; it changes how a negative result should be read.
Why it matters
Most negative blood cultures in a septic patient are answering a question about the sample, not about the patient.
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