Capillary or Dermal Blood Specimens: Principles and Procedures

Capillary or Dermal Blood Specimens

Introduction

Capillary blood specimen collection, also known as dermal or skin puncture, is a fundamental technique in clinical laboratory practice. It is especially important for pediatric patients and in situations where venipuncture is not feasible. This method provides a mixture of blood from arterioles, venules, capillaries, and tissue fluids, and is used for a variety of diagnostic tests.

Indications for Skin Puncture

• Benefits:

  • Diminished risk of iatrogenic anemia, sharps injury, and patient injury (e.g., nerve damage).

  • Less painful than venipuncture (perceived by patients).

  • Less costly and faster than venipuncture.

  • Easy for self-monitoring diabetic patients.

• Pediatric Use:

  • Primary use for children and infants to reduce risk of complications such as anemia, cardiac arrest, hemorrhage, venous thrombosis, and reflex arteriospasm.

  • Prevents gangrene, tissue/organ damage, infections, and injuries from restraining children.

• Adult Use:

  • Recommended for patients with severe burns, thrombotic tendencies, fragile veins (geriatrics), or obesity.

  • Helps save veins for therapy and enables home or point-of-care (POC) testing.

• Contraindications:

  • Not recommended when large blood volumes are needed, in swollen areas (risk of dilution with interstitial fluid), or in dehydrated/poorly circulated patients.

• Tests Not Suitable for Capillary Blood:

  • Coagulation studies, erythrocyte sedimentation rate (ESR), and blood cultures due to dilution and volume requirements.

Common Laboratory Tests Using Capillary Blood

• Blood smears for manual WBC differential

• Complete blood count (CBC), hemoglobin, and hematocrit (H&H)

• Electrolytes

• Neonatal blood gases, bilirubin, and screening

• Point-of-care or home/self-testing (e.g., glucose)

Composition of Capillary Blood

• Mixture of blood from arterioles, venules, capillaries, and intracellular/interstitial fluids.

• Arterial pressure is stronger than venous pressure, resulting in a slightly higher proportion of arterial blood.

Example:

Capillary blood is preferred for glucose testing because its composition more closely reflects arterial blood, which is important for accurate glucose measurement.

Preparation for Skin Puncture

• Emotional preparation and patient comfort are essential.

• Standard precautions: hygiene, gloves, safe positioning.

• Patient identification and cleanliness are critical.

• Assess patient for contraindications and site suitability.

Basic Technique for Collecting Capillary Blood Specimens

1. Greet, assess, and identify the patient; position for safety and comfort.

2. Perform hand hygiene and don gloves.

3. Prepare supplies and microcollection device.

4. Check for diet restrictions, latex sensitivity, and fainting risk.

5. Select and warm the site if necessary; cleanse with 70% isopropanol and allow to dry.

6. Open sterile puncture device in view of patient.

7. Perform puncture, wipe away first drop (unless for POC tests), and collect specimen.

8. Apply gentle pressure with gauze; discard lancet in biohazard container.

9. Label specimen and prepare for transport; ensure bleeding has stopped.

Supplies for Skin Puncture

• Puncture devices: needle/blade (vertical) or incision (slice into capillary bed).

• Disposable gloves, sterile retractable safety devices, warming packs, disinfectant pads, hypoallergenic bandages, gauze pads.

• Glass microscope slides, diluting fluids, microcollection tubes/capillary tubes, lab requisitions/labels, marking pen, biohazard container.

Skin Puncture Sites

• Preferred Sites:

  • Central palmar surface of distal phalanx of third (middle) or fourth (ring) finger of nondominant hand.

  • For infants <1 year: lateral or medial plantar surface of the heel.

• Sites Not Recommended:

  • Earlobe, central arch/posterior curve of infant's heel, fingers of newborns/infants <1 year, fifth (pinky) finger, thumb, index finger, swollen/infected/previously punctured sites, fingers on side of mastectomy, plantar surface of big toe.

Improving Site Selection

• Do not use cold fingers; warm site to increase arterial blood flow (e.g., warm towel at 42°C for 3–5 minutes).

• Lower the arm to pool blood in fingertips.

Cleansing the Skin Puncture Site

• Use 70% isopropanol pad; allow to dry to prevent hemolysis, stinging, and poor blood drop formation.

• Avoid iodine tincture (may falsely elevate potassium, phosphorus, uric acid).

Order of Collection

• After first drop is removed:

  1. Blood gases (if ordered)

  2. EDTA specimen for hematology tests (CBC, differential, platelet, Hgb/Hct)

  3. Other tubes with additives (green, gray, gold)

  4. Nonadditive tubes (red)

Blood Smears/Slides for Microscopic Analyses

• Manual blood smears confirm automated results and detect abnormalities.

• Slides must be free of ridges, lines, or holes; errors include too large a drop, delay in making smear, blowing on slide, or using chipped slides.

• Label and transport slides appropriately; discard unusable slides and biohazardous waste.

Other Considerations for Capillary Blood Samples

• Venous and capillary blood test results differ; note specimen type on lab results.

• Glucose concentration is higher in capillary blood; potassium, total protein, and calcium are lower compared to venous blood.

• Microhematocrit: collect into capillary tube with heparin, fill two-thirds full, seal immediately.

• Hematology specimens: use tubes with EDTA anticoagulants; avoid heparinized tubes unless only PCV is needed.

• Blood pH and gas determinations: warm site, use heparinized tubes without air bubbles, seal quickly.

Do Not Cause Hemolysis!

• Causes: residual alcohol, excessive milking/massaging, excessive mixing, increased RBC fragility, high PCV (especially in newborns/infants).

Summary Table: Capillary vs. Venous Blood Properties

Property	Capillary Blood	Venous Blood
Glucose	Higher	Lower
Potassium	Lower	Higher
Total Protein	Lower	Higher
Calcium	Lower	Higher

Additional info:

Capillary blood collection is a critical skill for clinical laboratory professionals and is relevant for microbiology students studying specimen collection, blood composition, and diagnostic testing.