Punktionsstelle an der Fingerbeere

A capillary puncture looks like the simplest part of a blood draw, yet it is the part where technique decides most. The Punktionsstelle is the side of the finger pad, roughly where the pad curves round towards the nail edge, and not the centre of the pad and not the very tip. The centre and the tip carry the densest nerve endings, so a puncture there hurts more than it has to, and on a hand that works the skin is often thicker there as well.

Which finger is used follows from the same reasoning.

• The middle or ring finger of the less used hand is the usual choice.
• The thumb and the index finger are avoided at a capillary puncture.
• The little finger has too little tissue over the bone.

Blood flow at the fingertip decides the rest, and the sequence that follows is fixed.

1. The hand is warmed first, by letting it hang down for a moment, by rubbing it, or with a warm cloth where one is available.
2. The disinfectant is applied and then allowed to dry completely, because a wet surface stings on puncture and dilutes the blood that follows.
3. The puncture is made in a single movement with an Einmalstechhilfe held flat against the skin.
4. The used device goes straight into the disposal container afterwards.
5. The first bead of blood is wiped away with a dry swab, because it is mixed with tissue fluid and with the remains of the disinfectant.
6. Everything after it is collected, with each bead allowed to form on its own and the vessel touched to the bead rather than to the skin.

That first bead is also the one least representative of the blood the laboratory measures, which is the reason it is discarded rather than collected.

The important error to avoid is squeezing the finger. Milking the finger to force blood out presses interstitial fluid into the sample and damages red cells on the way, which is how a capillary sample arrives showing Hämolyse and a potassium value that means nothing. Diluted, haemolytic capillary blood is the classic reason a kapillare Blutentnahme has to be repeated, and the repeat is considerably more unpleasant than getting the first attempt right. If the blood does not come, the answer is a warmer hand, the hand held lower and a fresh puncture at a different site, never more pressure on the finger.

All of this belongs to Präanalytik, before any analysis begins, and it is written down rather than left to habit, so that every person doing the draw chooses the same site and follows the same sequence. Hygiene bei der Blutentnahme covers the parallel requirements around the same few minutes, from gloves and skin disinfection through to disposal of the used lancet.