Capillary (Dermal) Puncture Technique

Capillary puncture — also called dermal puncture or skin puncture — collects small volumes of blood from the capillary beds just beneath the skin. It is an essential alternative to venipuncture for specific patient populations and testing situations.

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When to Use Capillary Puncture

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Capillary Puncture Sites

Adults and Children Over 1 Year: Finger

The preferred site is the fleshy pad of the middle (3rd) or ring (4th) finger of the non-dominant hand:

Sites to AVOID (Fingers)

• Index finger — heavily used; more nerve endings; calloused
• Thumb — has a pulse (arterial); too thick
• Pinky (5th finger) — too thin; close to bone
• Fingers that are cold, cyanotic, swollen, or infected
• Fingers with previous puncture sites that have not healed

Infants Under 1 Year: Heel

The preferred site is the medial or lateral plantar surface of the heel:

Sites to AVOID (Heel)

• Center of the heel (posterior curvature) — too close to the calcaneus bone
• Arch of the foot — nerves, tendons, and ligaments
• Toes — too small; risk of bone damage
• Previous puncture sites — allow healing between sticks
• Areas that are bruised, edematous, or infected

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Capillary Puncture Equipment

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Capillary Puncture Procedure — Step by Step

1. Verify patient identity — two identifiers
2. Select the site — finger (adults/children) or heel (infants)
3. Warm the site for 3-5 minutes if circulation is poor (increases blood flow up to 7x)
4. Clean the site with 70% isopropyl alcohol; allow to air dry completely
5. Perform the puncture:
   • Hold the finger or heel firmly
   • Place the lancet perpendicular to the skin (across fingerprint lines for fingers)
   • Activate the lancet with firm, quick pressure
6. Wipe away the FIRST drop of blood — it contains tissue fluid (interstitial fluid) that dilutes the specimen
7. Collect specimens using gentle, intermittent pressure — do NOT squeeze or "milk" the finger/heel (this forces tissue fluid into the blood and dilutes/contaminates the specimen)
8. Follow the capillary order of collection:
   • 1st: EDTA tubes (lavender) — for CBC, etc.
   • 2nd: Other additive tubes
   • 3rd: Tubes without additives (serum)
9. Cap and mix microcollection tubes by gentle inversion
10. Apply pressure with clean gauze until bleeding stops
11. Apply adhesive bandage (NOT on infants under 2 years — choking hazard)
12. Label specimens at the bedside
13. Dispose of the lancet in a sharps container

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Capillary vs. Venipuncture Order of Collection

Why EDTA first in capillary collection? EDTA tubes for capillary specimens must be filled first because platelets aggregate (clump) quickly in capillary blood. Filling the EDTA tube first ensures accurate platelet counts and CBC results.

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Newborn Screening (Heel Stick Blood Spot Cards)

Most states require newborn screening within 24-48 hours of birth for metabolic and genetic disorders:

• Collected by heel puncture onto specially designed filter paper (blood spot cards)
• Completely fill each circle on the card with a single drop of blood (applied from one side only)
• Allow cards to air dry horizontally for at least 3 hours — do NOT stack, heat, or touch the blood spots
• Tests include: PKU (phenylketonuria), hypothyroidism, sickle cell disease, cystic fibrosis, and 30+ other conditions depending on the state

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Capillary Blood Composition Differences

Capillary blood is a mixture of arterial blood, venous blood, and interstitial (tissue) fluid. Because of this mixed composition, certain test results may differ between capillary and venous specimens:

Tests NOT Suitable for Capillary Specimens

• Blood cultures — require venous blood for adequate volume and sterility
• Coagulation tests (PT, PTT) — tissue fluid activates coagulation factors
• ESR (erythrocyte sedimentation rate) — requires venous blood
• Blood bank specimens — type and screen requires venous blood
• Tests requiring large volumes of blood

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Common Capillary Puncture Errors