Capillary or Dermal Blood Specimens: Principles and Procedures

Capillary or Dermal Blood Specimens

Introduction

Capillary blood collection, also known as dermal puncture, is a minimally invasive method for obtaining blood specimens. It is especially useful in pediatric, geriatric, and certain adult populations where venipuncture is impractical or poses higher risk. This chapter outlines the indications, procedures, and considerations for capillary blood collection, as well as the differences between capillary and venous blood samples.

Indications for Skin Puncture

Benefits and Uses

• Reduced risk of complications: Lower chance of iatrogenic anemia, sharps injury, and nerve damage compared to venipuncture.

• Less pain: Generally perceived as less painful by patients.

• Cost and convenience: Requires less equipment and is faster, making it suitable for self-monitoring (e.g., diabetic patients).

• Pediatric use: Preferred for children and infants to reduce risks such as anemia, cardiac arrest, hemorrhage, venous thrombosis, and reflex arteriospasm.

• Special adult populations: Used for patients with severe burns, thrombotic tendencies, fragile veins, or obesity; also for home or point-of-care (POC) testing.

Contraindications

• Not suitable when: Large blood volumes are needed, the area is swollen (risk of dilution with interstitial fluid), or the patient is dehydrated or has poor circulation.

• Tests not suitable for capillary blood: Coagulation studies, erythrocyte sedimentation rate (ESR), and blood cultures due to volume or contamination concerns.

Common Laboratory Tests Using Capillary Blood

• Blood smears for manual white blood cell (WBC) differential

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

• Electrolytes

• Neonatal blood gases and bilirubin

• Neonatal screening

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

Composition of Capillary Blood

Differences from Venous Blood

• Capillary blood is a mixture of blood from arterioles, venules, capillaries, and interstitial/intracellular fluids.

• It contains a slightly higher proportion of arterial blood due to stronger arterial pressure.

• This composition can affect laboratory test results, making it important to note the sample type on lab reports.

Preparation for Skin Puncture

• Be emotionally prepared and ensure patient comfort.

• Follow standard precautions: hand hygiene, gloves, and safe positioning.

• Focus on accurate patient identification and cleanliness.

• Assess the patient for any contraindications or special needs.

Basic Technique for Collecting Capillary Blood

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, if needed, warm the puncture site; cleanse with 70% isopropanol and allow to dry.

6. Open sterile puncture device in view of the patient.

7. Perform the puncture, wipe away the first drop (unless otherwise specified), and collect the specimen.

8. Apply gentle pressure with gauze, discard lancet in biohazard container, label specimen, and ensure bleeding has stopped.

Supplies for Skin Puncture

• Puncture devices: Needle/blade (vertical puncture) or incision devices (small slice).

• Other supplies: Disposable gloves, warming packs, disinfectant pads, hypoallergenic bandages, gauze pads, glass slides, diluting fluids, microcollection tubes, labels, marking pen, and biohazard container.

Skin Puncture Sites

Preferred Sites

• Fleshy, central palmar surface of the distal phalanx of the third (middle) or fourth (ring) finger of the 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 infants <1 year, fifth (pinky) finger, thumb, index finger, swollen/infected/previously punctured sites, fingers on the side of a mastectomy, and plantar surface of the big toe.

Improving Site Selection

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

• Lower the arm to allow gravity to pool blood in the fingertips.

Cleansing the Skin Puncture Site

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

• Avoid iodine tincture preparations, which can falsely elevate potassium, phosphorus, or uric acid results.

Order of Collection for Capillary Blood

• After removing the first drop:

  • Blood gases (if ordered)

  • EDTA specimens for hematology (CBC, differential, platelet count, hemoglobin, hematocrit)

  • Other tubes with additives (green, gray, gold)

  • Nonadditive tubes (red)

Blood Smears/Slides for Microscopic Analyses

Purpose and Procedure

• Used for manual WBC differential and confirmation of automated results.

• Slides must be clean and free of ridges, lines, or holes.

• Errors can result from too large a drop, delay in making the smear, blowing on the slide, or using a chipped slide.

• Label and transport slides appropriately; discard unusable slides and waste in biohazard containers.

Other Considerations for Capillary Blood Samples

• Venous and capillary blood test results may differ; always note the sample type.

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

• Microhematocrit can be collected directly into heparinized capillary tubes, filled two-thirds full and sealed immediately.

• Hematology specimens should use EDTA tubes; heparinized tubes only if PCV is the sole test.

• For blood pH and gases, warm the site and avoid air bubbles in heparinized tubes.

Prevention of Hemolysis

• Causes include residual alcohol, excessive milking, over-mixing, and increased RBC fragility (notably in newborns/infants).

Summary Table: Capillary vs. Venous Blood

Parameter	Capillary Blood	Venous Blood
Glucose	Higher	Lower
Potassium, Total Protein, Calcium	Lower	Higher
Sample Composition	Arterial, venous, capillary, interstitial fluids	Primarily venous blood

Example:

Capillary blood collection is commonly used for neonatal screening tests, where only a small volume is needed and venipuncture would be too invasive or risky.

Additional info: Capillary blood collection is a critical skill in clinical microbiology and laboratory medicine, as it enables rapid, minimally invasive testing for a variety of analytes, especially in vulnerable populations.