A 59-year-old woman in the emergency department, breathing 40% oxygen by mask, has an arterial sample taken into a 3-mL plastic syringe with dry balanced heparin. It reaches the laboratory 55 minutes later at room temperature, with a 0.4-mL air bubble trapped under the cap. The doctor wants pH, pCO2, pO2 and lactate to guide treatment for suspected sepsis. The laboratory follows CLSI guidance: air is expelled at once after collection, and plastic syringes kept at room temperature are analyzed within 30 minutes. What should the technologist do?

Reject it and request a new anaerobic sample

Report the lactate and withhold the gases

Expel the bubble, remix and analyze it now

Report pH and pCO2 and withhold the pO2

A 3-mL syringe coated with dry, non-balanced lithium heparin is filled with only 0.8 mL of arterial blood and analyzed within 5 minutes, with no air. Why is the ionized calcium falsely low?

Excess heparin has bound some of the calcium

Red cells have taken up calcium in the syringe

Glycolysis has lowered the sample's pH

The dry heparin has diluted the sample

A stable ICU patient's arterial-line samples, collected in dry-heparin syringes, give: one hour ago Hb 118 g/L, Cl- 103 mmol/L, K+ 4.2 mmol/L, pCO2 5.3 kPa (40 mm Hg); now Hb 86 g/L, Cl- 116 mmol/L, K+ 3.1 mmol/L, pCO2 3.9 kPa (29 mm Hg). The most likely cause is:

Flush solution drawn into the sample

Acute blood loss since the last sample

Venous blood drawn instead of arterial

Hyperventilation during the sampling

Air bubbles in a freshly drawn arterial blood gas syringe should be expelled:

Straight away, before the syringe is capped and mixed

After the syringe has been mixed for 2 minutes

At the analyzer, just before the sample is injected

Only if analysis will be delayed beyond 30 minutes

Blood gas sampling and preanalytical errors

10 DOH Abu Dhabi (HAAD) lab licensing practice questions on Blood gas sampling and preanalytical errors, part of Clinical Chemistry. The first 10 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Hard
A 59-year-old woman in the emergency department, breathing 40% oxygen by mask, has an arterial sample taken into a 3-mL plastic syringe with dry balanced heparin. It reaches the laboratory 55 minutes later at room temperature, with a 0.4-mL air bubble trapped under the cap. The doctor wants pH, pCO2, pO2 and lactate to guide treatment for suspected sepsis. The laboratory follows CLSI guidance: air is expelled at once after collection, and plastic syringes kept at room temperature are analyzed within 30 minutes. What should the technologist do?
1. AReport the lactate and withhold the gases
2. BReject it and request a new anaerobic sample
3. CExpel the bubble, remix and analyze it now
4. DReport pH and pCO2 and withhold the pO2
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Answer: B · Reject it and request a new anaerobic sample
Two separate faults affect every requested result, so the sample cannot be rescued. The trapped air exchanges gas with the blood: CO2 escapes into the bubble, which lowers pCO2 and raises pH, and pO2 moves toward the bubble's value of about 20 kPa (150 mm Hg). During 55 minutes at room temperature the cells keep consuming oxygen and making lactate by glycolysis, so lactate rises and the gases drift further. Expelling the bubble now cannot undo changes that have already happened. The rejection and its reason are documented and the ward is told at once, because a septic patient needs a reliable result quickly.
- A: Lactate keeps rising from glycolysis while blood stands at room temperature, so it is unreliable too.
- C: Gas exchange with the bubble and the long delay have already changed the results; expelling air now cannot reverse them.
- D: The bubble and the delay also change pCO2 and pH, so these results are unreliable as well.
Question 2 · Medium
A 3-mL syringe coated with dry, non-balanced lithium heparin is filled with only 0.8 mL of arterial blood and analyzed within 5 minutes, with no air. Why is the ionized calcium falsely low?
1. ARed cells have taken up calcium in the syringe
2. BGlycolysis has lowered the sample's pH
3. CExcess heparin has bound some of the calcium
4. DThe dry heparin has diluted the sample
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Answer: C · Excess heparin has bound some of the calcium
Heparin is a negatively charged polymer that binds ionized calcium, and the effect grows as the heparin-to-blood ratio rises. Underfilling a syringe meant for 3 mL raises the heparin concentration nearly fourfold, so the ionized calcium reads low. Syringes for ionized calcium should contain electrolyte-balanced (calcium-titrated) heparin and be filled to the stated volume. Dry heparin adds no liquid, which is why it is preferred over liquid heparin.
- A: Red cells do not take up a measurable amount of calcium within minutes.
- B: A lower pH would raise ionized calcium, and 5 minutes is too short for glycolysis to matter.
- D: Dry heparin adds no liquid, so there is no dilution.
Question 3 · Medium
A blood gas syringe stands for 15 minutes and is then injected without being mixed again. Which reported result is most at risk?
1. AIonized calcium
2. BSodium and potassium
3. CTotal hemoglobin
4. DpCO2 and pO2
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Answer: C · Total hemoglobin
Red cells settle in a syringe within minutes, so the portion the analyzer aspirates may hold too many or too few cells. Total hemoglobin and the co-oximetry fractions depend directly on how many red cells are sampled, so they are the results most affected. Gases and electrolytes are measured in the plasma phase and change little with settling. Syringes should be rolled between the palms and inverted thoroughly just before injection.
- A: Ionized calcium is measured in plasma and is not affected by red cell settling.
- B: Electrolyte electrodes respond to plasma activity, which settling does not change.
- D: Gas tensions are the same in settled and mixed blood.
Question 4 · Hard
A stable ICU patient's arterial-line samples, collected in dry-heparin syringes, give: one hour ago Hb 118 g/L, Cl- 103 mmol/L, K+ 4.2 mmol/L, pCO2 5.3 kPa (40 mm Hg); now Hb 86 g/L, Cl- 116 mmol/L, K+ 3.1 mmol/L, pCO2 3.9 kPa (29 mm Hg). The most likely cause is:
1. AAcute blood loss since the last sample
2. BVenous blood drawn instead of arterial
3. CFlush solution drawn into the sample
4. DHyperventilation during the sampling
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Answer: C · Flush solution drawn into the sample
Hemoglobin, potassium and pCO2 have all fallen by about the same proportion (roughly 27%), while chloride has risen toward the 154 mmol/L of 0.9% saline. This is the signature of flush saline from the arterial line contaminating the sample, which happens when too little dead-space volume is discarded first. Saline contains no CO2 and has a pO2 close to that of room air, so it can also pull pO2 toward about 20 kPa (150 mm Hg). The sample should be recollected after withdrawing a discard volume of several times the line's dead space.
- A: Bleeding does not lower hemoglobin this fast, and it would not raise chloride while lowering potassium and pCO2.
- B: Venous blood has a higher pCO2, not a lower one.
- D: Hyperventilation lowers pCO2 but cannot change hemoglobin, potassium or chloride.
Question 5 · Easy
Air bubbles in a freshly drawn arterial blood gas syringe should be expelled:
1. AStraight away, before the syringe is capped and mixed
2. BAfter the syringe has been mixed for 2 minutes
3. CAt the analyzer, just before the sample is injected
4. DOnly if analysis will be delayed beyond 30 minutes
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Answer: A · Straight away, before the syringe is capped and mixed
Air in the syringe starts exchanging gas with the blood at once, so bubbles must be removed immediately, before capping and mixing. Mixing first breaks the air into tiny bubbles that are hard to expel and speeds up equilibration. Room air has a pO2 near 20 kPa (150 mm Hg) and almost no CO2, so the sample's pO2 moves toward that value and its pCO2 falls. The cap is then fitted and the syringe rolled gently to mix in the heparin.
- B: Mixing first spreads the air through the blood and speeds gas exchange.
- C: By the time the sample reaches the analyzer, the gases have already shifted.
- D: Even a short delay with air in the syringe changes pO2 and pCO2.
Question 6 · Hard
An arterial sample for gases and electrolytes was kept in ice water for 2 hours; its potassium is 4.9 mmol/L. A lithium heparin tube drawn at the same time and spun within 30 minutes gives 4.1 mmol/L. The syringe had no clots or air, and plasma from both shows no hemolysis. What best explains the difference?
1. APlatelets released K+ as the syringe sample clotted
2. BCold slowed the Na+/K+ pump and K+ leaked out of cells
3. CLithium heparin in the syringe added extra K+
4. DCO2 loss raised the pH and pushed K+ out of cells
Answer and explanation in LearnFree account
Question 7 · Easy
By CLSI guidance, an arterial sample in a plastic syringe kept at room temperature should be analyzed within:
1. A30 minutes
2. B60 minutes
3. C4 hours
4. D2 hours
Answer and explanation in LearnFree account
Question 8 · Easy
At what temperature does a blood gas analyzer measure pH, pCO2 and pO2?
1. AThe patient's entered temperature
2. BA fixed 30 °C
3. CA fixed 37 °C
4. DA fixed 25 °C
Answer and explanation in LearnFree account
Question 9 · Medium
An arterial sample in a plastic syringe is left in ice water for 45 minutes before analysis. The result most likely to be falsely changed by the icing itself is:
1. ApO2, falsely raised
2. BpO2, falsely lowered
3. CpCO2, falsely raised
4. DpH, falsely lowered
Answer and explanation in LearnFree account
Question 10 · Medium
How should a sample for blood lactate be collected and handled?
1. AStraight after vigorous exercise
2. BLeft at room temperature for 2 hours before separation
3. CIn a gel serum tube stored overnight
4. DFluoride-oxalate tube on ice, analyzed promptly
Answer and explanation in LearnFree account

Blood gas sampling and preanalytical errors

A 3-mL syringe coated with dry, non-balanced lithium heparin is filled with only 0.8 mL of arterial blood and analyzed within 5 minutes, with no air. Why is the ionized calcium falsely low?

A blood gas syringe stands for 15 minutes and is then injected without being mixed again. Which reported result is most at risk?

A stable ICU patient's arterial-line samples, collected in dry-heparin syringes, give: one hour ago Hb 118 g/L, Cl- 103 mmol/L, K+ 4.2 mmol/L, pCO2 5.3 kPa (40 mm Hg); now Hb 86 g/L, Cl- 116 mmol/L, K+ 3.1 mmol/L, pCO2 3.9 kPa (29 mm Hg). The most likely cause is:

Air bubbles in a freshly drawn arterial blood gas syringe should be expelled:

By CLSI guidance, an arterial sample in a plastic syringe kept at room temperature should be analyzed within:

At what temperature does a blood gas analyzer measure pH, pCO2 and pO2?

An arterial sample in a plastic syringe is left in ice water for 45 minutes before analysis. The result most likely to be falsely changed by the icing itself is:

How should a sample for blood lactate be collected and handled?