Phlebotomy Test Arterial Blood Gas Collection 1 — Questions and Answers

Question 1: The Allen test is performed before radial artery puncture to assess:

The depth of the radial artery
Adequacy of collateral circulation from the ulnar artery (Correct answer)
The patient's pain threshold before arterial puncture
Patency of the brachial artery

Correct answer: Adequacy of collateral circulation from the ulnar artery

The Allen test determines whether the ulnar artery can adequately perfuse the hand if the radial artery is damaged during arterial puncture, ensuring collateral circulation is sufficient.

The Allen test (or modified Allen test) is a required pre-procedure assessment before radial artery puncture for ABG collection or arterial line placement. It tests the adequacy of collateral blood flow from the ulnar artery to the hand. Procedure: (1) Compress both the radial and ulnar arteries while the patient makes a tight fist; (2) Patient opens hand—it should be pale/blanched; (3) Release ulnar artery compression only; (4) Observe hand for return of color (flush). A positive Allen test (color returns within 5–15 seconds) indicates adequate ulnar collateral circulation, and it is safe to proceed with radial puncture. A negative Allen test (color does not return within 15 seconds) indicates inadequate collateral circulation—the radial artery is the dominant supply, and puncture could compromise hand perfusion, leading to ischemia. In this case, an alternative site (brachial or femoral artery) should be used.

Question 2: What angle of needle insertion is recommended for radial artery puncture for ABG collection?

15–30 degrees
45–60 degrees (Correct answer)
90 degrees
10–15 degrees

Correct answer: 45–60 degrees

A needle insertion angle of 45–60 degrees is recommended for radial artery puncture, providing the optimal approach to enter the artery at its depth without excessive angulation.

For radial artery puncture, the recommended needle insertion angle is 45–60 degrees (most commonly cited as 45 degrees in many protocols). This angle is determined by the depth and position of the radial artery, which lies just beneath the skin surface at the wrist but at a slight depth requiring an angled approach. A shallower angle (15–30 degrees, used for venipuncture) may not adequately enter the artery, while too steep an angle (90 degrees) increases the risk of going through the artery (trans-fixation) or hitting the periosteum (bone covering). Some protocols specify 30–45 degrees depending on patient anatomy. The needle should be advanced slowly with the bevel up until pulsatile blood fills the syringe under arterial pressure without aspiration.

Question 3: After collecting an ABG sample, the syringe should be gently rotated/rolled for at least 5 seconds before capping. This step ensures:

The sample cools to room temperature before analysis
The heparin anticoagulant is thoroughly mixed with the blood (Correct answer)
Air bubbles rise to the top for easier removal
The sample clots before being placed on ice

Correct answer: The heparin anticoagulant is thoroughly mixed with the blood

Rolling the syringe mixes the heparin anticoagulant throughout the blood sample, preventing clot formation that would interfere with blood gas analysis.

ABG syringes contain a small amount of heparin (either liquid or lyophilized/dry) pre-loaded to prevent blood from clotting during collection and transport. After collection, the syringe must be gently rolled between the palms (not shaken vigorously) for at least 5–10 seconds to ensure the heparin is thoroughly mixed with the blood. This prevents micro-clots from forming in the sample. Micro-clots can obstruct the blood gas analyzer's sample pathway, produce erroneous results, and damage the analyzer. Shaking rather than rolling causes hemolysis and introduces air bubbles. Liquid heparin (as opposed to dry/lyophilized) can dilute the sample if excessive volumes are used, affecting PO2, PCO2, and pH values—which is why most modern ABG syringes use pre-measured dry heparin.

Question 4: A patient presents with uncontrolled hypertension and is on anticoagulation therapy. When preparing to collect an ABG, the phlebotomist should recognize that anticoagulation therapy requires:

Using a larger bore needle to ensure adequate sample
Applying pressure for an extended period (5–10 minutes or more) after collection (Correct answer)
Avoiding the radial artery and using femoral access only
No special precautions—the procedure is unchanged

Correct answer: Applying pressure for an extended period (5–10 minutes or more) after collection

Patients on anticoagulants have impaired clotting; pressure must be held for a prolonged time (at least 5 minutes, often 10+ minutes) at the arterial puncture site to achieve hemostasis.

Arterial puncture sites require significantly more post-procedure pressure than venipuncture sites because arteries are under much higher pressure. Under normal circumstances, firm, continuous pressure is applied for at least 3–5 minutes after radial artery puncture and hemostasis is confirmed before the patient is left. In patients on anticoagulation therapy (warfarin, heparin, direct oral anticoagulants, antiplatelet agents) or with coagulopathies, the clotting response is impaired, requiring prolonged pressure of 10–15 minutes or more. Failure to achieve hemostasis leads to hematoma formation that can compress surrounding structures including the median nerve, causing permanent neurological damage. The site must be inspected before the phlebotomist leaves, and any developing hematoma requires immediate additional pressure.

