3.4 Capillary Puncture and Dermal Blood Collection

3.4 Capillary Puncture and Dermal Blood Collection

Capillary puncture (also known as dermal puncture or skin puncture) is an essential phlebotomy technique utilized when venipuncture is contraindicated, technically infeasible, or hazardous. Understanding skin anatomy, lancet depth limits, correct order of draw, and proper site selection is essential to avoid severe complications such as bone infection.

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1. Clinical Indications for Capillary Puncture

Capillary puncture involves collecting a mixture of blood from arterioles, venules, and capillaries, along with interstitial and intracellular fluids.

Primary Indications

1. Pediatric Patients (Infants < 1 Year): Venipuncture in infants presents severe risks, including cardiac arrest from restraint, deep vein laceration, nerve damage, and iatrogenic anemia (anemia caused by excessive blood withdrawal). Dermal collection is the preferred method for infants.
2. Adult Patients with Compromised Vasculature:
   • Extremely fragile, superficial, or thrombosed veins.
   • Severe burns, scarring, or bilateral edema covering upper extremities.
   • Obesity where antecubital veins cannot be identified.
   • Mastectomy on one side and an active IV line on the other.
3. Frequent Self-Monitoring & Point-of-Care (POC) Testing: Bedside blood glucose monitoring, microhematocrit, hemoglobin, HbA1c, and cholesterol screening.
4. Patients with Severe Thrombotic Tendencies: Patients at high risk for deep vein thrombosis where venipuncture poses vessel trauma.

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2. Equipment Selection & Puncture Depth Standards

Dermal collection relies on sterile single-use retractable micro-collection lancets. Selecting the correct puncture depth is critical to ensure adequate blood flow without contacting underlying bone.

Depth & Location Standards

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3. Anatomical Site Selection Rules

Adult & Child (> 1 Year) Fingerstick Selection

• Preferred Fingers: The 3rd (middle) and 4th (ring) fingers of the non-dominant hand.
• Puncture Site: The palmar surface of the distal phalanx, slightly off-center from the fleshy tip.
• Orientation: The puncture line MUST be executed PERPENDICULAR to the fingerprint whorls (ridges). Puncturing parallel to fingerprint ridges causes blood to flow down the grooves of the finger rather than forming a neat, rounded drop.
• Sites to Avoid:
  • 1st (Thumb): Too thick, calloused, and has an active pulse.
  • 2nd (Index Finger): Highly sensitive with dense nerve endings and callouses.
  • 5th (Little Finger): Tissue depth above the phalanx bone is too thin (<2.0 mm).
  • Swollen, cyanotic, scarred, infected, or cold fingers.

Infant (< 1 Year) Heelstick Selection

• Approved Zone: The medial or lateral plantar surface of the heel (the outer boundaries of the heel sole).
• Sites Strictly Prohibited:
  • Posterior Curvature / Center of Heel: High risk of calcaneus bone damage.
  • Arch of the Foot: Contains nerves and tendons.
  • Swollen, bruised, or previously punctured heel sites.
  • Fingers of infants under 1 year: Distance from skin to bone is extremely short.

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4. Step-by-Step Capillary Puncture Procedure

Step 1: Site Preparation & Warming

• Site Warming: Warming the site for 3 to 5 minutes using a warm washcloth or commercial warming pack (max 42°C) increases capillary blood flow up to 7-fold.
• Antisepsis: Cleanse the site with a 70% isopropyl alcohol pad.
• Crucial Rule: Allow alcohol to air dry completely. Do NOT use Povidone-Iodine (Betadine) for skin punctures, as povidone-iodine falsely elevates laboratory values for uric acid, phosphorus, potassium, and bilirubin (BURP tests).

Step 2: Puncture Execution & Initial Drop Disposal

• Execution: Position the lancet firmly against the prepared site (perpendicular to whorls on finger, or on outer plantar heel). Depress the release button to deploy the safety blade. Dispose of the lancet immediately into a biohazard sharps container.
• CRITICAL STEP — Wipe Away the First Drop: Always wipe away the very first drop of blood using a clean, dry sterile gauze pad.
  • Clinical Rationale: The first drop contains intracellular fluids, tissue thromboplastin, and residual alcohol that contaminate the specimen, cause hemocoagulation, and alter cell counts.

Step 3: Specimen Collection & Microcontainer Handling

• Collection Technique: Apply gentle, intermittent pressure proximal to the puncture site to encourage blood flow. Do NOT squeeze ("milk") the site continuously, as excessive squeezing causes hemolysis and dilutes the blood sample with interstitial tissue fluid.
• Collection Containers: Touch the tip of the microcollection tube (microtainer) or capillary tube to the blood drop, allowing blood to flow into the container by capillary action.
• Mixing: Tap EDTA microtainers gently throughout collection to prevent microclots, and invert additive microtainers 8 to 10 times immediately after sealing.

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5. Dermal Order of Draw vs. Venipuncture Order

The order of draw for capillary specimens is fundamentally different from the venipuncture order of draw. Capillary action and tissue trauma induce rapid platelet activation. Therefore, hematology specimens must be collected first to prevent microclot formation.