Phlebotomy Test Arterial Blood Gas Collection 2 — Questions and Answers
Question 1: For an ABG specimen, which anticoagulant is used in the collection syringe?
- EDTA (ethylenediaminetetraacetic acid)
- Sodium citrate
- Heparin (liquid or lyophilized) (Correct answer)
- Sodium fluoride
Correct answer: Heparin (liquid or lyophilized)
Heparin (as liquid sodium heparin or dry/lyophilized heparin) is the anticoagulant used in ABG syringes because it does not affect blood gas values when used in proper amounts.
ABG collection syringes are pre-loaded with heparin as the anticoagulant. Heparin works by activating antithrombin III, which inhibits thrombin and factor Xa, preventing clot formation in the collected specimen. Modern ABG syringes typically use lyophilized (dry) heparin rather than liquid heparin to avoid dilution errors—liquid heparin can dilute the sample and falsely lower PO2, PCO2, and electrolyte values if too much is used. The dry heparin dissolves when it contacts blood during collection, providing anticoagulation without dilution. The amount of heparin used is precisely measured by the manufacturer to avoid electrolyte interference (excess heparin can affect ionized calcium and pH measurements). EDTA is used for hematology but is not appropriate for blood gases. Sodium fluoride is used for glucose preservation only.
Question 2: How should an ABG specimen be transported to the laboratory if analysis will be delayed beyond 30 minutes?
- At room temperature in a sealed syringe cap
- In iced slush or ice water to slow cellular metabolism (Correct answer)
- In a warming block at 37°C to simulate body temperature
- In a biohazard bag without ice at ambient temperature
Correct answer: In iced slush or ice water to slow cellular metabolism
ABG specimens must be transported in iced slush/ice water to slow cellular metabolism (O2 consumption and CO2 production) that would otherwise alter blood gas values.
Blood continues to undergo cellular metabolism after collection: white blood cells and platelets consume oxygen (decreasing PO2) and produce carbon dioxide (increasing PCO2) and lactate (decreasing pH). At 37°C, these changes occur rapidly. Cooling the specimen in an ice water slush (approximately 0°C to 4°C) significantly slows metabolic activity, minimally affecting blood gas values for up to 30–60 minutes. However, even on ice, samples should be analyzed as soon as possible (within 30 minutes if possible, 60 minutes maximum). If the specimen has a high WBC count (leukocytosis >50,000) or platelet count (thrombocytosis), metabolic consumption is even more rapid, and specimens should be iced immediately and analyzed within 15–20 minutes. Modern ABG analyzers process specimens in 60–90 seconds, so transport time is the critical variable.
Question 3: What is the significance of a 'negative Allen test' before radial artery puncture?
- The radial artery is too deep to puncture safely
- There is inadequate ulnar collateral circulation; the radial artery should not be punctured (Correct answer)
- The ulnar artery is blocked and must be punctured instead
- The test failed and must be repeated
Correct answer: There is inadequate ulnar collateral circulation; the radial artery should not be punctured
A negative Allen test means the ulnar artery cannot adequately supply the hand if the radial artery is damaged; proceeding with radial puncture risks hand ischemia.
A negative Allen test result is defined as failure of color (circulation) to return to the hand within 5–15 seconds after releasing ulnar artery compression (while maintaining radial artery compression). This indicates that the ulnar artery cannot provide adequate collateral blood flow to the hand, meaning the radial artery is the dominant or sole supply. If the radial artery is subsequently damaged during arterial puncture (thrombosis, prolonged spasm, or inadvertent laceration), the hand has no collateral circulation to fall back on, putting the patient at risk for hand ischemia, necrosis, and potential amputation. With a negative Allen test, the phlebotomist should not use the radial artery on that side. Alternative options include: the contralateral radial artery (if Allen test is positive on that side), the brachial artery, or the femoral artery.
Question 4: A phlebotomist notes pulsatile blood flow filling the ABG syringe spontaneously. This indicates:
- Venous blood was accidentally collected instead of arterial
- The needle is correctly positioned within the artery (Correct answer)
- The patient has abnormally high venous pressure
- The syringe plunger must be pulled to aspirate arterial blood
Correct answer: The needle is correctly positioned within the artery
Spontaneous pulsatile blood flow into the ABG syringe without aspiration confirms correct arterial needle placement, as arterial pressure drives blood into the syringe.
One of the key indicators of successful arterial puncture is the spontaneous, pulsatile filling of the ABG syringe without the operator pulling the plunger. Arteries carry blood under high pressure (systolic pressure 120 mmHg in a normotensive patient), and this pressure is sufficient to drive blood into the syringe against minimal resistance. The pulsatile pattern (surges with each heartbeat) confirms arterial placement. In contrast, venous blood (which is under much lower pressure, typically 5–15 mmHg) will not fill the syringe spontaneously unless the needle is in a very large vein. If the plunger must be actively pulled to obtain blood, this suggests possible venous placement or arterial spasm. Venous blood is also darker (deoxygenated), while arterial blood is typically bright red (oxygenated), though this distinction is less reliable in severely hypoxic patients.
