Capillary Collection

Role and Rationale for Capillary Collection

Capillary collection (also called skin puncture or microcollection) is a blood-drawing technique used to obtain small volumes of blood through dermal capillaries. It is the preferred method for infants, children, elderly patients with fragile veins, patients with difficult venous access, point-of-care testing, and when only a small volume of blood is needed.^[1]

For the phlebotomy certification exam, you must know the correct sites, equipment, order of draw for capillary specimens, and common errors. Capillary blood differs slightly from venous blood in composition (higher glucose, slightly lower protein), so you need to understand those differences.^[2]

Core Equipment, Sites, and Capillary Order of Draw

• Capillary blood: A mixture of arterial and venous blood from dermal capillaries, arterioles, and venules. Contains higher glucose and electrolyte values than venous blood.^[2]
• Microcollection: Technique using capillary tubes or microtainers (plastic tubes with small capacities, e.g., 200–600 µL).
• Lancet: Sterile, disposable device used to puncture the skin. Safety-engineered lancets are now standard to avoid needlestick injuries.^[3]
• Order of draw (capillary): Because capillary blood is not subject to the same additive interactions as venipuncture, the order is slightly different: blood gas samples (if using heparinized capillary tubes) first, then EDTA, other anticoagulants, and finally serum specimens.^[4]
• Site selection: Most common sites are the fingertip (palmar surface of the distal phalanx, avoiding the sides and tip) and heel (medial or lateral plantar surface) for infants <1 year old.^[1]

Standard Capillary Collection Steps and Neonatal Modifications

Step-by-Step Capillary Collection Procedure

1. Verify patient identification using two identifiers (name, date of birth) per facility policy.^[5]
2. Select the appropriate site: For patients over 6 months, the third or fourth finger (ring or middle finger) is preferred—avoid the index finger (callused) and little finger (thin tissue).^[1] For infants, use the lateral or medial plantar surface of the heel; never use the central arch to avoid bone injury.^[6]
3. Warm the site for 3–5 minutes using a warm towel or commercial warming device (not hot water) to increase blood flow up to 7×.^[2]
4. Clean the site with 70% isopropyl alcohol and allow it to air-dry completely to avoid hemolysis and contamination.^[3]
5. Perform the puncture using a sterile lancet perpendicular to the fingerprint lines on a finger, or at an appropriate depth on a heel (typically 2.0 mm or less for preterm infants).^[6]
6. Wipe away the first drop (contains tissue fluid and may be contaminated) unless performing a blood smear or certain POCT tests that allow the first drop.^[4]
7. Collect blood gently by allowing it to flow freely into the microcollection tube; do not “milk” the finger (excessive squeezing causes hemolysis and dilution with tissue fluid).^[2]
8. Follow the correct order of draw: Blood gas → EDTA → other anticoagulants → serum specimens.^[4]
9. Mix additives by inverting tubes gently (e.g., EDTA tubes 8–10 times) to prevent clotting.^[5]
10. Apply pressure with a sterile gauze to the puncture site until bleeding stops — do not apply adhesive bandages to infants or patients with sensitivity.^[3]
11. Label all tubes immediately at the bedside with patient identifiers, date, time, and collector initials.^[5]

Special Considerations for Infants

• Heel-puncture depth: maximum 2.0 mm for preterm infants, 2.4 mm for full-term infants to avoid calcaneal bone puncture.^[6]
• Use automated lancets designed for heel sticks — they have controlled depth (usually 1.75–2.0 mm).^[6]
• Warming the heel is especially important; use a commercial heel warmer or warm cloth (≤42°C).^[1]

Characteristic Differences and Quality Indicators

Capillary blood specimens have some characteristic differences from venous blood that phlebotomists must recognize:

• Higher glucose, lactate, and potassium compared to venous blood — important for interpreting results.^[2]
• May show hemolysis if excessive squeezing was used, causing falsely elevated potassium and hemolysis index flags.
• Clotting indicates improper mixing with anticoagulants or failure to invert tubes.
• Insufficient volume (short samples) lead to rejection; always fill to the marked line on microtainers.

Identifying Common Procedural Errors and Pitfalls

On the exam, you may be asked to evaluate a capillary collection scenario for errors:

• Check site condition: Puncturing through calluses, scars, or skin infections is contraindicated.^[1]
• Check warming technique: Inadequate warming yields insufficient flow and hemoconcentration.
• Evaluate order of draw: Mixing EDT A tubes before blood gas tubes can contaminate the gas sample with anticoagulant.
• Assess for contamination: If alcohol is not dry, the sample may be hemolyzed or diluted.

Troubleshooting Difficult Collections and Patient Aftercare

• For difficult collections: Re-warm the site; do not puncture more than twice — if unsuccessful, consider alternative sites or venipuncture.^[5]
• For bleeding patients (e.g., anticoagulated): Apply firm pressure for at least 2 minutes; monitor for hematoma formation.^[3]
• After collection: Properly dispose of lancet into a sharps container, remove gloves, and perform hand hygiene.^[3]

Key Safety Hazards and Preventive Measures

• Needlestick injury: Use safety-engineered lancets that retract after activation — never manually remove the lancet cap.^[3]
• Infection: Perform hand hygiene before and after, and wear clean gloves for each patient.
• Bone injury (heel stick): Avoid puncturing the posterior curvature of the heel (calcaneus). Depth should be controlled.^[6]
• Hemolysis: Avoid excessive squeezing, allow alcohol to dry, and ensure collected blood flows freely.
• Burns: If using warming devices, do not exceed 42°C for any site.^[1]

Certification Exam Emphasis and Quick Recall Mnemonics

• Remember the order of draw for capillary: Blood gas (if using heparin capillary tubes) → EDTA → other additive tubes → serum.
• When to choose capillary over venipuncture: Infants, elderly, burned patients, patients with difficult veins, tests requiring small volumes (e.g., glucose, bilirubin in newborns).^[2]
• Why wipe away the first drop? To remove tissue fluid and possible alcohol contamination — improves accuracy of results.^[4]
• Common exam scenarios: A patient with cold hands — warm the site before puncture. Patient who is anxious — ensure proper positioning and support.
• Mnemonic for capillary order of draw: "B.E.S.T." — Blood gas, EDTA, Serum (and other additives).

References & Sources

1. Clinical and Laboratory Standards Institute (CLSI). Procedures and Devices for the Collection of Diagnostic Capillary Blood Specimens; Approved Standard. 7th ed. CLSI document GP42-A7. Wayne, PA: CLSI; 2018. https://clsi.org
2. McCall RE, Tankersley CM. Phlebotomy Essentials. 7th ed. Burlington, MA: Jones & Bartlett Learning; 2020. https://books.google.je/books?id=Rjav-IpKCwQC&printsec=frontcover#v=onepage&q&f=false
3. Centers for Disease Control and Prevention (CDC). Workbook for Designing, Implementing, and Evaluating a Sharps Injury Prevention Program. 2008. https://www.cdc.gov/niosh/docs/2008-101/
4. Garza D, Becan-McBride K. Phlebotomy Handbook: Blood Specimen Collection from Basic to Advanced. 10th ed. Upper Saddle River, NJ: Pearson; 2019. https://www.pearson.com
5. World Health Organization (WHO). WHO guidelines on drawing blood: best practices in phlebotomy. Geneva: WHO; 2010. https://www.who.int/publications/i/item/9789241599221
6. American Academy of Pediatrics (AAP), Committee on Fetus and Newborn. Prevention and Management of Procedural Pain in the Neonate: An Update. Pediatrics. 2016;137(2):e20154271. https://doi.org/10.1542/peds.2015-4271