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 minimally invasive method for obtaining blood samples. It is commonly used in clinical laboratories for specific patient populations and test types, offering distinct advantages and limitations compared to venipuncture.

Indications for Skin Puncture

• Benefits:

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

  • Less painful than venipuncture (often perceived as such by patients).

  • Lower cost in terms of equipment and supplies.

  • Faster procedure and suitable for self-monitoring (e.g., diabetic patients).

• Pediatric Use:

  • Preferred 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 during procedures.

• Adult Use:

  • Indicated 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 Puncture:

  • Coagulation studies (dilution effect).

  • Erythrocyte Sedimentation Rate (ESR) due to volume requirements.

  • Blood cultures (risk of contamination and insufficient volume).

Common Laboratory Tests Using Capillary Blood

• Manual WBC differential blood smears.

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

• Electrolyte analysis.

• Neonatal blood gases and bilirubin.

• Neonatal screening tests.

• Point-of-care or home glucose testing.

Composition of Capillary Blood

• Capillary blood is a mixture of:

  • Blood from arterioles, venules, and capillaries.

  • Intracellular and interstitial (tissue) fluids.

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

• Clinical Impact:

  • Glucose concentration is higher in capillary blood than venous blood.

  • Potassium, total protein, and calcium concentrations are lower in capillary blood.

Preparation for Skin Puncture

• Emotional preparation and patient comfort are essential.

• Standard precautions: hygiene, gloves, and safe positioning.

• Patient identification and cleanliness are critical.

• Assess patient for contraindications and 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 likelihood of fainting.

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 clean 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 devices (vertical entry).

  • Incision devices (small slice into capillary bed).

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

• Glass microscope slides, diluting fluids, microcollection tubes/capillary tubes, labels, marking pen, and 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 or neonates: 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% aqueous isopropanol pad; allow to dry thoroughly.

• Alcohol can cause hemolysis, contaminate glucose tests, cause stinging, and prevent round drops.

• Iodine tincture not recommended (can 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 are used for confirmation, detecting abnormalities, or as a backup method.

• Slides are treated with special stains to distinguish WBC morphology; blood drop is thinned out on slide.

• Errors include using too large a drop, delay in making smear, blowing on slide, or using chipped slides.

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

Other Considerations for Capillary Blood Samples

• Venous and capillary blood test results differ; laboratory results should note specimen type.

• Microhematocrit can be collected directly into capillary tube with heparin; fill at least two-thirds full and seal immediately.

• Hematology specimens should be collected in tubes with EDTA anticoagulant; heparinized tubes only for PCV.

• For blood pH and gas determinations, warm site to increase arterial flow and avoid air bubbles in heparinized tubes.

Prevention of Hemolysis

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

Summary Table: Capillary vs. Venous Blood

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

Additional info: Capillary blood sampling is essential for pediatric and point-of-care testing, but careful technique and site selection are critical to avoid complications and ensure accurate results.