Capillary blood collection (also called dermal collection or fingerstick) is one of the most frequently performed procedures in healthcare, yet it is often taught as an afterthought. Mistakes here are especially costly because capillary samples are often used for point-of-care testing, where results drive immediate clinical decisions. You need to master site selection, needle technique, and specimen handling before test day.
When to Use Capillary vs. Venipuncture
Capillary collection is the method of choice for:
- Neonates and infants under 12 months (heelstick is standard)
- Patients with severely limited venous access (burns, bilateral mastectomy, lymphedema)
- Point-of-care testing (blood glucose, lactate, blood gases)
- Tests requiring small volumes (newborn screening, HbA1c screening)
- Obtaining specimens when repeated venipuncture would be traumatic
Capillary samples are NOT appropriate for blood cultures, coagulation studies (PT/PTT/INR), or tests sensitive to hemoconcentration.
Approved Capillary Collection Sites in Adults
In adults, there are only two acceptable sites for capillary collection:
- Middle or ring finger (lateral edge) - preferred for conscious, cooperative adults
- Thumb and index finger - only if middle and ring fingers are unavailable (scarred, injured, infected)
Common mistakes: Many people attempt collection from the pad of the finger (which has fewer capillaries and more nerve endings). Always collect from the lateral edge, below the nail bed, in the area roughly between the first and second knuckles.
Capillary Collection Sites in Infants
For neonates and infants under 12 months, the lateral heel is the only approved site. This prevents damage to the plantar arch and the calcaneal bone. The medial heel or the sole is never acceptable and puts the child at risk of osteomyelitis and permanent structural damage.
Site landmarks: The heel collection site should be limited to the outer edge of the heel, below the line that connects the top of the medial and lateral malleoli. Never collect above this line (risk of calcaneal bone injury) or in the center of the heel.
Specimen Quality Issues in Capillary Collection
Capillary specimens have inherent quality challenges that differ from venous samples:
Hemoconcentration
When you perform a capillary stick, you are collecting blood from arterial, venous, and tissue fluid compartments simultaneously. If you squeeze the area heavily to generate flow, you will dilute the specimen with tissue fluid, producing falsely low analyte values. This is why some tests (like potassium) are contraindicated on capillary samples when higher accuracy is critical.
Microclot Formation
Capillary tubes, especially plain glass tubes, are not anticoagulated. If you do not fill them immediately or move them end-to-end (to ensure mixing with any anticoagulant), microclots can form and cause spurious results. This is especially problematic for hematology samples.
Air Bubbles
Air in a capillary tube can affect blood gas results. Always expel air bubbles before sealing the tube and submitting it to the lab.
Proper Capillary Collection Technique
Step 1: Prepare the Site
- Wash the site with soap and warm water. Alcohol is acceptable but warm water is preferred because it increases blood flow more effectively.
- Allow the site to air dry completely (do not blot with a cloth, which can re-contaminate).
- Do not use a lancet if the site is still wet; alcohol interferes with capillary flow and increases pain.
Step 2: Select the Lancet Depth
Lancet depth is standardized by patient age:
- Adults: 1.5–2.0 mm depth (most automated lancets have this setting)
- Infants 1–12 months: 0.85–1.1 mm depth
- Newborns (< 24 hours): 0.65–0.85 mm depth
Going too deep increases pain, bruising, and the risk of hitting bone (especially critical in heelsticks). Too shallow a depth produces insufficient blood flow and requires re-sticking, which patients experience as pain and increases contamination risk.
Step 3: Make the Puncture
- The puncture should be perpendicular to the fingerprint lines (not parallel). This helps prevent the blood from running along a ridge and missing the collection container.
- Discard the first drop of blood on a sterile gauze. The first drop is enriched with tissue fluid and can affect test results.
- Allow blood to flow freely. Do NOT squeeze excessively; gentle pressure around the site is acceptable, but aggressive squeezing dilutes the specimen.
Step 4: Fill Collection Containers in the Correct Order
When collecting multiple capillary tubes, follow this order (similar to venipuncture order of draw):
- 1. EDTA (lavender) for CBC/hematology
- 2. Plain or serum separator for chemistry
- 3. Heparin (green) for plasma studies if needed
Fill capillary tubes to the marked line. For EDTA tubes, mix immediately by tilting the tube side-to-side several times to ensure the anticoagulant is distributed throughout the blood.
Step 5: Seal and Label
- Both ends of capillary tubes must be sealed (with clay, caps, or sealing wax) to prevent leakage and evaporation.
- Label the tube with patient name, date, time, and phlebotomist initials immediately before transport to the lab.
Common Capillary Collection Errors on Exams
- "What is the approved heel collection site for infants?" Answer: Lateral heel, below the line connecting the malleoli.
- "Why do you discard the first drop?" Answer: It is diluted with tissue fluid and can affect test results.
- "A patient's capillary potassium is reported as critically low, but the patient is clinically well. What happened?" Possible answer: Hemoconcentration from aggressive squeezing, or the sample was hemolyzed.
- "What lancet depth is appropriate for a 6-month-old?" Answer: 0.85–1.1 mm (not 1.5–2.0 mm, which is for adults).
Practice Summary
Capillary collection is not a "simple" procedure. The test writers know that, and they will ask about site selection, specimen quality, and the reasons for protocol steps. Clinically, precise capillary collection is critical because these samples often drive immediate care decisions in emergency and primary-care settings. Master this technique, and you master a core phlebotomy competency.
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