8.1 Capillary Puncture Including Infants

CMLA Work Area II lists capillary work as two competencies: perform capillary punctures (II.3.B.9) and perform capillary punctures on infants (II.3.B.10). The supporting standard on AMT CMLA-REF-2025-1 is CLSI GP42, Collection of Capillary Blood Specimens, 7th edition (2020). GP42 is not PRE02. PRE02 is venous collection; GP42 is skin puncture. Reciting the venous order of draw does not make a correct heelstick.

Quick Answer: Older children and adults: palmar distal 3rd or 4th finger. Infants: medial or lateral plantar heel—never the posterior curvature, never the arch, never an infant finger. Use a device-controlled safety lancet so you do not hit bone. Warm, wipe the first drop, fill EDTA microtainers first, and air-dry newborn screening cards without stacking them wet.

When capillary is the right collection

Capillary puncture (skin puncture, fingerstick, or heelstick) samples mixed arteriolar, venous, and interstitial blood from a dermal incision. Choose it when the method is validated on capillary blood, the volume is small, venous access is poor or restricted (neonates, burns, oncology, repeated glucose checks), or the order is a newborn screening (NBS) filter-paper card. Do not use capillary as a shortcut for tests that require venous or arterial blood: routine coagulation (PT/aPTT), blood cultures, most erythrocyte sedimentation rates, and chemistry panels that exceed microcollection volume.

Capillary blood is not "tiny venous blood." Glucose may run slightly higher on capillary than venous samples, especially after a meal. Hemolysis is more common because of squeezing and small-bore collection, so potassium, lactate dehydrogenase, and aspartate aminotransferase can read falsely high. Do not interchange a fingerstick result with a venous reference interval unless the method and the laboratory directory say you may.

Adult and pediatric finger sites

For patients with adequate finger tissue (classically children older than about one year who are walking, and adults), puncture the palmar surface of the distal segment of the third (middle) or fourth (ring) finger. Stay slightly off the pad center. Align the incision across (perpendicular to) the fingerprint ridges so blood forms a round drop instead of running along the grooves.

Do not use the thumb (pulse, thicker callus), the index finger (callus, used constantly), the fifth (pinky) finger (thin tissue, bone closer to skin), a swollen, cyanotic, scarred, burned, or infected finger, the same hole repeatedly, or the earlobe (not a CLSI-recommended diagnostic site).

Clean with 70% isopropyl alcohol and air-dry. Residual alcohol hemolyzes cells and stings. Povidone-iodine left on the skin can contaminate capillary samples and falsely elevate potassium, phosphorus, and uric acid—do not substitute it for alcohol on a routine fingerstick unless a legal-collection SOP says otherwise (Section 8.3).

Infant heel sites — the high-stakes map

Infants, especially neonates, do not have enough tissue on the fingers. Finger punctures in this group risk osteomyelitis (bone infection) and bone injury. The approved site is the medial or lateral plantar surface of the heel—the sides of the plantar heel, not the back and not the bottom center.

Never puncture the posterior curvature of the heel (the calcaneus is closest to the skin there), the arch of the foot (nerves, tendons, cartilage), fingers or toes of a neonate, previous punctures, bruises, or edematous skin. If flow is poor, warm again; do not "just try the back of the heel."

Imagine a heel-print. The safe zones are the outer and inner edges of that print, still on the plantar surface—not the Achilles-adjacent curve.

Depth, devices, and osteomyelitis

Depth is controlled by choosing the correct single-use safety lancet, not by pressing harder. Infant heel lancets make a shallow, device-limited incision so the blade does not reach the calcaneus. Typical teaching associated with GP42 keeps infant heel depth at or below 2.0 mm; premature and low-birth-weight infants need still shallower, device-specific lancets. Adult finger devices are not interchangeable with neonatal heel devices.

Osteomyelitis of the calcaneus is the named complication of puncturing too deep or in the wrong place. Related injuries include cartilage damage and calcaneal osteochondritis. If blood does not flow, you do not "go deeper" with a longer blade. You rewarm, lower the heel below the heart, and, if needed, select a new approved site with a new device. Activate the safety feature; used lancets go in a sharps container. OSHA treats them as contaminated sharps.

Warming, first drop, and milking

Warm the site about 3–5 minutes with a commercial heel warmer or a warm (not scalding) cloth. The goal is increased arterial flow (arterialization), not a burn. Manufacturer warmers are built not to exceed a safe skin temperature (often about 42°C); a microwave-hot washcloth is not a substitute.

Hold the finger or heel dependent. Puncture, then wipe away the first drop. That drop is mixed with tissue fluid and leftover alcohol. Collect later drops into microcollection tubes (microtainers), into capillary tubes when allowed, or onto filter paper. Some waived glucose meters specify a different first-drop rule in the instructions for use (IFU); unless that IFU says otherwise, wipe.

Do not milk the site with a pumping squeeze. Gentle, intermittent pressure is not wringing the finger. Excess squeezing hemolyzes red cells and dilutes the specimen with interstitial fluid—both of which change glucose, potassium, and cell counts.

Microtainer order of fill (common CLSI/practice)

Capillary additive order is not the venous order of draw. Platelets in capillary blood aggregate quickly, so hematology cannot wait while you fill a serum tube. Common CLSI GP42 / laboratory practice—label it as such on CMLA; the facility IFU still wins—is:

Mix additive microtainers by inversion immediately. Underfilling EDTA still changes morphology and counts. Do not scrape dried blood off the skin into a tube.

Newborn screening filter paper

State NBS programs use dried-blood-spot cards. Fill each printed circle completely so blood soaks through from one side. Do not layer extra drops on a partially dried circle, do not paint with a capillary tube unless the program IFU allows a named applicator, and do not contaminate circles with lotion, powder, or alcohol-wet gloves.

Air-dry the card horizontally at ambient temperature—commonly at least 3 hours—away from sunlight and heat. Do not stack wet cards; stacking smears spots and can transfer blood between patients. Do not dry with a hair dryer or incubator. Once dry, sleeve the card per the state kit and ship as the program specifies. Timing (often after 24 hours of age, with repeat rules for early collection) is a state protocol, not a number you invent.

Capillary versus venous results

• Glucose: capillary may be higher than venous, especially postprandial.
• Hemolysis risk: higher with skin puncture and squeezing.
• Hematocrit/hemoglobin: often slightly higher in capillary samples.
• Potassium and some enzymes: falsely high if hemolyzed.
• Volume: you will run out of blood; plan the tubes, EDTA first.

Exam traps

• Infant fingers are not small adult fingers.
• Posterior heel and arch are forbidden, not backup sites.
• Venous order of draw is the wrong answer for microtainers.
• The first drop is discarded, not the best drop.
• Wet NBS cards are never stacked to save space.

When CMLA shows a heel or a microtainer rack, name the site, the device, the first tube, and the bone you are trying not to hit.