Arterial Blood Gas Collection: Scope, Safety, and Collector Role
Objective. Students will define the phlebotomy technician's scope of practice regarding arterial blood gas (ABG) collection, describe the modified Allen test for collateral circulation assessment, and identify pre-analytical variables that affect ABG results.
This lesson, its worked examples and its practice questions has not yet been verified by a licensed clinician in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it to make decisions about any patient or medication. Always follow your instructor, your supervising professional, and current official references and regulations.
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Arterial Blood Gas Collection: Scope, Safety, and Collector Role
Arterial blood gas (ABG) testing measures the partial pressures of oxygen and carbon dioxide, as well as pH and bicarbonate, in arterial blood. Clinicians use these values to evaluate a patient's respiratory function and acid-base status. Because arterial puncture carries greater risk than venous puncture — arteries are under higher pressure, spasm more readily, and complications such as hematoma or arterial occlusion are more serious — the roles involved in ABG collection are carefully divided among health care disciplines.
Scope of Practice for the Phlebotomy Technician
In most US states, the phlebotomy technician's role in the ABG process is supportive and pre/post-analytical, not the arterial puncture itself. Arterial puncture is typically performed by a respiratory therapist, a registered nurse with documented competency, a physician, or another licensed practitioner as defined by your state's practice act and facility policy.
A phlebotomy technician may typically:
- Prepare and label the heparinized syringe and other supplies before the procedure.
- Assist with patient identification and positioning.
- Perform the modified Allen test to assess collateral circulation before the puncture (in many facilities this is delegated to the phlebotomist as a safety check).
- Immediately process and transport the specimen after collection: expel air bubbles, cap the syringe, mix gently by rolling, place on ice (or use a refrigerating transport system per facility policy), and deliver to the laboratory within the time frame specified by your facility.
- Document collection time, patient temperature, oxygen delivery method and flow rate, and patient position on the specimen label or requisition.
A phlebotomy technician may not:
- Perform the arterial puncture unless a specific state law or facility policy explicitly grants that permission and the technician holds documented competency.
- Interpret ABG results or alter a patient's oxygen therapy based on values.
Because scope varies by state, always consult your state's phlebotomy practice guidelines and your employer's policies. When in doubt, defer to the licensed practitioner. Zero-tolerance rule: never perform a procedure outside your legal scope.
The Modified Allen Test
Before any radial artery puncture, collateral circulation to the hand must be confirmed. The radial and ulnar arteries both supply the hand via the palmar arch. If the radial artery is damaged or goes into spasm, the ulnar artery must be able to take over. The modified Allen test is the standard bedside method for checking this.
Procedure:
1. Ask the patient to make a tight fist with the chosen hand.
2. Apply firm pressure over both the radial and ulnar arteries simultaneously to occlude flow.
3. Ask the patient to open the hand — it should appear pale.
4. Release pressure over the ulnar artery only while continuing to compress the radial artery.
5. Observe the time for color (flush) to return to the hand and fingers.
Interpreting results:
- Positive result (acceptable): Color returns within 5–10 seconds (some references allow up to 15 seconds depending on facility policy). This indicates adequate ulnar collateral circulation. Proceed with radial artery puncture.
- Negative result (not acceptable): Color returns slowly (beyond the facility's cutoff) or not at all. This indicates poor collateral circulation. Do not puncture the radial artery on that side. Notify the ordering clinician; an alternate site or alternate artery must be considered.
Document the result and which hand was tested. This documentation is a patient safety record.
Pre-Analytical Variables That Affect ABG Results
Pre-analytical errors — those occurring before the specimen reaches the analyzer — are a leading cause of inaccurate ABG results. As the person who handles the specimen immediately after collection, the phlebotomy technician has direct influence over several of these variables.
Supplemental oxygen: If a patient is on supplemental oxygen and the ordering clinician has requested a room-air baseline, the sample must be drawn only after the patient has breathed room air for the time specified (commonly 20–30 minutes). Failing to document or control oxygen status will falsely elevate PaO₂.
Air bubbles in the syringe: Room air has a high PO₂ and low PCO₂ relative to arterial blood. Air bubbles that remain in the syringe will raise PaO₂ and lower PaCO₂. Bubbles must be expelled immediately after collection.
Specimen not placed on ice (or equivalent cooling): Red blood cells and white blood cells in the sample continue to metabolize oxygen and produce CO₂ at room temperature. Delayed cooling causes PaO₂ to fall and PaCO₂ and lactate to rise. Specimens should be analyzed within 15–30 minutes of collection, or placed on ice/wet ice slurry if transport will exceed that window (follow your facility's protocol and the analyzer manufacturer's guidance).
Prolonged transport time: Extends the metabolic effect described above, compounding errors in PaO₂, PaCO₂, and pH.
Excess heparin in the syringe: Heparin is acidic. Excess liquid heparin dilutes the sample and can lower measured pH and PCO₂. Pre-filled heparinized syringes or dry-balanced heparin formulations minimize this risk.
Patient temperature: Analyzers calibrate to 37 °C. Hypothermic patients have higher dissolved gas levels than the machine will report; hyperthermia has the opposite effect. Document the patient's actual temperature so the clinician can apply a temperature correction if needed.
Patient anxiety or hyperventilation: Can lower PaCO₂ and raise pH before or during the draw.
Patient position: Sitting versus supine can affect oxygenation values in some patients. Document position at time of draw.
Safety Summary
- Always confirm patient identity with two identifiers before any specimen-related activity.
- Use standard precautions; arterial blood carries the same infectious risk as venous blood.
- Never perform arterial puncture outside your legal scope.
- A negative Allen test result is a stop signal — communicate it immediately and do not proceed.
- Process and transport ABG specimens rapidly; time is a critical pre-analytical variable.
Key terms10
Materials: State phlebotomy practice guideline printout, ASCP scope-of-practice reference, Allen test demonstration handout, pre-analytical variable worksheet, phlebotomy textbook ABG chapter
Check: T-chart evaluated for accuracy — no scope-of-practice boundaries may be misclassified (zero-error tolerance on items that would place the student outside legal scope); Allen test demonstration rated satisfactory on a 4-step checklist; worksheet minimum 80%.
Practice check8
Answer each question, then check it. Nothing is recorded; this is your own practice.
