4.3 Capillary Collection & Complications
Key Takeaways
- Capillary (dermal) blood is collected when veins must be preserved, in newborns and infants, for point-of-care glucose, and from patients who are difficult venous draws or severely burned
- The capillary order of draw is reversed from venous: collect EDTA (lavender/hematology) first, then other additives, then serum, to minimize platelet clumping and clotting
- Wipe away the first drop of capillary blood because it is contaminated with tissue fluid that dilutes the specimen and can cause clotting
- A heel stick must be on the medial or lateral plantar surface and no deeper than 2.0 mm to avoid the calcaneus (heel bone) and osteomyelitis
- Shooting or radiating pain means a nerve was struck: remove the needle immediately; for syncope, stop the draw, lower the head, and protect the patient
Capillary Collection: When and How
Capillary (dermal/skin) puncture collects a mix of arterial, venous, and capillary blood plus tissue fluid. It is indicated when:
- The patient is a newborn or young infant (small total blood volume; deep venipuncture is dangerous).
- Only a small volume is needed (point-of-care glucose, newborn screening, hematocrit).
- Veins must be preserved (oncology, severe burns, IV therapy in both arms).
- The patient is severely obese, has thrombotic tendency, or is at risk from venipuncture.
Capillary is not appropriate for coagulation studies, blood cultures, or large-volume chemistry panels.
Site Selection
| Patient | Site | Rationale |
|---|---|---|
| Adult / child > 1 yr | Palmar surface of distal middle or ring finger, slightly off-center | Avoids the bone, nail, and the very tip |
| Infant < 1 yr | Medial or lateral plantar surface of the heel | Finger tissue is too thin; heel is safe if correct area used |
Heel Stick Safety
For an infant heel stick, puncture only the medial or lateral plantar (bottom) surface and no deeper than 2.0 mm. The central plantar area and the posterior curve overlie the calcaneus (heel bone); striking bone can cause osteomyelitis (bone infection). Never use the arch or a previous puncture site.
Capillary Technique and the Reversed Order of Draw
- Warm the site if needed (a warm compress for 3-5 minutes increases blood flow up to sevenfold).
- Clean with 70% isopropyl alcohol and let it air dry — residual alcohol causes hemolysis and stinging.
- Puncture with a sterile lancet in one firm motion, across (perpendicular to) the fingerprint lines so the drop forms a bead rather than running down the grooves.
- Wipe away the first drop with clean gauze. The first drop contains tissue fluid that dilutes the sample and promotes clotting and platelet clumping.
- Collect using gentle, intermittent pressure — do not strongly squeeze ("milk") the site, which causes hemolysis and tissue-fluid contamination.
- Fill containers in the capillary order of draw.
Capillary Order of Draw (Reversed)
The capillary order is the reverse of the venous order with respect to hematology: collect the EDTA (lavender) hematology specimen first, because capillary blood begins clotting immediately and platelets clump fastest.
| Order | Specimen |
|---|---|
| 1 | Blood gas (capillary, if collected) |
| 2 | EDTA (lavender) — CBC/hematology |
| 3 | Other additive tubes (heparin) |
| 4 | Serum tubes (last) |
The rule to memorize: venous = light blue first; capillary = EDTA first. Reversing them is a classic NHA distractor.
Complications: Recognition and Response
The exam gives a scenario and asks for the immediate correct action. Know the trigger and the response for each.
| Complication | Recognition | Immediate Response |
|---|---|---|
| Nerve injury | Sharp, shooting, radiating, or electric pain; numbness/tingling | Remove the needle immediately, apply pressure, document, report |
| Syncope (fainting) | Pale, sweaty, lightheaded, loss of consciousness | Stop the draw, remove the needle, lower the head / raise legs, protect from falling, alert staff |
| Hematoma | Rapid swelling and bruising at the site during the draw | Release the tourniquet, withdraw the needle, apply firm pressure 2+ minutes |
| Hemolysis | Pink/red serum or plasma after spin; falsely high potassium | Recollect; avoid small-gauge needles, vigorous mixing, and frothing |
| Hemoconcentration | History of long tourniquet/fist pumping; falsely elevated protein, calcium, potassium | Recollect with tourniquet under 1 minute and no pumping |
| Failed draw | No blood return, vein missed or collapsed | Reposition slightly; maximum two attempts, then ask a colleague |
Key Rules
- Shooting or radiating pain = nerve. Stop and remove the needle now; do not reposition through pain.
- Hemolysis falsely elevates potassium (and LDH and AST) because these are concentrated inside red cells that have ruptured. A hemolyzed potassium is an absolute recollect.
- Hemoconcentration vs hematoma: hemoconcentration is plasma shifting out of the vessel from a long tourniquet (concentrated results); a hematoma is whole blood leaking into tissue (visible swelling). Same family of distractors — read carefully.
- Two-attempt limit: make no more than two venipuncture attempts; then hand the patient off to another phlebotomist rather than probing or causing injury.
- Never probe laterally with the needle hunting for a vein — this is the leading cause of nerve injury and hematoma.
During a venipuncture the patient suddenly reports a sharp, electric, shooting pain radiating down the forearm. What should the phlebotomist do first?
What is the correct order of draw for a capillary (dermal) collection, and why does it differ from venous?
An infant requires a heel stick. Which site and depth are correct?
A serum specimen appears pink-red after centrifugation and the potassium result is markedly elevated. What most likely happened and what is the response?