Capillary Puncture: Fingerstick and Heelstick Technique

What Is Capillary Puncture?

Capillary puncture — also called skin puncture — is a method of collecting a small volume of blood from the capillary bed just beneath the skin. Blood obtained this way is a mixture of capillary blood, a small amount of arterial and venous blood, and interstitial fluid. Because the volume is limited, capillary collection is used for point-of-care testing, newborn screening, blood glucose monitoring, and any test requiring only a few drops of blood. The two most common sites are the fingertip (fingerstick) in adults and older children, and the plantar surface of the heel (heelstick) in neonates and infants.

Site Selection: Where and Why It Matters

Fingerstick: The preferred sites are the fleshy pads of the third (middle) or fourth (ring) finger of the non-dominant hand. These fingers have adequate tissue depth to protect underlying bone. The puncture should be made slightly off-center on the pad — not directly on the fingertip or along the very center line — and never on the finger's side, thumb, index finger, or fifth finger. The index finger is calloused from frequent use, making puncture harder; the thumb has a pulse and is avoided. The fifth finger has very little tissue depth. Avoid any site that is bruised, swollen, infected, or has compromised circulation.

Heelstick: In neonates and infants under approximately one year (or per institutional policy for weight), the plantar surface of the heel is used. Acceptable zones are the medial and lateral plantar surfaces — never the posterior curve of the heel, the arch, or the central plantar surface. Puncturing outside the safe zones risks bone injury (calcaneal osteomyelitis), which is a serious and preventable complication.

Lancet Selection and Depth

Safety lancets are engineered to retract automatically after a single use, preventing needlestick injuries. Lancets come in different blade depths. For adults, a depth of about 1.5–2.0 mm is typical; for neonatal heelsticks, lancets designed specifically for that use are selected to limit puncture depth and reduce the risk of bone injury. Always use the lancet that matches the patient type and site — never use a lancet intended for adult fingers on a neonate's heel.

The Fingerstick Procedure Step by Step

1. Gather and verify supplies: gloves, alcohol prep pad, gauze, appropriate lancet, microcollection tube(s), sharps container, bandage.

2. Wash hands and don gloves.

3. Warm the site if circulation is poor; a warm compress for up to three to five minutes can increase blood flow.

4. Cleanse the site with an alcohol prep pad using a circular motion and allow it to air-dry completely — typically 30 to 60 seconds. Residual alcohol causes stinging, hemolyzes red cells, and can contaminate the specimen.

5. Position the finger with the pad facing upward; apply gentle, steady support.

6. Puncture by pressing the lancet firmly against the cleansed site and activating it. The lancet retracts automatically.

7. Discard the lancet immediately into the sharps container — do not recap.

8. Wipe away the first drop with a clean, dry gauze pad. The first drop contains excess tissue fluid released by the puncture trauma and should never be collected.

9. Collect by allowing subsequent drops to form naturally. Hold the microcollection tube below the site so gravity assists flow. Touch the tube tip gently to the drop and let blood run in by capillary action.

10. Apply gentle pressure and release in a rhythmic fashion if needed to encourage flow — this is not milking.

11. Cap and mix the tube according to the additive type (invert gently the number of times specified for the tube).

12. Label the tube at the bedside or point of care immediately after collection.

13. Apply gauze and pressure to the site; apply a bandage when bleeding stops.

The Critical Rule: No Milking

Milking means squeezing the finger repeatedly from the base toward the tip to force blood out. This action does two harmful things: it dilutes the specimen with tissue fluid (interstitial fluid), which can falsely lower analyte concentrations, and it ruptures red blood cells, causing hemolysis. A hemolyzed specimen appears visibly darker or pink-tinged in the tube and will produce inaccurate results for many tests including potassium, lactate dehydrogenase, and others. If blood flow is poor, warm the site, reposition, or repuncture — do not milk.

Microcollection Tubes

Microcollection tubes (sometimes called capillary collection devices or "bullets") follow a color-coding system similar to standard evacuated tubes, indicating the additive inside. A general draw order for capillary collection is: blood gas specimens first, then EDTA (lavender) tubes for hematology, then other additive tubes, and finally serum tubes. Follow your facility's policy and the current CLSI guideline.

Safety and Scope of Practice

• Always use a safety lancet; never use a blade or non-retracting lancet device.
• Dispose of lancets immediately after use into an approved sharps container.
• Never perform a heelstick on the posterior heel curve in any patient.
• Never puncture through a previous puncture site; choose a fresh location.
• Patient identification must be confirmed before every collection.
• Report any accidental self-puncture immediately per your facility's exposure control plan.