Question 5: If an air bubble enters the ABG syringe during collection, the IMMEDIATE action should be to:

Place the syringe on ice to prevent gas exchange
Expel the air bubble immediately before capping the syringe (Correct answer)
Mix the syringe contents to incorporate the air bubble
Discard the entire sample and recollect

Correct answer: Expel the air bubble immediately before capping the syringe

Air bubbles must be expelled immediately because atmospheric air (high O2, low CO2) will equilibrate with the blood sample, falsely raising PO2 and lowering PCO2.

Air bubbles in an ABG syringe are a critical source of error because air contains approximately 21% oxygen (PO2 ~155 mmHg at sea level) and almost no CO2 (PCO2 ~0.3 mmHg). Blood typically has PO2 of 80–100 mmHg and PCO2 of 35–45 mmHg. When air contacts the blood sample, gas exchange occurs according to partial pressure gradients: O2 from air diffuses into blood (falsely raising PO2) and CO2 from blood diffuses into air (falsely lowering PCO2). These changes also affect the calculated pH. If the air bubble is small and noticed immediately (within seconds), it should be expelled by pointing the syringe upward and gently pushing the plunger. If significant time has passed or the bubble is large, the sample may need to be discarded. This is why ABG syringes are designed to minimize air entry and why immediate capping after collection is essential.

Question 6: The preferred site for ABG collection in most adult patients is the:

Femoral artery
Brachial artery
Radial artery (Correct answer)
Dorsalis pedis artery

Correct answer: Radial artery

The radial artery is preferred for ABG collection because it is superficial and easy to palpate, has collateral circulation from the ulnar artery, and complications can be quickly identified.

The radial artery at the wrist is the preferred site for arterial blood gas collection in adults for several reasons: (1) It is superficial and easily palpable against the radius bone; (2) The hand has collateral circulation from the ulnar artery (confirmed by Allen test), providing a safety margin if radial artery injury occurs; (3) The relatively small caliber reduces bleeding risk compared to the brachial or femoral arteries; (4) Complications (hematoma, spasm) are easily visible and accessible for pressure application; (5) There are no major adjacent nerves or veins at high risk. The brachial artery is an alternative when the radial is inaccessible, but there is no collateral circulation at the elbow. The femoral artery is used as a last resort in emergency situations due to its large size, deep location, and proximity to the femoral nerve and vein, with higher complication risks.

Question 7: An ABG result shows pH 7.52, PCO2 28 mmHg, HCO3 22 mEq/L. This pattern is consistent with:

Respiratory acidosis
Metabolic alkalosis
Respiratory alkalosis (Correct answer)
Metabolic acidosis

Correct answer: Respiratory alkalosis

pH 7.52 (alkalotic), low PCO2 (28 mmHg—hyperventilation), and normal bicarbonate indicate respiratory alkalosis, caused by excessive CO2 elimination.

Interpreting ABG results requires assessing pH, PCO2, and HCO3 systematically. Normal values: pH 7.35–7.45, PCO2 35–45 mmHg, HCO3 22–26 mEq/L. In this case: pH 7.52 is alkalotic (>7.45). The PCO2 of 28 mmHg is below normal (indicating hyperventilation—blowing off CO2). The HCO3 of 22 mEq/L is normal. Since the primary abnormality that explains the alkalosis is the low PCO2 (respiratory parameter), this is respiratory alkalosis. CO2 + H2O ⇌ H2CO3 ⇌ H⁺ + HCO3⁻: when CO2 decreases, fewer H+ ions are produced, raising pH. Respiratory alkalosis is caused by hyperventilation (anxiety, hypoxia, fever, salicylate toxicity, mechanical ventilator settings). If HCO3 were elevated with normal or slightly elevated PCO2, it would be metabolic alkalosis.

Question 8: After an ABG collection, which of the following complications should the phlebotomist monitor for BEFORE leaving the patient?

Needle phobia and vasovagal response only
Hematoma formation, continued bleeding, and adequacy of distal pulse (Correct answer)
Changes in the patient's blood pressure only
Allergic reaction to the heparin in the syringe

Correct answer: Hematoma formation, continued bleeding, and adequacy of distal pulse

Post-ABG, the phlebotomist must confirm hemostasis, check for hematoma, ensure there is no continued bleeding, and verify the distal pulse is intact before leaving.

After ABG collection, the phlebotomist or clinician must remain at the bedside maintaining pressure for at least 3–5 minutes (longer for anticoagulated patients) and then perform a post-procedure assessment before leaving: (1) Confirm hemostasis—no active bleeding from the puncture site; (2) Check for hematoma formation—a developing lump under the skin indicates arterial bleeding into tissue, requiring more pressure; (3) Assess distal circulation—check the radial pulse distal to the puncture (or capillary refill in fingers) to ensure arterial blood flow is intact; (4) Assess for signs of arterial spasm—pallor, pain, paresthesia, or cold hand; (5) Ensure the patient is comfortable and aware of warning signs. Arterial spasm (vasospasm) can temporarily reduce blood flow. Prolonged hematoma can compress the median nerve. These complications require immediate nursing/medical attention.

Phlebotomy Test Arterial Blood Gas Collection 1

The Allen test is performed before radial artery puncture to assess:

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