A 71-year-old man with COPD on 2 L/min nasal oxygen has a pulse oximetry reading of 88%. His arterial sample is drawn into a dry, balanced-heparin syringe and arrives by pneumatic tube with a large air bubble and froth at the tip; it is analyzed 10 minutes after collection. Results: pH 7.50, pCO2 44 mm Hg [5.9 kPa], pO2 104 mm Hg [13.9 kPa], HCO3- 33 mmol/L. Two days earlier, on the same oxygen and with the same oximetry reading, his results were pH 7.39, pCO2 60 mm Hg [8.0 kPa], pO2 56 mm Hg [7.5 kPa], HCO3- 35 mmol/L. The most likely explanation for today's results is:

Room air mixing with the sample in the syringe

Glycolysis during a delay in analysis

Venous blood drawn instead of arterial

Acute improvement in his ventilation

In a blood gas analyzer, the Severinghaus pCO2 electrode detects CO2 as:

A pH change in a bicarbonate solution

A change in the conductivity of blood

Light absorbed at several wavelengths

A current from reducing CO2 at a cathode

A 3-mL syringe holding 0.5 mL of liquid heparin was filled with only 0.6 mL of arterial blood. Results: pH 7.38, pCO2 26 mm Hg [3.5 kPa], HCO3- 15 mmol/L. A chemistry sample drawn at the same time has total CO2 25 mmol/L. The most likely explanation is:

