Phlebotomy Test Capillary and Dermal Puncture 2 — Questions and Answers
Question 1: Which lancet depth is recommended for an adult fingerstick to ensure adequate blood collection without causing undue pain?
- 0.5–1.0 mm
- 1.5–2.0 mm (Correct answer)
- 2.4–3.0 mm
- 3.5–4.0 mm
Correct answer: 1.5–2.0 mm
A lancet depth of 1.5–2.0 mm is typically sufficient for adult fingerstick collection, providing adequate blood flow while minimizing pain and avoiding deep tissue.
CLSI H04 guidelines recommend lancet penetration depths based on the patient population and site. For adult fingersticks, a depth of approximately 1.5–2.0 mm is typical for most commercially available safety lancets. This depth penetrates through the epidermis into the upper dermis where the capillary network is richest, providing adequate blood flow without reaching deeper structures. Some areas of skin with calluses or thicker epithelium (common in manual laborers) may require a slightly deeper lancet. Excessively deep punctures cause more pain, larger wounds that are harder to stop bleeding, and increased tissue fluid contamination. Always use the appropriate lancet gauge and depth for the patient's skin condition and collection volume needed.
Question 2: A capillary blood glucose result is typically considered to be which of the following compared to a venous plasma glucose?
- 10–15% lower than venous plasma glucose
- Approximately equal to or slightly higher than venous plasma glucose (Correct answer)
- Always 20% higher due to arterial mixing
- Identical—there is no difference
Correct answer: Approximately equal to or slightly higher than venous plasma glucose
Capillary blood glucose is approximately equal to or slightly higher (up to 10–15% higher) than venous blood glucose because capillary blood contains more arterial blood with higher glucose.
Capillary blood is a mixture of blood from arterioles, venules, and capillaries, making it closer to arterial blood in composition than venous blood. After a meal (postprandial state), arterial blood has significantly more glucose than venous blood because the tissues have not yet extracted the glucose from arterial blood. As a result, postprandial capillary glucose can be 10–15% higher than venous plasma glucose. In the fasting state, the difference is minimal. This is an important consideration when comparing fingerstick glucose values to venous laboratory values, particularly in diabetes management. Most glucose meters are designed to account for this and report plasma-equivalent values, but some older meters may report whole blood values.
Question 3: Which of the following is an absolute contraindication for a fingerstick on a specific finger?
- Cold fingers
- Fingers with nail polish
- Fingers with edema or poor circulation (Correct answer)
- Dominant hand fingers
Correct answer: Fingers with edema or poor circulation
Fingers with edema have excess interstitial fluid that will dilute the blood sample, and poor circulation means inadequate blood flow for collection—both are contraindications.
Edematous fingers contain excess interstitial (tissue) fluid that will mix with the blood sample during collection, diluting analyte concentrations and producing inaccurate results. Poor circulation (due to Raynaud's phenomenon, peripheral vascular disease, shock, or hypothermia) means insufficient blood flow to the capillary bed, making sample collection difficult or impossible and requiring excessive squeezing that further compromises sample quality. Other contraindications include: infected or bruised sites (risk of introducing infection or contamination), healed burns or scars (poor blood supply and abnormal tissue), heavily calloused areas (difficult to puncture), and previously mastectomy side (lymphedema risk). Nail polish does not prevent fingerstick but should be noted for pulse oximetry. Cold fingers can be warmed to improve circulation.
Question 4: For a neonatal PKU screening (newborn screening), the blood specimen is collected on which type of collection device?
- Micro-collection tube (Microtainer)
- Blood spot filter paper card (Guthrie card) (Correct answer)
- Standard EDTA tube (lavender top)
- Heparinized capillary tube
Correct answer: Blood spot filter paper card (Guthrie card)
Newborn metabolic screening uses filter paper blood spot cards (Guthrie cards), where blood drops are applied to designated circles and dried for analysis.
Newborn screening for metabolic disorders (PKU, hypothyroidism, galactosemia, sickle cell disease, and dozens of others) uses a specialized filter paper card, historically called a Guthrie card. Blood is obtained via heelstick and allowed to soak into designated circles on the filter paper by touching the card to a blood drop. The filled circles must be single-layer saturated spots—no layering, no streaking. The card is dried at room temperature, then mailed to the state public health laboratory for analysis using tandem mass spectrometry and other methods. Timing of collection is critical: most states require collection after 24 hours of life (and before discharge) to ensure adequate feeding has occurred, which affects metabolite levels.
