Capillary blood sampling: finger-stick and neonatal heel-stick

Capillary blood collection — a skin puncture rather than a vein puncture — is the right tool when the volume needed is small, when a vein is difficult or unnecessary, or when the patient is an infant. It is the method behind a bedside glucose, a newborn screening card, and most point-of-care testing. It looks simpler than a venepuncture, and it is not: the site, the depth, and the handling of the first drop all change the result.

Where to puncture, by age

The single most important decision is the site, and it is decided by the patient's age.

Two sites are avoided in adults and older children. The thumb has its own pulse and can mislead; the fifth finger has too little tissue over the bone. In infants, the rule is firmer still.

Lancet depth

The lancet is chosen by depth, and the depth is matched to the patient so the puncture reaches the capillary bed without reaching bone:

• Premature infants: around 0.85 mm.
• Full-term infants: up to 2.0 mm.
• Children and adults: 1.5 to 2.4 mm.

A deeper lancet does not give a better sample; it gives a more painful puncture and, in a small heel, a dangerous one.

The technique that protects the sample

1. Warm the site if blood flow is poor — a warm compress can increase flow several-fold.
2. Clean with 70% alcohol and let it dry completely.
3. Puncture once, with a single deliberate motion.
4. Wipe away the first drop — it is diluted with tissue fluid and will skew the result.
5. Collect the drops as they form, without squeezing the site hard.

The two habits that matter most are wiping the first drop and not squeezing. Heavy squeezing — "milking" the finger or heel — forces tissue fluid into the sample and ruptures red cells, producing the same haemolysis that ruins a venous draw. Let the drops form under their own pressure.

The capillary order of draw is different

In a venous draw the additive tubes follow a set order of draw. Capillary collection uses a different order. The haematology sample is collected first, then chemistry, then any blood-bank sample — because platelets begin to clump at the puncture site the moment it is made, and the haematology sample is the one that clumping ruins. Treat this as a sequence that differs from the venous order rather than a strict mirror of it; the exact order follows CLSI H04-A6 and your local protocol.

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This guide is a free extract from PhlebMastery's phlebotomy theory course, with content aligned to published WHO guidance. The full treatment — capillary sampling alongside blood-culture, arterial, paediatric, and donor collection — is in Module 7: Special Collections & Advanced Techniques. New here? Start with the free Module 1, or see the whole course — full access is a one-time purchase.