Dermal (Capillary) Puncture – Study Notes

Why Dermal Puncture?

Infants & Children (<2 years):

  • Superficial veins are too small; veins reserved for IV therapy

  • Deep vein puncture (e.g., femoral) risks: cardiac arrest, venous thrombosis, hemorrhage, infection, reflex arteriospasm (→ gangrene), tissue/organ damage

  • Small blood volumes — a 2-lb infant has only ~150 mL total blood; excessive draws → anemia

  • Certain tests require capillary blood (newborn screening, capillary blood gases)

Adults — Indications:

  1. Burned or scarred patients

  1. Chemotherapy patients (reserve veins for therapy)

  1. Thrombotic tendencies

  1. Fragile veins (geriatric)

  1. Inaccessible veins

  1. Obese patients

  1. Apprehensive patients

  1. Home glucose monitoring / point-of-care testing

Importance of Correct Collection

  • Small volumes → highly susceptible to hemolysis, contamination, clot formation

  • Hemolysis causes: excessive squeezing ("milking"), residual alcohol, vigorous mixing, difficult collection; newborn RBCs are more fragile

  • Contamination: tissue fluid, interstitial fluid, residual alcohol if first drop not wiped

  • Microclot formation: from slow collection or failure to mix anticoagulated tubes immediately

Key practices: correct site selection, let antiseptic dry, wipe first drop, avoid squeezing, mix tubes promptly

Composition of Capillary Blood

A mixture of:

  • Arterial blood

  • Venous blood

  • Capillary blood

  • Interstitial fluid

  • Intracellular fluid

Capillary blood resembles arterial blood more than venous (due to arterial pressure). Warming increases the arterial component (blood flow ↑ up to 7×).

Capillary vs. Venous Blood Differences

Test

Capillary vs. Venous

Glucose

Higher

Potassium

Lower

Calcium

Lower

Total Protein

Lower

⚠️ Always indicate collection method on requisition forms. Don't alternate between venipuncture and dermal puncture for serial results.

Equipment

  • Lancets (safety, retractable)

  • Microcollection tubes (~600 µL; color-coded caps; invert 5–10× after collection)

  • Capillary tubes (~50–75 µL) — for microhematocrit & blood gases; sealed with clay/plastic caps

  • Red = Heparinized

  • Blue = Plain

  • Glass slides, alcohol pads, gauze, heel warmers, sharps containers

Lancet Key Points

  • Max heel puncture depth: ≤ 2.0 mm (to protect calcaneus)

  • Incision width > depth — wider incision = better capillary blood flow

  • Surgical blades are prohibited

  • Types: contact-activated, color-coded safety, heelstick, fingerstick, laser lancets (up to 100 µL)

Warming the Puncture Site

  • Purpose: vasodilation → ↑ blood flow → easier collection, more arterial-like sample

  • Indications: cold/cyanotic extremities, multiple capillary samples, capillary blood gases

  • Method: warm moist towel or commercial heel warmer at ~42°C for 3–5 minutes

  • ⚠️ Excessive heat → burns or altered results

Cleansing the Site

  • Use 70% isopropyl alcohol in circular motion

  • Must air dry completely — residual alcohol causes hemolysis, stinging, dilution, poor blood drop formation

  • Avoid povidone-iodine — falsely ↑ bilirubin & potassium

Performing the Puncture

  • Support site firmly, do NOT squeeze

  • Position lancet perpendicular to skin grooves (fingerprints/heel lines) → produces rounded blood drop

  • Discard lancet immediately into sharps container

Sample Collection

  • Wipe away first drop (contains tissue fluid, alcohol, damaged cells)

  • Collect from free-flowing drops — no milking/squeezing

  • Touch container lightly to blood drop; never scrape the skin

Order of Collection (Dermal Puncture)

  1. Capillary blood gases

  1. Blood smears

  1. EDTA tubes

  1. Other anticoagulant tubes

  1. Serum tubes

(Order matters due to platelet accumulation at puncture site)

Post-Collection & Completion

  • Apply gauze + pressure; elevate site until bleeding stops

  • No bandages for infants <2 years (choking hazard)

  • Label specimens at bedside

  • Dispose of sharps, remove gloves, hand hygiene

  • Document: collection method, time, collector ID

  • Transport specimens to lab promptly

Puncture Sites

Heel Puncture (infants <1 year)

  • Acceptable: medial & lateral plantar surfaces of heel

  • Locate safe zone using 2 imaginary lines from big toe and 4th/5th toes to heel

  • Unacceptable:

    • Posterior curvature (back of heel) — calcaneus too close

    • Arch of foot — nerves, tendons, cartilage

    • Bruised, swollen, infected, or previously punctured areas

  • Complications of wrong site: osteomyelitis, osteochondritis, permanent bone damage

Finger Puncture (adults & children >1 year)

  • Preferred: fleshy distal segment of 3rd or 4th finger

  • Palmar surface, nondominant hand, slightly off-center (avoid fingertip)

  • Not for infants <1 year — insufficient tissue depth, risk of bone contact