Capillary Collection

Capillary Collection as a Core Phlebotomy Skill

Capillary collection is a fundamental phlebotomy skill used to obtain small volumes of blood for point-of-care testing (POCT) or when venipuncture is not feasible (e.g., fragile veins, infants, geriatric patients). This method relies on the capillary bed—a network of arterioles, venules, and capillaries—to provide a specimen that is a mixture of arterial and venous blood. For the medical assistant, mastery of capillary puncture ensures accurate test results, minimizes patient discomfort, and reduces the risk of hemolysis or tissue fluid contamination. High-yield exam topics include site selection, order of draw for microcollection, and safety precautions.[1][2]

Clinical Vocabulary and Microcollection Tube Standards

  • Capillary blood: A combination of arterial and venous blood with slightly higher oxygen content than venous blood. It is used for tests such as glucose, hemoglobin, hematocrit, and newborn screening.
  • Microcollection containers: Small tubes (e.g., Microtainer®) containing additives (EDTA, heparin, fluoride). Color coding differs from standard tubes.
  • Lancet: A sterile, single-use device that makes a shallow incision. Safety-engineered lancets are required for injury prevention.[3]
  • First drop discard technique: The initial blood drop is wiped away because it may contain excess tissue fluid and microbes. The second drop is used for testing (except for certain glucose meters).
  • Order of draw for capillary collections: To prevent additive carryover, follow: blood gas tubes → EDTA tubes → other additive tubes → serum tubes (without additives).[1]

Anatomical Site Selection and Collection Protocol

Site Selection

  • Finger (adults and children over 1 year): Use the middle or ring finger—lateral aspect of the fingertip (avoid the pad). The index and pinky fingers are less ideal due to calluses or thin skin.[2]
  • Heel (infants under 1 year): The medial or lateral plantar surface of the heel. Avoid the posterior curve (calcaneus) to prevent bone injury. Maximum puncture depth: 2.0 mm.[4]
  • Earlobe (rarely used): Permitted for certain specialized tests (e.g., acid-base balance).

Step-by-Step Capillary Puncture Procedure

  1. Gather supplies: Clean gloves, lancet, alcohol swab, gauze, microcollection tubes, bandage, sharps container.
  2. Identify the patient using at least two identifiers per facility policy.
  3. Warm the site (optional): Apply a warm compress (42 °C) for 3–5 minutes to increase blood flow.
  4. Clean the site with 70% isopropyl alcohol. Allow to dry completely to prevent hemolysis and stinging.
  5. Select a lancet depth based on site and patient age (e.g., 2.0 mm for pediatric heel, 1.8 mm for adult finger).
  6. Perform the puncture: Hold the lancet perpendicular to the skin. Press firmly and activate the device; then discard the lancet immediately into the sharps container.
  7. Wipe away the first drop of blood with dry gauze.
  8. Collect blood: Gently milk the finger (do not squeeze excessively to avoid hemolysis). Fill microcollection tubes according to the manufacturer’s fill line.
  9. Mix additive tubes by gentle inversion (usually 8–10 times).
  10. Apply pressure to the puncture site with gauze until bleeding stops. Do not use alcohol after puncture.
  11. Label specimens immediately at the patient’s side, including date, time, and collector initials.
  12. Transport specimens promptly according to test stability requirements.[1][2]

Infection Control and Specimen Integrity Safeguards

  • Infection control: Use standard precautions. Dispose of all sharps in a puncture-resistant container immediately after use.[3]
  • Hematoma: Avoid puncturing too deeply or through a previous puncture site. Apply adequate pressure post-procedure.
  • Hemolysis: Causes include excessive squeezing, inadequate drying of alcohol, and using a cold site. Hemolyzed specimens are rejected for most tests (e.g., potassium, LDH).
  • Contamination with tissue fluid: Wiping the first drop and preventing excessive milking reduces this risk.
  • Bone injury (infants): Never use the central heel; limit depth to 2.0 mm for neonates.[4]
  • Needlestick injury prevention: Use only safety-engineered lancets that retract after use.

Testable Priorities and Common Pitfalls

  • Preferred finger: Middle or ring finger. The little finger has thin tissue; the index finger is more callused and sensitive.
  • First drop discard: Required for most capillary tests (except some glucose meters). Know that the first drop may contain elevated potassium or decreased glucose due to tissue fluid.
  • Order of draw for microcollection: Blood gas (heparin) → EDTA (lavender) → other additives (green, gray) → red (serum). This is regularly tested on the CMA (AAMA) exam.[1]
  • Heel puncture: Only the lateral or medial plantar surfaces; avoid the arch and posterior heel. Maximum depth 2.0 mm for infants.
  • Common complications: Hemolysis, clotting, insufficient volume, and site infection.
  • Patient comfort: Warm the site, use a sharp lancet, and avoid excessive squeezing. Provide distraction for pediatric patients.
  • CLSI guidelines: The Clinical and Laboratory Standards Institute (CLSI) document H04-A6 is the gold standard for capillary puncture procedures.[5]

References

  1. McCorry, L. K., & Mason, D. J. (2021). Phlebotomy: A Competency-Based Approach (5th ed.). McGraw-Hill Education. https://www.mheducation.com/highered/product/phlebotomy-competency-based-approach-mccorry-mason/M9781260030892.html
  2. Ernst, D. J., & Ballance, L. O. (2020). Applied Phlebotomy (4th ed.). Wolters Kluwer. https://shop.lww.com/Applied-Phlebotomy/p/9781975108677
  3. Occupational Safety and Health Administration (OSHA). (2019). Bloodborne Pathogens Standard (29 CFR 1910.1030). https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.1030
  4. Clinical and Laboratory Standards Institute (CLSI). (2017). Procedures and Devices for the Collection of Diagnostic Capillary Blood Specimens (7th ed.). CLSI guideline H04-A6. https://clsi.org/standards/products/hematology/documents/h04/
  5. Pagana, K. D., & Pagana, T. J. (2021). Mosby's Manual of Diagnostic and Laboratory Tests (7th ed.). Elsevier. https://www.elsevier.com/books/mosbys-manual-of-diagnostic-and-laboratory-tests/pagana/978-0-323-69423-5

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