CCMA · Phlebotomy · 8 min read
Capillary Puncture and Infant Heel Sticks
- Identify the clinical indications, physiological composition of capillary blood, and anatomical site selection criteria for adult and infant capillary collections.
- Execute proper patient preparation, cleansing, lancet activation, and microcollection techniques while preventing hemolysis and sample contamination.
- Apply the correct capillary order of draw, follow tube handling and labeling protocols, and carry out specialized newborn screening blood spot procedures safely within the clinical medical assistant scope of practice.
Clinical Indications, Capillary Blood Composition, and Patient Assessment
In an outpatient clinic, the medical assistant frequently performs capillary collection as a delegated clinical task during patient rooming or point-of-care testing under the supervision of a licensed provider. Capillary puncture is indicated when venipuncture is difficult, when a patient's veins are fragile, or when only small volumes of blood are required for the ordered diagnostic tests. For instance, an ambulatory provider might order point-of-care testing such as hemoglobin, hematocrit, or blood glucose during a routine chronic care visit. The medical assistant collects the capillary specimen and conducts the ordered test, reporting the quantitative findings directly to the provider without diagnosing conditions or independently interpreting the clinical results.
To appreciate how capillary collection differs from standard venipuncture, the clinical medical assistant must understand the unique physiological nature of the specimen. Capillary blood is not purely venous blood; rather, it is a physiological mixture of arterial blood, venous blood, capillary blood, and intracellular and interstitial fluids. Because of local arterial pressure, capillary blood more closely resembles arterial blood than venous blood. Understanding this composition helps explain why certain analyte values differ between capillary and venous samples, reinforcing the need to adhere strictly to standardized collection protocols.
Patient assessment is a crucial preliminary step before initiating any dermal puncture. The medical assistant must evaluate whether capillary puncture is clinically appropriate for the specific patient. Severely dehydrated patients or patients with poor peripheral circulation are not ideal candidates for capillary puncture because diminished peripheral perfusion impedes blood flow to the extremities, making adequate specimen collection difficult and potentially altering laboratory values. For nonwalking infants, heel puncture is commonly used to obtain small-volume capillary specimens when the ordered test permits capillary collection, although venipuncture may still be needed for other tests ordered by the supervising provider.
Common mistakes
- Milking, squeezing, or rubbing the puncture site excessively, which induces hemolysis and contaminates the sample with tissue fluid.
- Using povidone-iodine to cleanse the site, leading to falsely elevated laboratory values for bilirubin, uric acid, phosphorus, and potassium.
- Layering multiple drops of blood onto a single printed circle on a newborn screening card instead of saturating the circle with one continuous, large drop.
- Applying adhesive bandages to infants or children younger than two years of age, creating hazardous risks of aspiration and skin tears.
Key points
- Capillary blood is a physiological mixture of arterial, venous, and capillary blood, as well as intracellular and interstitial fluids, resembling arterial blood more closely due to arterial pressure.
- Adult fingersticks are performed on the palmar surface of the distal phalanx of the middle or ring finger, cut perpendicular to fingerprint whorls to a depth not exceeding 2.4 mm.
- Infant heel punctures are restricted to the medial and lateral plantar surfaces of the heel with depths up to 2.0 mm in full-term infants (0.85 mm or less in very small premature infants), strictly avoiding the posterior curvature and central arch.
- Cleansing requires 70% isopropyl alcohol air-dried completely; povidone-iodine is avoided because it falsely elevates bilirubin, uric acid, phosphorus, and potassium.
- Wipe away the first drop of blood unless the specific test manufacturer's instructions direct otherwise, and do not scrape collection rims against the skin.
- Capillary order of draw requires blood gas specimens first, followed by EDTA (lavender), other anticoagulants, and finally serum tubes, with immediate gentle inversions for additive tubes.
- Newborn screening cards require saturating each circle with a single drop of blood without layering, followed by horizontal air-drying at room temperature away from heat and sunlight.
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