Dilution by the excess liquid heparin

Blood gas test procedures and preanalytical errors

11 ASCP MLS practice questions on Blood gas test procedures and preanalytical errors, part of Chemistry. The first 11 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 71-year-old man with COPD on 2 L/min nasal oxygen has a pulse oximetry reading of 88%. His arterial sample is drawn into a dry, balanced-heparin syringe and arrives by pneumatic tube with a large air bubble and froth at the tip; it is analyzed 10 minutes after collection. Results: pH 7.50, pCO2 44 mm Hg [5.9 kPa], pO2 104 mm Hg [13.9 kPa], HCO3- 33 mmol/L. Two days earlier, on the same oxygen and with the same oximetry reading, his results were pH 7.39, pCO2 60 mm Hg [8.0 kPa], pO2 56 mm Hg [7.5 kPa], HCO3- 35 mmol/L. The most likely explanation for today's results is:
1. ARoom air mixing with the sample in the syringe
2. BGlycolysis during a delay in analysis
3. CVenous blood drawn instead of arterial
4. DAcute improvement in his ventilation
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Answer: A · Room air mixing with the sample in the syringe
A pO2 of 104 mm Hg cannot be right in a patient whose oximetry still reads 88%, which matches a pO2 in the mid-50s, as it did two days ago. Room air has a pO2 of about 150 mm Hg and almost no CO2, so a large bubble shaken in a pneumatic tube pulls pO2 up and pCO2 down, and pH rises as CO2 leaves the sample. Bicarbonate changes little when CO2 escapes, which is why it stays near his usual value while pCO2 falls by 16 mm Hg. A real improvement in ventilation would also raise the oximetry reading, a venous sample would give a lower pO2 and a higher pCO2, and glycolysis during a delay lowers pH and pO2. The sample should be rejected and redrawn, with bubbles expelled at once and the syringe capped.
- B: Glycolysis lowers pH and pO2 and raises pCO2, the opposite of what is seen.
- C: Venous blood would show a lower pO2 and a higher pCO2, not the reverse.
- D: Better ventilation would raise the oximetry reading too, but it is unchanged at 88%.
Question 2 · Medium
In a blood gas analyzer, the Severinghaus pCO2 electrode detects CO2 as:
1. AA change in the conductivity of blood
2. BLight absorbed at several wavelengths
3. CA current from reducing CO2 at a cathode
4. DA pH change in a bicarbonate solution
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Answer: D · A pH change in a bicarbonate solution
CO2 diffuses from the sample through a gas-permeable membrane into a thin layer of bicarbonate electrolyte. There it forms carbonic acid, and a glass pH electrode measures the pH change, which is proportional to log pCO2. It is therefore a potentiometric measurement. The Clark pO2 electrode, by contrast, is amperometric: oxygen is reduced at a platinum cathode and the current is measured.
- A: Conductivity is used to estimate hematocrit, not pCO2.
- B: Absorbance at several wavelengths is co-oximetry.
- C: Reduction at a cathode with a measured current describes the Clark pO2 electrode.
Question 3 · Hard
A 3-mL syringe holding 0.5 mL of liquid heparin was filled with only 0.6 mL of arterial blood. Results: pH 7.38, pCO2 26 mm Hg [3.5 kPa], HCO3- 15 mmol/L. A chemistry sample drawn at the same time has total CO2 25 mmol/L. The most likely explanation is:
1. ADilution by the excess liquid heparin
2. BVenous blood in the syringe
3. CDelay before analysis
4. DCompensated metabolic acidosis
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Answer: A · Dilution by the excess liquid heparin
Liquid heparin contains almost no CO2 and is roughly in equilibrium with air, so a large volume relative to blood dilutes the CO2 and bicarbonate in the sample. pCO2 and the calculated HCO3- fall together, so their ratio and the pH change little, which mimics a compensated metabolic acidosis. The normal total CO2 on the chemistry sample shows the patient's true bicarbonate is normal. Use dry heparin, or expel excess liquid heparin and fill the syringe to its stated volume.
- B: Venous blood would have a higher pCO2, not a lower one.
- C: A delay raises pCO2 rather than lowering it.
- D: A true metabolic acidosis would also lower the chemistry total CO2.
Question 4 · Medium
An arterial sample in a capped plastic syringe sits at room temperature for 90 minutes before analysis. Which pattern of change is expected?
1. ApH down, pCO2 up, pO2 down
2. BpH down, pCO2 down, pO2 up
3. CpH up, pCO2 down, pO2 up
4. DpH up, pCO2 up, pO2 down
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Answer: A · pH down, pCO2 up, pO2 down
Leukocytes, platelets and reticulocytes keep metabolizing in the syringe. They consume oxygen and produce CO2 and lactic acid, so pO2 falls, pCO2 rises and pH falls; glucose falls and lactate rises too. CLSI advises analyzing samples in plastic syringes within 30 minutes at room temperature. The changes are faster at higher temperature and with very high white cell or platelet counts.
- B: Falling pCO2 and rising pO2 fit dilution by excess liquid heparin.
- C: Rising pO2 and falling pCO2 fit air contamination, not metabolism.
- D: Rising pCO2 from cell metabolism lowers the pH, not raises it.
Question 5 · Easy
An arterial sample from a patient breathing room air (pO2 70 mm Hg [9.3 kPa]) arrives with a large air bubble. What will the bubble most likely do?
1. ALower both pO2 and pCO2
2. BLower pO2 and raise pCO2
3. CRaise both pO2 and pCO2
4. DRaise pO2 and lower pCO2
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Answer: D · Raise pO2 and lower pCO2
Room air has a pO2 of about 150 to 160 mm Hg and almost no CO2. Gases move between the bubble and the blood toward equilibrium, so pO2 rises toward the air value and pCO2 falls, which also raises the pH. The error grows with bubble size, time and agitation, for example in a pneumatic tube. Expel any air and cap the syringe at once after collection.
- A: pO2 rises here because the sample pO2 is below that of air.
- B: pO2 falls only when the sample pO2 is above that of room air.
- C: Air contains almost no CO2, so pCO2 falls.
Question 6 · Hard
A ventilated patient on 100% oxygen has a true arterial pO2 of 450 mm Hg [60 kPa]. The sample is sent by pneumatic tube with a large air bubble. What will the reported pO2 most likely be?
1. AFalsely high, above 450 mm Hg
2. BFalsely low, nearer 150 mm Hg
3. CFalsely low, below 40 mm Hg
4. DUnchanged, since the syringe is capped
Answer and explanation in LearnFree account
Question 7 · Medium
A patient cooled to 32 °C during cardiac surgery has a blood gas measured at the analyzer's 37 °C. How do his actual values at 32 °C compare with the 37 °C results?
1. AThe same pH, pCO2 and pO2
2. BLower pH, higher pCO2 and pO2
3. CHigher pH, higher pCO2 and pO2
4. DHigher pH, but lower pCO2 and pO2
Answer and explanation in LearnFree account
Question 8 · Easy
For arterial blood gas analysis, the preferred collection device is a syringe containing:
1. ADry lithium heparin
2. BBuffered sodium citrate
3. CLiquid potassium EDTA
4. DSodium fluoride/oxalate
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Question 9 · Hard
An "arterial" sample from a calm patient with SpO2 98% on room air and WBC 7.0 × 10^3/µL [7.0 × 10^9/L] was analyzed within 10 minutes, with no bubbles: pH 7.36, pCO2 48 mm Hg [6.4 kPa], pO2 38 mm Hg [5.1 kPa], O2 saturation 71%. What is the most likely explanation?
1. AA vein was sampled by mistake
2. BExcess liquid heparin diluted it
3. CThe patient has true hypoxemia
4. DCells in the syringe used the oxygen
Answer and explanation in LearnFree account
Question 10 · Hard
A man with acute myeloid leukemia (WBC 180 × 10^3/µL [180 × 10^9/L]) is comfortable, with pulse oximetry 97% on room air. His arterial sample, analyzed 40 minutes after collection, shows pO2 48 mm Hg [6.4 kPa]. What is the most likely cause?
1. AAn air bubble trapped in the sample
2. BA falsely high pulse oximeter reading
3. COxygen use by leukocytes in the syringe
4. DExcess liquid heparin in the syringe
Answer and explanation in LearnFree account
Question 11 · Medium
An arterial sample will not reach the analyzer for about 60 minutes. Following CLSI guidance, the sample should be collected and transported:
1. AIn a plastic syringe in ice water
2. BIn a glass syringe in ice water
3. CIn a heparin vacuum tube on ice
4. DIn a plastic syringe at room temperature
Answer and explanation in LearnFree account

