2.3 Capillary / Skin Puncture Protocols & Order of Draw

Capillary / Skin Puncture Protocols & Order of Draw

Capillary skin puncture (also known as dermal puncture) involves obtaining blood specimens from small arterioles, venules, and capillaries by puncturing the cutaneous layer of the skin. Because capillary blood represents a mixture of arterial blood, venous blood, and interstitial fluid, test results for certain analytes (e.g., glucose, total protein, potassium) differ slightly from venous blood. Mastering dermal puncture mechanics, anatomical selection, and the unique capillary order of draw is essential for certified phlebotomists.

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Section 1: Clinical Indications & Contraindications

Skin puncture is a preferred or mandatory collection method under specific clinical circumstances, particularly when venipuncture poses severe physiological or psychological risks.

Primary Clinical Indications

1. Infants Under 1 Year of Age: Deep venipuncture in infants can cause cardiac arrest, nerve damage, venous thrombosis, hemorrhage, or tissue infection. Furthermore, removing large blood volumes risks inducing iatrogenic anemia.
2. Adult Patients with Severely Compromised Veins: Patients undergoing extensive chemotherapy, burn victims, severe geriatric skin fragility, or individuals with severe scarring/thrombosis.
3. Extreme Obesity: Where deep antecubital veins are non-palpable.
4. Point-of-Care Testing (POCT): Bedside capillary blood glucose monitoring, hemoglobin A1c, or rapid micro-hematocrit determination.
5. Patient Apprehension: Extreme phobia of venipuncture needles.

Absolute Contraindications

• Severe Dehydration or Peripheral Edema: Edematous tissue contains excess fluid that dilutes capillary blood, yielding inaccurate blood counts.
• Poor Peripheral Circulation / Shock: Reduced perfusion impairs blood flow to distal extremities.
• Tests Requiring Large Blood Volumes: Blood cultures, ESR (Erythrocyte Sedimentation Rate), and coagulation studies requiring sodium citrate (PT/aPTT) cannot be drawn via capillary puncture.

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Section 2: Anatomical Puncture Sites & Depth Limits

Selecting the correct anatomical site and adhering to strict lancet depth limits prevents catastrophic bone injuries such as osteomyelitis (bone infection) or osteochondritis (bone/cartilage inflammation).

Adult & Child Puncture Site (>1 Year Old)

• Approved Site: The palmar surface of the distal phalanx of the 3rd (middle) finger or 4th (ring) finger of the non-dominant hand.
• Puncture Alignment: Perform the puncture perpendicular to the fingerprint whorls (ridges). Puncturing perpendicular allows blood to form a rounded, standing drop. Puncturing parallel to the ridges causes blood to run down the grooves of the finger, making collection impossible.
• Sites to Avoid: Index finger (calloused and highly sensitive), pinky finger (tissue is too thin over bone), thumb (has a pulse and callouses), and big toe.

[Fingerprint Ridges: ||||||]  ===>  [Lancet Blade Cut: ------ (Perpendicular)]  ===>  Forms Round Drop

Infant Heel Puncture Site (<1 Year Old)

• Approved Site: The medial or lateral plantar surfaces of the heel (the outer boundaries of the bottom of the heel).
• Strict Boundary Rule: Draw an imaginary line extending backward from the middle of the great toe to the heel, and another from between the 4th and 5th toes to the heel. Puncture only outside these lines.
• Sites to Avoid: The central posterior curve of the heel, the back of the heel, or previous puncture sites. Puncturing the central calcaneus area risks striking the heel bone.

Lancet Blade Depth Limits

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Section 3: Step-by-Step Capillary Puncture Protocol

Step 1: Site Warming (Thermal Hyperemia)

Warming the site increases arterial blood flow up to 7-fold, transforming the capillary bed into an "arterialized" specimen. Apply a warm washcloth or commercial heel-warmer pack (warmed to no more than 42°C) to the site for 3 to 5 minutes.

Safety Alert: Temperature must never exceed 42°C (107.6°F) to prevent severe thermal skin burns on delicate infant skin.

Step 2: Sanitization & Air Drying

Cleanse the site with 70% isopropyl alcohol. Allow the skin to air-dry completely. Residual alcohol causes intense burning, destroys red blood cells (hemolysis), and prevents blood from forming a rounded drop.

Step 3: Puncture Execution & Safety Disposal

Position the safety lancet firmly against the skin perpendicular to the fingerprint whorls or on the outer plantar heel. Depress the trigger mechanism, instantly retracting the blade. Discard the used lancet immediately into a sharps container.

Step 4: The Mandatory First Drop Wipe

Wipe away the very first drop of blood with a clean, dry 2x2 gauze pad.

• Clinical Rationale: The first drop contains tissue thromboplastin, cellular debris, and interstitial fluid released from damaged skin cells, along with potential residual alcohol. Including the first drop dilutes the sample, causes false low platelet/RBC counts, and triggers premature clotting.

Step 5: Specimen Collection & Flow Enhancement

Apply gentle, intermittent pressure proximal to the puncture site to encourage blood drop formation. Never squeeze, pump, or milk the finger/heel forcefully. Excessive squeezing causes hemolysis and forces interstitial fluid into the sample (tissue contamination).

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Section 4: Capillary Order of Draw

The capillary order of draw is fundamentally different from the venipuncture order of draw. Platelets aggregate rapidly at skin puncture sites to form microscopic clots. Therefore, hematology tubes containing EDTA must be collected first before microclotting begins.

Venipuncture Order: Blood Cultures --> Light Blue --> SST/Red --> Green --> Lavender --> Gray
Capillary Order:    Lavender (EDTA) FIRST --> Green/Other Additives --> Red/SST (Serum) LAST