When venipuncture is not feasible, such as in infants, elderly patients with fragile veins, or patients requiring frequent glucose monitoring, capillary or dermal puncture may be performed to obtain a small blood specimen. In infants under one year of age, or until the child is walking, the heel stick is preferred over a finger stick because the fingers lack sufficient tissue depth. The puncture is performed on the medial or lateral plantar surface of the heel, never on the posterior curvature, where penetration of the calcaneus bone could lead to osteomyelitis. The maximum depth of a heel stick puncture is 2.0 mm. The site should not have any previous puncture wounds, bruising, infection, or edema, and warming the heel before the puncture increases blood flow by as much as sevenfold, arterializing the specimen and improving its quality.
In older children and adults, the preferred capillary puncture site is the distal, palmar surface of the middle or ring finger. The thumb and index finger should be avoided because they have more calluses and nerve endings, while the pinky lacks sufficient tissue depth. The puncture should be made perpendicular to the fingerprint ridges rather than parallel to them, which allows blood to bead up on the surface for easier collection. Maximum depth for an adult fingerstick is approximately 2.0 to 3.0 mm, depending on the device used. A retractable lancet with a safety mechanism is typically used to control depth and reduce the risk of needlestick injury.
The order of draw for capillary specimens differs from that used in venipuncture because capillary blood clots quickly. The recommended sequence is blood gases first, followed by EDTA tubes for hematology (such as CBC and slides), then other additive tubes, and finally serum tubes. Drawing the EDTA tube early minimizes the formation of microclots that could interfere with cell counts. The first drop of blood produced after the puncture is wiped away, as it often contains excess tissue fluid that would dilute the specimen and skew results. Squeezing or "milking" the finger or heel to obtain additional blood is discouraged, as this causes hemolysis and contaminates the specimen with tissue fluid, producing inaccurate results.