When a heelstick is the right choice

Heelstick capillary sampling is the standard collection method for infants under 1 year of age when:

• A venous sample is not feasible (no accessible vein, fragile patient).
• A small-volume blood test is needed (newborn screening, glucose, bilirubin, lead).
• Multiple capillary collections are required (NICU monitoring).

Heelsticks are not appropriate for older children, for tests requiring large volumes, or for cultures (capillary collection is not sterile enough for blood cultures). The exam tests whether the candidate knows the age threshold (under 1 year) and the most common tests (newborn screening panel, bilirubin for jaundice monitoring).

The site: medial and lateral plantar surfaces

The CLSI GP42-A6 standard specifies the medial and lateral plantar surfaces of the heel as the safe puncture site. These areas contain capillaries but no major nerves or arteries. The center of the heel, the Achilles tendon area, and the toes must be avoided:

• Center of the heel — the calcaneus bone is close to the skin; puncture can cause osteochondritis or osteomyelitis.
• Posterior curvature of the heel — high risk of nerve and bone injury.
• Toes — in infants under 1 year, toe puncture is not recommended; the bones are too small and the tissue too fragile.
• Previous puncture sites — these should be avoided to prevent infection and scarring.

The exam tests this site restriction with at least one question per administration.

The depth limit

The incision depth should not exceed 2.0 mm. Most lancets used for infant heelsticks are 1.5 mm or 1.8 mm. The depth limit exists to prevent puncture of the calcaneus bone — a serious complication that can lead to chronic osteomyelitis.

A high-quality lancet designed for infant heelsticks is mandatory. Adult fingerstick lancets are not appropriate — they are typically 1.8 to 2.4 mm and are designed for thicker adult tissue.

The warming step

The heel should be warmed before puncture to increase capillary blood flow. Standard practice:

• Warm towel or heel warmer — wrap the heel for 3 to 5 minutes before the puncture.
• Temperature should be no more than 42 °C (108 °F) — higher temperatures can burn the infant's skin.
• Warming increases blood flow and reduces the number of punctures needed to obtain an adequate sample.

The exam tests whether the candidate knows the warming step is necessary, the temperature limit (42 °C), and the typical duration (3 to 5 minutes).

The puncture technique

1. Position the infant supine with the foot lower than the body to allow gravity to pool blood in the heel. Some facilities have the parent hold the infant; others use a padded pediatric tray.
2. Warm the heel for 3 to 5 minutes.
3. Clean the site with 70 percent isopropyl alcohol. Allow to air-dry completely — alcohol residue can hemolyze the specimen and cause a burning sensation.
4. Hold the foot firmly with the non-dominant hand, encircling the ankle with the thumb and index finger to stabilize.
5. Position the lancet on the medial or lateral plantar surface, avoiding the center of the heel.
6. Activate the lancet with a single swift motion — perpendicular to the skin.
7. Wipe away the first drop of blood, which contains tissue fluid and can contaminate the specimen.
8. Allow free-flowing blood to collect into the capillary tube or onto the test strip. Do not squeeze the heel — squeezing causes hemolysis and dilutes the specimen with tissue fluid.
9. Apply pressure to the site with gauze until bleeding stops (typically 1 to 2 minutes).
10. Bandage the heel if the infant will be returned to the parent; some facilities prefer no bandage to allow observation.

Common errors and their consequences

1. Squeezing the heel. Produces hemolyzed or tissue-fluid-diluted specimens. The lab may reject the sample and the infant will be re-punctured.

2. Puncturing the center of the heel. Risks bone injury (osteochondritis, osteomyelitis), a serious complication.

3. Skipping the warming step. Results in insufficient blood flow, requiring a second puncture and additional trauma.

4. Using an adult lancet. Too deep; risks bone injury.

5. Not wiping the first drop. First-drop blood contains tissue fluid and can falsely elevate glucose, bilirubin, and other tests.

6. Not allowing alcohol to dry. Alcohol residue hemolyzes the specimen and causes a chemical burn to the puncture site.

Specimen collection and handling

Heelstick blood is collected into:

• Capillary tubes for hematocrit and other small-volume tests.
• Microtainer tubes with EDTA for CBC.
• Filter paper for newborn screening (Guthrie cards).
• Direct application to test strips for point-of-care glucose.

Each collection container has specific volume and handling requirements. Underfilling a microtainer affects the anticoagulant:blood ratio and can produce inaccurate CBC results. The exam tests whether the candidate knows the specific volume requirements for each container type.

The exam's favorite heelstick questions

Question type 1: "Where is the appropriate puncture site for an infant heelstick?" — Answer: medial or lateral plantar surface of the heel. Avoid the center of the heel, the posterior curve, and the toes.

Question type 2: "What is the maximum depth for an infant heelstick puncture?" — Answer: 2.0 mm. Most lancets are 1.5 to 1.8 mm.

Question type 3: "Why is the heel warmed before puncture?" — Answer: to increase capillary blood flow, allowing an adequate specimen with a single puncture.

Question type 4: "The first drop of blood is wiped away. Why?" — Answer: the first drop contains tissue fluid and can dilute the specimen, falsely altering test results.

A practical closing note

Heelstick is a high-skill procedure on a fragile patient. Parents are often watching, the infant can feel pain, and the specimen is small and prone to error. The discipline of warming, depth control, free-flowing collection, and proper container filling is what separates a clean sample from a rejected one. The exam tests these details deliberately. Master them.