Capillary Puncture in Phlebotomy: Fingerstick and Heel Stick Procedures

What Is Capillary Puncture?

Capillary puncture — also called dermal puncture — is a method of blood collection that samples blood from the capillary bed just beneath the skin rather than from a vein. When you puncture the skin, blood from capillaries, arterioles, and venules mixes at the surface, producing what labs call a capillary specimen. The composition is slightly different from venous blood, which matters for certain analytes, but the technique is the right choice in a wide range of clinical situations.

On the NHA CPT exam, capillary puncture appears in the Specimen Collection domain. You need to know when to use it, how to perform a fingerstick and a heel stick correctly, which microcollection containers to use, and — critically — how the order of draw differs from venipuncture.

When Capillary Puncture Is the Right Choice

Capillary puncture is not a substitute for venipuncture in every situation, but there are specific clinical indications where it is preferred or required:

• Point-of-care testing (POCT): Glucose monitoring and hemoglobin/hematocrit screening are the most common examples. These tests require only a drop or two of blood and are performed at the bedside or in a clinic using handheld analyzers.
• Newborn screening: State-mandated metabolic panels (PKU, congenital hypothyroidism, galactosemia, and others) are collected via heel stick onto filter paper cards. The blood volume from a heel stick is sufficient for these dried blood spot tests, and venipuncture is not appropriate for neonates.
• Burn patients: When a patient has burns over large areas of the body, intact skin for capillary collection may still be available even when veins are inaccessible or compromised in burned areas.
• Severe obesity: Deep, difficult-to-palpate veins can make venipuncture unreliable. In these patients, a fingerstick may yield a faster, less traumatic result for POCT tests.
• Fragile or inaccessible veins: Elderly patients, patients receiving chemotherapy, and those with advanced peripheral vascular disease often have veins that collapse or roll. If the test can be run on a capillary specimen, dermal puncture avoids repeated venipuncture attempts.
• Small required volume: When only a tiny amount of blood is needed — such as for a single POCT test or a CBC from a pediatric patient — a fingerstick avoids the discomfort and risk of venipuncture.

One thing to keep in mind: capillary specimens are not acceptable for blood cultures, coagulation studies (PT/INR, aPTT), or tests requiring large volumes. Know the limitations, not just the indications.

Fingerstick Procedure: Step by Step

The fingerstick is the standard capillary collection method for adults and children over about one year of age. Follow these steps in order:

1. Verify the patient and prepare supplies. Confirm two patient identifiers. Gather your lancet (single-use safety lancet), alcohol prep pads, gauze, microcollection tubes appropriate for the ordered tests, and gloves.
2. Select the correct finger. Use the ring finger or middle finger of the non-dominant hand. Avoid the pinky (too thin, poorly perfused), the thumb (thick skin, has a pulse — risk of arterial sample), and the index finger (callused, frequently used, more sensitive). Never puncture through a previous puncture site, across a fingernail, or on a swollen or cyanotic finger.
3. Warm the finger. A warm finger increases blood flow by vasodilation, producing a free-flowing sample. Use a commercial heel/finger warmer or a warm moist towel at 40°C (104°F) for 3–5 minutes. This step is especially important for patients with poor peripheral circulation.
4. Clean and dry the site. Wipe the fingertip with a 70% isopropyl alcohol prep pad. Allow it to air dry completely. Residual alcohol contaminates the specimen, causes stinging, and can hemolyze red cells.
5. Puncture perpendicular to the fingerprint lines. Position the lancet across the fingerprint lines (not parallel to them). Puncture perpendicular to the whorls or ridges so the wound opens and bleeds freely. Punctures that run parallel to the print lines tend to close up and bleed poorly. Use a single-use safety lancet — twist-activated or press-activated styles are both acceptable.
6. Wipe away the first drop. The first drop of blood contains excess tissue fluid from the puncture itself, which can dilute the specimen and alter results. Wipe it away with dry gauze before collecting.
7. Collect the specimen. Apply gentle, intermittent pressure — do not squeeze or milk the finger repeatedly, as this forces tissue fluid into the sample and causes dilution and hemolysis. Touch the microcollection tube to the blood drop; do not scrape. Hold the finger downward to let gravity assist.
8. Fill tubes in the correct order of draw (see section below).
9. Mix and label. Cap and gently invert each additive tube 8–10 times. Label at the bedside before leaving the patient.
10. Apply pressure and inspect. Press gauze to the site until bleeding stops. Apply a bandage if appropriate.

Heel Stick for Neonates

The heel stick is used for infants — typically newborns and babies up to about 12 months. In premature and term infants, the heel is the only approved site for capillary blood collection. Do not use the fingers of neonates — the bones are close to the surface and the tissue volume is too small to puncture safely.

Where to Puncture on the Heel

This is one of the highest-yield facts for the exam. You must puncture on the medial or lateral plantar surface of the heel — the curved bottom sides. Picture drawing a line from the middle toe straight back to the heel; the safe zones are the fleshy areas on either side of that centerline.

Never puncture the center of the heel (the plantar surface directly beneath the calcaneus) or the back of the heel (the posterior curvature). The calcaneal bone is extremely close to the skin surface in those regions. Puncturing too deep there risks osteomyelitis (bone infection), a serious and permanent complication.

Lancet Depth for Neonates

• Premature infants: Maximum lance depth of 0.65 mm (some sources state up to 0.85 mm depending on gestational age and heel thickness). Use a lancet specifically designed for preemies.
• Term neonates (full-term newborns): Maximum lance depth of 2.0 mm. Do not exceed this — the calcaneus is typically 2.4 mm below the skin at the medial/lateral sites in term neonates, leaving very little margin.

Before puncturing, warm the heel with a commercial warmer at 40°C for 3–5 minutes to increase blood flow. Wipe clean with alcohol and allow it to dry. Wipe away the first drop, then collect onto the filter paper card in smooth, even circles, or into microcollection tubes as needed. Do not press the card against the heel — touch it to the drop and let it absorb.

Capillary Order of Draw

The order of draw for capillary specimens is different from the venipuncture order of draw, and this distinction is a frequent exam question. Here is the capillary order:

1. EDTA tubes (lavender/pink) first
2. Other additive tubes (green/heparin, gold/SST, gray/fluoride-oxalate)
3. Plain tubes (red/no additive) last

Why is EDTA first for capillary collections? Capillary blood clots faster than venous blood because the puncture triggers the clotting cascade almost immediately and tissue thromboplastin mixes into the sample. If you fill a plain tube first, the blood may start to clot before you even reach the EDTA tube — and once clotted blood enters an EDTA tube, the CBC will be invalid. Filling EDTA first ensures the anticoagulant captures the blood before clotting begins.

Compare this to venipuncture order of draw, where blood cultures go first, then light blue (citrate), then SST, then green, then lavender, then gray. The logic is flipped for the first tubes because the circumstances are different.