11.1 Arterial & Capillary Blood Sample Collection

Why sampling skill is scored on the RPFT

Domain II — Procedures is the largest slice of the NBRC Pulmonary Function Technologist Examination (about 44% of scored items). Under the blood-gas cluster, the outline separates selecting sampling methods (II.A.4), performing collection (II.B.4), and evaluating validity of the sample (II.C.4). Equipment setup for blood gas analyzers (Domain I.A.2) and for the arterial/venous collection gear itself (I.A.16) is covered in Chapters 3 and 5; here the focus is how you obtain and protect the specimen so that analyzer numbers are clinically usable.

Form construction note: NBRC may cap blood gas analysis items (often cited as a maximum of about 3 analysis items on a form), but sampling and procedural validity still appear under the procedures domain. High-cut (RPFT) items favor vignettes: failed Allen test, air in the syringe, delayed uniced sample, excess heparin, or capillary versus arterial choice.

Site selection and the modified Allen test

Radial artery is the first-choice site for most elective ABGs because it is superficial, compressible against bone, and has collateral flow via the ulnar artery and palmar arches in most patients. Brachial and femoral sites are alternatives when radial access fails or anatomy forbids radial puncture—each carries higher complication risk and usually requires stricter policy and skill verification.

Before radial puncture, perform a modified Allen test (or an accepted collateral-flow assessment used by your lab):

1. Explain the test; patient clenches a fist while you compress both radial and ulnar arteries.
2. Patient opens the hand (avoid hyperextension of the fingers, which can blanch the palm artifactually).
3. Release ulnar compression only, keeping radial compressed.
4. Observe return of color to the palm. Adequate collateral flow is typically return within about 5–15 seconds (lab policies may state a specific cutoff such as ≤10–15 s).
5. If color returns slowly or not at all, do not puncture that radial artery; try the other wrist after testing, or choose another site per protocol.

Exam trap: Releasing the radial instead of the ulnar, or skipping Allen testing when collateral assessment is indicated, is a classic error item.

Radial ABG technique overview

Positioning and landmarks

• Extend the wrist slightly over a rolled towel or board; support the forearm.
• Palpate the strongest pulse; cleanse per aseptic protocol (alcohol or chlorhexidine per policy).
• Use a heparinized ABG syringe (dry balanced lithium or sodium heparin designed for blood gases) with an appropriate needle gauge (often 22–25 G for radial).

Needle angle and entry

• Hold the syringe like a pencil; bevel up.
• Approach the artery at roughly a 45-degree angle (some protocols allow a slightly steeper angle for deeper arteries); advance slowly toward the pulse.
• Arterial pressure usually fills the syringe; do not force aspiration that draws venous blood or tissue fluid.
• Obtain the volume required by the analyzer (often ~1–3 mL depending on device and co-ox needs)—do not underfill relative to the syringe’s heparin design volume.

Anaerobic handling after draw

1. Expel air bubbles immediately—air equilibrates with the sample and typically raises PO2 toward room air and lowers PCO2 if delay occurs.
2. Cap the syringe with an anaerobic tip cap; never leave an open needle hub.
3. Mix gently by inversion/rolling so heparin contacts the entire sample and prevents microclots without vigorous shaking that can hemolyze red cells.
4. Label at the bedside: patient ID, FiO2 or O2 device and flow, temperature if relevant, time of draw, and site.
5. Transport promptly. If analysis will be delayed, use ice slurry when required by specimen type and lab policy (historically important for glass syringes and longer delays); many plastic ABG syringes are validated for room-temperature short delays—follow manufacturer and current AARC/ATS-aligned lab SOPs rather than memorizing a single universal ice rule.

Patient aftercare

Apply firm pressure for several minutes (longer if the patient is anticoagulated), then check for hematoma and distal perfusion (color, temperature, capillary refill, sensation). Document the attempt and any complication.

Capillary (arterialized) sampling: use cases and limits

Capillary blood gas (CBG) sampling from a warmed heel (neonates/infants) or fingertip/earlobe (selected older patients) is used when arterial puncture is technically difficult, repeatedly refused, or disproportionate to the clinical need—common in pediatrics and some monitoring contexts.

Arterialization (warming the site) increases local blood flow so capillary values better approximate arterial pH and PCO2. PO2 from capillary samples is less reliable than arterial PO2 and should not be treated as interchangeable with ABG PO2 for precise oxygenation decisions when an arterial sample is obtainable and required.

Validity (II.C.4): Reject or qualify samples with air gaps in capillary tubes, excessive milking of the site, delayed analysis, or clear venous contamination.

Anticoagulant (heparin) effects and sample integrity

ABG syringes use heparin to prevent clotting in the sample path and electrodes.

• Correct fill volume matches the dry heparin amount engineered into the syringe.
• Excess liquid heparin (overfilling the dead space with liquid heparin or underfilling blood relative to liquid heparin) dilutes the sample: expect falsely ↓ PCO2, dilution of electrolytes/Hct if measured, and distortion of calculated HCO3/BE related to PCO2/pH shifts.
• Insufficient mixing → clots → analyzer flags, clogs, or rejected runs.
• Hemolysis (traumatic draw, vigorous shaking) can alter potassium and some optical readings; treat as integrity failure when flags appear.

Integrity checklist before analysis:

• No visible air after expelling bubbles
• Adequate volume, well mixed, no clots
• Correct patient/oxygen context documented
• Within allowed time/temperature window

Complications: recognize and respond

Selection skill (II.A.4): choose capillary versus arterial versus delay/defer based on order, patient age, collateral flow, anticoagulation, and whether PO2 precision is required. Performance skill (II.B.4): execute Allen testing, puncture, anaerobic handling, and aftercare. Validity skill (II.C.4): decide whether the specimen is acceptable to analyze or must be redrawn.

/practice/rpftPractice questions with detailed explanations