Question 5: Which of the following patients would have a CONTRAINDICATION to radial artery puncture at a specific site?
- A patient with a history of anxiety
- A patient with a negative Allen test at that wrist (Correct answer)
- A patient on blood pressure medications
- A patient who prefers not to have blood drawn from the wrist
Correct answer: A patient with a negative Allen test at that wrist
A negative Allen test at that wrist indicates absent ulnar collateral circulation, making radial artery puncture at that site contraindicated due to risk of hand ischemia.
The modified Allen test is performed specifically to identify contraindications to radial artery puncture. A negative Allen test (failure of hand flushing within 5–15 seconds of ulnar release) is an absolute contraindication to radial artery puncture at that wrist because adequate collateral circulation is absent. Other contraindications to radial artery puncture at a specific site include: (1) Active infection or cellulitis over the site; (2) Previous surgical procedure that damaged the artery; (3) Presence of an AV fistula or graft in that arm (used for dialysis); (4) Evidence of peripheral arterial disease with absent distal pulse; (5) Burns or traumatic injury to the area. Patient anxiety, blood pressure medications, or patient preference alone do not represent absolute contraindications, though they require consideration and patient preparation.
Question 6: When preparing a glass ABG syringe with liquid heparin, how should excess heparin be handled?
- Leave the excess heparin in the syringe to ensure adequate anticoagulation
- Expel all but a thin film coating the inside of the syringe and needle (Correct answer)
- Remove the heparin and add it after blood is collected
- Use the excess heparin to wet the syringe tip before insertion
Correct answer: Expel all but a thin film coating the inside of the syringe and needle
Excess liquid heparin must be expelled, leaving only a thin film; excess heparin dilutes the sample and causes significant errors in PO2, PCO2, and electrolyte measurements.
When using glass ABG syringes with liquid heparin (1,000 units/mL sodium heparin is standard): (1) Draw up 0.5–1.0 mL of heparin to coat the syringe barrel; (2) Rotate/roll the syringe to coat all interior surfaces; (3) Hold the syringe tip up and expel ALL excess heparin, leaving only the thin film coating and the dead space of the needle filled with heparin. Excess liquid heparin left in the syringe will mix with the blood sample and cause: (1) Dilution of all analytes—falsely lower PO2, PCO2, and electrolytes; (2) Ionic heparin can directly bind calcium, falsely lowering ionized calcium; (3) Falsely lower pH due to the heparin solution's pH. Modern pre-heparinized ABG syringes with lyophilized (dry) heparin have largely eliminated this problem in clinical practice.
Question 7: A phlebotomist accidentally punctures the accompanying vein instead of the radial artery. Which finding would help distinguish a venous from an arterial sample?
- Sample color and flow characteristics (Correct answer)
- Patient complaint of pain at the puncture site
- Sample temperature
- The amount of blood collected
Correct answer: Sample color and flow characteristics
Arterial blood is bright red and fills the syringe pulsatilely without aspiration; venous blood is darker and requires plunger aspiration, distinguishing the two specimen types.
The primary ways to distinguish arterial from venous blood during ABG collection: (1) Color: arterial blood (oxygenated) is bright red; venous blood is darker crimson-burgundy. However, in severely hypoxic patients, arterial blood may also be dark, making color unreliable. (2) Flow: arterial blood fills the syringe spontaneously and pulsatilely (driven by cardiac systole); venous blood requires active aspiration (plunger pulling) and fills steadily without pulsation. (3) Pressure: arteries are under high pressure; veins under low pressure. If there is uncertainty about sample source, the blood gas results themselves will clarify: a true arterial PO2 of 80–100 mmHg (on room air) versus a venous PO2 of 35–45 mmHg. pH is also slightly lower in venous blood (7.31–7.41) compared to arterial (7.35–7.45).
Question 8: Normal arterial blood gas values at sea level breathing room air include which of the following pH ranges?
- 7.25–7.35
- 7.35–7.45 (Correct answer)
- 7.45–7.55
- 7.30–7.40
Correct answer: 7.35–7.45
Normal arterial pH is 7.35–7.45; values below this range indicate acidosis and above indicate alkalosis.
Normal arterial blood gas reference values (breathing room air at sea level): pH 7.35–7.45 (normal physiological range), PaO2 80–100 mmHg (partial pressure of oxygen), PaCO2 35–45 mmHg (partial pressure of CO2), HCO3 22–26 mEq/L (bicarbonate), SaO2 95–100% (oxygen saturation). Values outside these ranges indicate acid-base disturbances: pH <7.35 = acidosis (acidemia); pH >7.45 = alkalosis (alkalemia). The primary respiratory parameter is PCO2 (CO2 is an acid—increased PCO2 lowers pH, decreased PCO2 raises pH). The primary metabolic parameter is HCO3 (bicarbonate is a base—increased HCO3 raises pH, decreased HCO3 lowers pH). These values are fundamental to interpreting respiratory function and acid-base balance.
Phlebotomy Test Arterial Blood Gas Collection 2
For an ABG specimen, which anticoagulant is used in the collection syringe?