Capillary puncture

A capillary puncture, also called a dermal or skin puncture, collects blood from the capillary bed just beneath the skin rather than from a vein. The sample is a mixture of arterial blood, venous blood, and interstitial and intracellular fluid, with arterial blood predominating — which is why capillary and venous results can differ slightly for some analytes.

Site selection depends on age. In adults and older children, the standard site is the lateral or medial side of the fleshy pad of the third or fourth finger, punctured perpendicular to the fingerprint lines. In infants under about one year, the medial or lateral plantar surface of the heel is used, avoiding the central heel and the posterior curvature to prevent bone injury. Fingers are not used in young infants because the tissue is too shallow over the bone.

Technique affects specimen quality. Warming the site increases blood flow, the puncture is made with a sterile lancet of appropriate depth, and the first drop is wiped away because it is diluted with tissue fluid. Strong squeezing or milking of the site is avoided since it causes hemolysis and contaminates the sample with interstitial fluid. Order of draw differs from venipuncture: blood gas specimens are collected first, then EDTA tubes, then other additive tubes, and serum tubes last, because platelets aggregate rapidly at the puncture site. Common complications include hemolysis, insufficient sample volume, and — with excessive depth in infants — osteomyelitis.

Capillary puncture is directly tested on the CCMA certification exam within the blood collection domain. Expect questions on site selection by patient age, correct puncture depth and orientation, the capillary order of draw, why the first drop is discarded, and the legal and professional issues surrounding specimen labeling and handling.