Question 5: After a fingerstick, the patient should be instructed to hold their finger:
- Elevated above heart level with pressure applied (Correct answer)
- Dependent (hanging down) with gentle pressure applied
- In a fist to promote blood flow
- Submerged in cold water to stop bleeding
Correct answer: Elevated above heart level with pressure applied
Elevating the puncture site above heart level reduces blood flow and promotes hemostasis while applying gentle pressure; dependent positioning increases bleeding.
After completing a fingerstick blood collection, hemostasis should be achieved by applying gentle, continuous pressure with a clean dry gauze pad while elevating the finger above heart level. Elevation reduces hydrostatic pressure at the puncture site, decreasing blood flow and allowing the clotting process to occur more efficiently. Allowing the finger to hang dependent (below heart level) increases local blood pressure and prolongs bleeding. Having the patient make a fist can increase pressure within the fingertip and promote bleeding. Cold water immersion is not appropriate as it introduces contamination and excessive cold can damage tissue. Most fingerstick sites achieve hemostasis within 1–3 minutes with proper technique.
Question 6: Which micro-collection tube (Microtainer) color is used for hematology testing (CBC) in capillary collection?
- Gold (serum separator)
- Lavender/purple (EDTA) (Correct answer)
- Green (heparin)
- Light blue (sodium citrate)
Correct answer: Lavender/purple (EDTA)
Lavender/purple Microtainer tubes contain EDTA anticoagulant, the same additive used in full-draw lavender-top tubes for CBC and hematology testing.
Capillary Microtainer tubes (BD) and equivalent micro-collection devices use the same color-coding system as full-draw venipuncture tubes for consistency. Lavender/purple micro-collection tubes contain K2EDTA or K3EDTA as the anticoagulant, which chelates calcium to prevent clotting while preserving cell morphology and counts for complete blood count (CBC), differential, hemoglobin, hematocrit, platelet count, and other hematology studies. Gold Microtainer tubes contain a clot activator ± serum separator gel for chemistry testing. Green tubes contain lithium heparin for chemistry panels. Light blue contains sodium citrate for coagulation studies but is rarely used in capillary collection due to the critical 9:1 blood-to-anticoagulant ratio that is difficult to achieve in micro-collection.
Question 7: Why should a capillary blood gas specimen be collected in a heparinized capillary tube and immediately sealed at both ends?
- To prevent evaporation of the liquid sample
- To prevent exposure to atmospheric air that would alter CO2 and O2 levels (Correct answer)
- To keep the specimen at body temperature during transport
- To prevent clotting before analysis
Correct answer: To prevent exposure to atmospheric air that would alter CO2 and O2 levels
Sealing the capillary tube immediately prevents atmospheric oxygen from entering and CO2 from escaping, which would alter blood gas values and render the specimen invalid.
Capillary blood gas specimens must be collected in heparinized capillary tubes (containing dry heparin to prevent clotting) and immediately sealed at both ends with wax caps or clay to prevent any gas exchange with the atmosphere. Atmospheric air contains approximately 21% oxygen and 0.04% CO2. If air bubbles enter the sample or the tube is left open, atmospheric oxygen diffuses into the blood (falsely raising PO2) and CO2 diffuses out of the blood into the air (falsely lowering PCO2). These changes also affect pH calculation. For the same reason, any air bubbles visible in a blood gas syringe for arterial specimens must be expelled immediately. The specimen should also be placed in iced water and transported promptly (within 15–30 minutes) to minimize ongoing cellular metabolism that continues to consume O2 and produce CO2.
Question 8: A heelstick is performed on a 3-day-old full-term newborn. Which of the following actions is INCORRECT?
- Warming the heel for 3–5 minutes before puncture
- Puncturing the posterior curve (back) of the heel (Correct answer)
- Using a blade depth no greater than 2.0 mm
- Collecting the specimen in the medial plantar area
Correct answer: Puncturing the posterior curve (back) of the heel
The posterior curve of the heel is an unacceptable site because the calcaneus is very close to the skin surface there, posing a high risk of bone puncture and osteomyelitis.
CLSI H04 explicitly identifies the posterior curve of the heel as a forbidden site for neonatal heelstick. At the posterior curved surface, the calcaneus bone lies very close to the skin surface (less than 1 mm in many neonates), making bone puncture almost inevitable even with a compliant 2.0 mm lancet. Bone puncture leads to osteomyelitis (bone infection), calcification of soft tissue, and fibrosis—serious complications that have been documented in neonatal cases. The only acceptable heelstick sites are the medial (inner) and lateral (outer) plantar surfaces of the heel, where the distance to the calcaneus is adequate. All other options listed (warming, appropriate depth, medial plantar site) are correct neonatal heelstick practices.
Phlebotomy Test Capillary and Dermal Puncture 2
Which lancet depth is recommended for an adult fingerstick to ensure adequate blood collection without causing undue pain?