5.3 Capillary and Pediatric Collection

Capillary Collection Has Its Own Rules

Capillary (dermal) puncture is used for small-volume testing, point-of-care glucose, hemoglobin, newborn screening, and patients with poor venous access. The technique and even the order of draw differ from venipuncture, so the CCMA cannot simply transfer venipuncture habits.

Site Selection by Age

The fingertip puncture is made across the fingerprint lines, not parallel to them, so the drop forms instead of running down the grooves. The thumb and index finger are avoided because they are calloused and have more nerve endings; the fifth finger has too little tissue over the bone.

For infants, the puncture stays on the plantar heel medial or lateral to an imaginary line drawn from the great toe and from between the fourth and fifth toes. The lancet depth must not exceed roughly 2.0 mm (less for premature infants) so the blade does not strike the calcaneus, which can cause osteomyelitis.

Technique Steps

1. Warm the site for 3–5 minutes when needed to increase blood flow (especially for heelsticks).
2. Clean with alcohol and let it dry completely — residual alcohol hemolyzes the sample and causes a stinging, inaccurate glucose.
3. Puncture with a single firm motion using a retractable safety lancet, then dispose of it immediately in the sharps container.
4. Wipe away the first drop, which is contaminated with tissue fluid and can falsely dilute the result.
5. Collect subsequent drops without excessive squeezing.

Why Milking Is Wrong

Vigorously squeezing or "milking" the finger forces interstitial tissue fluid into the sample, diluting it and falsely lowering values and sometimes causing hemolysis. The correct technique applies gentle intermittent pressure well behind the puncture site and lets gravity assist.

Capillary Order of Draw

The order of draw reverses the additive priority for microtubes because platelets clump quickly at a small puncture:

Pediatric Comfort and Safety

A crying, moving child is a safety risk. Use a caregiver to comfort and gently secure the child, explain the steps in simple terms, and keep all sharps out of the child's reach before and after the puncture. Document the site used. A common scenario trap is choosing the infant heel arch or repuncturing the same spot, both of which raise injury and bruising risk.

Indications, Newborn Screening, and Comfort Technique

Knowing when to choose a capillary puncture over a venipuncture is itself tested. Capillary collection is preferred for point-of-care glucose monitoring, for patients with fragile or inaccessible veins, for severely burned or thrombotic patients, for those at risk from repeated venous draws, and especially for infants, in whom large venous draws can cause anemia. It is contraindicated when a large volume is required, for many coagulation studies, and for blood cultures, which need a sterile venous draw.

Newborn screening is a high-yield example. The heelstick blood is applied to a special filter-paper card by touching a large drop to each printed circle and letting it soak through to fill the circle completely from one side only. Layering drops, touching the paper to the heel, or filling from both sides invalidates the card and forces a repeat stick on the infant. The card must air-dry flat and be sent within the required window. These details appear on the exam because a botched screening card delays detection of conditions such as phenylketonuria and congenital hypothyroidism.

Capillary blood is a mixture of arterial, venous, and capillary blood with interstitial fluid, so reference ranges differ slightly from venous values; glucose tends to run a bit higher and some other analytes lower. The CCMA should know the specimen type is not interchangeable for every test.

Comfort and safety technique is also assessed. For infants and young children, swaddling or a parent's gentle hold steadies the limb; for older children, honest simple explanation and distraction reduce fear and movement. The lancet is a single-use retractable safety device that is activated once and dropped directly into the sharps container — never set down within reach of a child and never reused. Warming a heel with a commercial warmer or warm cloth (not hot) for a few minutes increases blood flow and improves the yield from a single stick, sparing the infant a repeat puncture.

Exam-Day Recall

The testable core of capillary work is site safety by age, lancet depth limits, the discard-the-first-drop rule, and the reversed microtube order with EDTA first. Pair each fact with its consequence: wrong heel site risks bone injury, too-deep lancet risks the calcaneus, milking dilutes with tissue fluid, and a delayed EDTA microtube clots. When a stem describes an infant or a difficult-access adult, confirm the chosen site and technique before reacting to the result, and keep every sharp controlled. These habits convert a memorized list into the applied judgment NHA rewards.