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🩸 How much blood
Volumes that change the result — cultures, cross-match, paediatrics
Volume is the part of phlebotomy nobody is taught and the part that most often decides whether the result is any use.
For blood cultures it is not a formality: the yield rises with the volume inoculated, and an under-filled set is the reason a bacteraemia comes back negative and the antibiotic is never narrowed.
Blood cultures
| Adult, per bottle | 8–10 mlRead the fill line on the bottle. Over-filling is also a problem — it can produce a false positive. |
|---|---|
| A "set" | 1 aerobic + 1 anaerobicBoth from the same venepuncture. If you only get enough for one, fill the aerobic. |
| Suspected endocarditis | 3 sets, separate sitesSeparated in time where the patient allows it. Do not take all three from one arm. |
| Before antibiotics | Always, if it does not delay themIn sepsis, cultures come before antibiotics — but never delay the antibiotic to chase a set. |
| Line sepsis | Paired, line and peripheralLabelled as such, so the lab can compare time to positivity. |
Cross-match and transfusion
| Group and save | One pink EDTA, filledHand-written at the bedside. See the Blood bottles card. |
|---|---|
| Two-sample rule | Two independent samplesMost trusts require a second confirmatory group from a separate venepuncture before non-emergency transfusion in a patient with no historical group. |
| Sample validity | Usually 3 daysShorter if transfused or pregnant within the last 3 months. Check local policy — a lapsed sample means a repeat bleed. |
Paediatric limits
| Single sample, well child | ≤ 1% of blood volumeCirculating volume is roughly 80 ml/kg — so about 8 ml/kg is total, and 1% of that is a small number. |
|---|---|
| Over 24 hours | ≤ 3% of blood volumeCumulative, and easy to breach on a sick neonate. Discuss with the lab before repeat sampling. |
| Neonates and small infants | Ask before you bleedPaediatric micro-tubes and minimum volumes vary by lab. A failed under-filled sample costs a second stab. |
Fill the culture bottles. If you take one thing from this card, it is that a pair of half-filled bottles from a septic patient is close to a wasted test, and the consequence lands days later when nobody can narrow the antibiotic.
Paediatric volume limits are not advisory — cumulative sampling anaemia in a sick neonate is a recognised iatrogenic harm.
Minimum volumes, sample validity and the two-sample rule are set by your local lab and transfusion policy and genuinely differ between trusts. The figures here are the common UK pattern — the chart on your wall is the one that governs.
Source: UK SMI B 37 blood culture investigation; BSH transfusion sample guidance; RCPCH paediatric sampling limits