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Capillary FBC: A Reliable Count from a Finger or Heel Prick

A finger-prick FBC spares a child a needle, but capillary blood clots faster, dilutes more easily and does not always match a venous sample. How to collect it, run it and know its limits.

Also called Capillary FBC, Finger-prick blood count, Heel-prick FBC, Microcollection EDTA tube

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When a capillary FBC makes sense

Capillary blood is a mix of blood from small arteries, veins and capillaries, with a little tissue fluid. For most FBCs on most adults, a venous sample remains the reference; capillary sampling earns its place where a vein is hard to reach or the blood volume is small.

  • Infants and young children, where veins are small and every millilitre counts.
  • Adults with very difficult veins, burns or other reasons a venepuncture is impractical.
  • Point-of-care testing where only an FBC is needed and the result is wanted during the visit.

Two ways to run it

Microcollection EDTA tube

A small lavender-top tube collects a few hundred microlitres from the puncture and is run in the analyser’s normal whole-blood mode. Check that your analyser can aspirate from that tube and that the volume is above its minimum.

Pre-diluted mode

A small, exactly measured volume of blood is mixed with a set volume of diluent and run in the analyser’s pre-diluted mode, which corrects for the dilution. It needs the least blood, but any pipetting error goes straight into the result, and the diluted sample should be run promptly.

Which to choose

Microcollection tubes are simpler and allow a rerun from the same tube; pre-diluted mode suits the smallest babies. Whichever you use, verify that mode on your own analyser and include it in your QC routine.

Verifying each sample mode

Collection technique

  • Warm the hand or heel first so blood flows freely without squeezing.
  • Fingers: use the side of the fingertip of the middle or ring finger. Babies: use the sides of the heel, never the back of the heel or the arch, where the bone lies close to the skin.
  • Use a single-use, retractable lancet made for the patient’s age and size.
  • Clean with alcohol, let it dry, puncture, then wipe away the first drop, which carries tissue fluid.
  • Let drops form and touch them to the tube. Avoid hard squeezing or “milking”, which adds tissue fluid and activates platelets.
  • Fill the EDTA tube first. WHO’s capillary job aid puts haematology samples first — the reverse of the venous order — because platelets start clumping at the wound within moments.
  • Mix the tube as you collect, fill to the marked range, cap it and label it beside the patient.

Read next Tube colours and the venous order of draw

How capillary results differ from venous

Studies do not agree on the size of the differences, and they vary with technique, analyser and patient group. These are the patterns most often reported:

ParameterTypical finding in capillary bloodWhy
PlateletsOften lowerPlatelets begin to clump at the puncture; slow collection makes it worse
Haemoglobin, haematocrit, red cellsSlightly higher in several studiesA different blood mix at the skin; tissue fluid from squeezing pulls the other way
White cellsSimilar in some studies, higher in othersCollection technique and patient group
RepeatabilityPoorer than venousEach drop differs, and small clots are easy to miss

Read next When a low platelet count is not real

Using capillary results safely

  • Keep one sample type for a patient you are following — a child with dengue, for instance. Switching between venous and capillary from day to day blurs the trend.
  • Treat a low capillary platelet count with caution: check the flags and histogram, and recollect, from a vein if possible, before anyone acts on it.
  • Write “capillary” on the request and the report so the result is read accordingly.
  • Draw a venous sample when a borderline, flagged or unexpected capillary result will decide referral or treatment.
  • Avoid capillary sampling in patients who are dehydrated, in shock or with poor circulation to the hands and feet; skin blood may not reflect the circulation.

Read next The FBC in dengue monitoringEDTA sample handling

Related guides

Blood Test Machine for Clinic: Choosing a Hematology Analyzer for Your GP PracticeA practical decision guide for GP clinics weighing an in-house FBC against sending samples to an outside lab.EDTA Blood Samples: Handling That Protects Every FBC ResultMost wrong FBC results start before the sample reaches the analyser. A step-by-step guide to the tube, the draw, the mix and the wait.Blood Tube Colours and the Order of Draw, ExplainedWhich tube each test needs, the order to fill them in, and why the wrong cap colour — an FBC in a citrate tube, say — gives a wrong result rather than no result.

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Frequently asked questions

Is a finger-prick FBC as accurate as a venous one?
It can be close when collected well, but capillary platelet counts often read lower and repeat samples agree less closely. Use venous blood when a borderline result will decide treatment or referral.
Why wipe away the first drop of blood?
The first drop carries tissue fluid and traces of the alcohol swab, which dilute the sample and can make cells clump. Wiping it away gives a more representative sample.
What is pre-diluted mode on a haematology analyser?
A mode in which a small, exactly measured volume of blood is diluted before it is run, and the analyser corrects for the dilution. It suits babies and other cases where very little blood is available.
Can a baby have an FBC from a heel prick?
Yes. Use the sides of the heel, never the back or the arch, with a lancet made for infants, and fill the EDTA microcollection tube first while mixing it.

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