Phlebotomy Test Capillary and Dermal Puncture 1 — Questions and Answers

Question 1: When performing a fingerstick on an adult patient, which finger is the PREFERRED site?

Thumb
Index finger
Middle or ring finger (Correct answer)
Little finger

Correct answer: Middle or ring finger

The middle (third) and ring (fourth) fingers are preferred for adult fingersticks because they have adequate tissue depth and fewer nerve endings than the index finger.

For adult fingerstick (capillary puncture), CLSI guidelines recommend the palmar surface of the distal phalanx (fingertip) of the middle (third) or ring (fourth) finger. These fingers are preferred because they have sufficient tissue depth to avoid hitting bone even with a deep lancet, and they have fewer concentrated nerve endings than the index finger or thumb. The index finger is more sensitive and frequently used, making it sore if punctured repeatedly. The little (fifth) finger has very little tissue depth and risks bone puncture. The thumb has a pulse and thicker skin. The puncture should be made slightly off-center to the fleshy pad, not the very tip or side of the finger.

Question 2: Why is the first drop of blood wiped away during a capillary puncture?

To ensure adequate blood flow by removing the clot
To remove tissue fluid that could dilute or contaminate the sample (Correct answer)
To sterilize the puncture site
To allow the lancet wound to seal partially

Correct answer: To remove tissue fluid that could dilute or contaminate the sample

The first drop contains tissue fluid released when the skin is punctured, which dilutes the blood sample and can alter test results; it must be discarded.

When the skin is punctured with a lancet, the initial blood that emerges is mixed with tissue fluid (interstitial fluid) released from disrupted cells and intercellular spaces. This tissue fluid dilutes the blood sample and can significantly alter the concentration of analytes, particularly potassium (falsely elevated from cell lysis), glucose (diluted), and proteins. CLSI standards require that the first drop be wiped away with a dry gauze pad before collecting the sample. Additionally, the site should have been cleaned with 70% isopropyl alcohol and allowed to air dry completely before puncture, both to disinfect and to prevent alcohol from contaminating the sample (alcohol can hemolyze RBCs).

Question 3: The correct order of draw for capillary specimens collected in micro-collection tubes is:

EDTA → other additive tubes → serum tube
Blood gases → EDTA → other additive tubes → serum tube (Correct answer)
Serum tube → EDTA → other additive tubes
Other additive tubes → EDTA → serum tube

Correct answer: Blood gases → EDTA → other additive tubes → serum tube

Capillary order of draw begins with blood gas specimens (if collected), then EDTA (lavender/pink) first to ensure accurate hematology results, then other additive tubes, then serum tubes last.

CLSI H04 guidelines specify a specific order of draw for capillary micro-collection specimens: (1) Blood gas specimens if required (collected anaerobically first), (2) EDTA tubes (lavender cap) — collected first to ensure the hematology sample is not contaminated by platelet activation or clotting from the puncture, (3) Other additive tubes (e.g., green/heparin, gray/fluoride), (4) Serum/clot activator tubes (red/gold cap) — collected last. This is importantly different from the venipuncture order of draw, where coagulation tubes come before EDTA. In capillary collection, EDTA first is critical because the first drops of blood are most representative for hematology and the tissue trauma-induced platelet activation makes later collection less accurate for CBC.

Question 4: What is the maximum recommended depth of a heel puncture in a full-term neonate?

1.0 mm
2.0 mm (Correct answer)
2.4 mm
3.0 mm

Correct answer: 2.0 mm

CLSI recommends a maximum lancet depth of 2.0 mm for full-term neonates to avoid penetrating the calcaneus (heel bone) and causing osteomyelitis.

According to CLSI H04 standards, the maximum depth for a heel stick lancet in a full-term neonate is 2.0 mm. The distance from the skin surface to the calcaneus (heel bone) in a full-term newborn is approximately 3.0–3.4 mm at the recommended medial and lateral plantar heel sites. Using a depth no greater than 2.0 mm provides a safe margin to avoid bone puncture, which can cause osteomyelitis (bone infection)—a serious complication that has been documented in neonates. Premature infants have even less tissue depth and require shallower devices. Only devices specifically designed and labeled for neonatal use should be used. Lancets designed for adult fingers should never be used on neonates.

Question 5: Which area of the neonatal heel is the CORRECT site for a heelstick?

Center of the heel pad (plantar surface)
Medial or lateral plantar surface of the heel (Correct answer)
Posterior curve of the heel
Top of the foot near the ankle

Correct answer: Medial or lateral plantar surface of the heel

CLSI guidelines specify the medial or lateral plantar surface of the heel as safe sites, avoiding the posterior curve and center where the calcaneus is close to the surface.

CLSI H04 guidelines precisely define the acceptable zones for neonatal heelstick: the medial (inner) and lateral (outer) plantar surface of the heel. These sites provide sufficient distance between the skin and the underlying calcaneus bone. Areas that must be avoided include: the posterior curve of the heel (where the calcaneus is closest to the skin surface and bone puncture risk is highest), the central plantar surface (also high risk), any area that has been previously punctured (increased scar tissue, infection risk), fingers (insufficient tissue in neonates), and any edematous areas (fluid dilutes specimen). The foot should be warmed prior to puncture to increase blood flow to the area.

Question 6: Warming the fingertip or heel before a capillary puncture primarily serves to:

Sterilize the skin surface
Increase local blood flow (arterialization) for adequate sample collection (Correct answer)
Reduce patient pain during puncture
Prevent the sample from clotting in the tube

Correct answer: Increase local blood flow (arterialization) for adequate sample collection

Warming vasodilates local capillaries and increases blood flow up to 7-fold, making sample collection easier and improving the quality of arterialized capillary blood.

Warming the site of capillary puncture (fingertip, heel, or earlobe) for 3–5 minutes using a commercially available heel warmer or warm moist towel (no hotter than 42°C / 107.6°F) causes vasodilation of the local capillary bed. This arterialization process increases blood flow to the area up to 7 times the baseline rate, making it much easier to obtain an adequate blood sample with minimal squeezing. Warming also improves the representativeness of the sample by increasing the proportion of arterial blood in the capillary specimen, making it more useful for blood gas approximations. Excessive squeezing (milking) should always be avoided as it introduces tissue fluid and causes hemolysis.

Question 7: A phlebotomist performs a fingerstick and squeezes the finger excessively to obtain enough blood. This practice can cause which test result to be FALSELY ELEVATED?

Hemoglobin
Potassium (Correct answer)
Glucose
Sodium

Correct answer: Potassium

Excessive squeezing (milking) causes cell lysis and release of intracellular potassium, and introduces tissue fluid—both of which falsely elevate potassium in the sample.

Potassium is the primary intracellular cation, with intracellular concentrations approximately 150 mEq/L compared to normal serum levels of 3.5–5.0 mEq/L. When a finger is squeezed excessively during capillary collection, two problems occur: (1) mechanical trauma lyses red blood cells, releasing their intracellular potassium into the sample (hemolysis), and (2) tissue fluid, which also contains higher potassium than plasma, is introduced into the drop. Both effects cause falsely elevated potassium results. This is why CLSI recommends gentle pressure only, good lancet technique to establish free-flowing blood, warming the site to improve blood flow, and discarding the first drop. Hemolysis from excessive squeezing also affects other analytes including LDH, AST, and phosphorus.

Question 8: In capillary puncture, the lancet should be positioned perpendicular to the fingerprint lines. Why?

To maximize lancet depth penetration
To ensure the wound opens freely and blood flows more easily (Correct answer)
To reduce the risk of nerve damage
To allow the wound to seal faster after collection

Correct answer: To ensure the wound opens freely and blood flows more easily

Puncturing perpendicular (across) the fingerprint lines allows the wound to gape slightly and blood to flow freely; puncturing parallel to the lines tends to seal the wound shut.

The fingerprint ridges on the skin are lines of Blaschko that contain elastic fibers oriented along their length. When a lancet punctures perpendicular (at a right angle) to these ridges, the wound crosses multiple ridges and the elastic tension pulls the wound open slightly, allowing blood to bead up and flow freely. When the puncture is made parallel to the fingerprint lines, the wound aligns with the elastic fiber tension and tends to close like a zipper, making blood flow poor and potentially requiring repeated squeezing. Proper lancet orientation—always across the fingerprint lines—is a key technique element that significantly improves blood flow and specimen quality in capillary collection.

Phlebotomy Test Capillary and Dermal Puncture 1

When performing a fingerstick on an adult patient, which finger is the PREFERRED site?

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