Blood gas test procedures and preanalytical errors

In a blood gas analyzer, the Severinghaus pCO2 electrode detects CO2 as:

A 3-mL syringe holding 0.5 mL of liquid heparin was filled with only 0.6 mL of arterial blood. Results: pH 7.38, pCO2 26 mm Hg [3.5 kPa], HCO3- 15 mmol/L. A chemistry sample drawn at the same time has total CO2 25 mmol/L. The most likely explanation is:

An arterial sample in a capped plastic syringe sits at room temperature for 90 minutes before analysis. Which pattern of change is expected?

An arterial sample from a patient breathing room air (pO2 70 mm Hg [9.3 kPa]) arrives with a large air bubble. What will the bubble most likely do?

A ventilated patient on 100% oxygen has a true arterial pO2 of 450 mm Hg [60 kPa]. The sample is sent by pneumatic tube with a large air bubble. What will the reported pO2 most likely be?

A patient cooled to 32 °C during cardiac surgery has a blood gas measured at the analyzer's 37 °C. How do his actual values at 32 °C compare with the 37 °C results?

For arterial blood gas analysis, the preferred collection device is a syringe containing:

An "arterial" sample from a calm patient with SpO2 98% on room air and WBC 7.0 × 10^3/µL [7.0 × 10^9/L] was analyzed within 10 minutes, with no bubbles: pH 7.36, pCO2 48 mm Hg [6.4 kPa], pO2 38 mm Hg [5.1 kPa], O2 saturation 71%. What is the most likely explanation?

A man with acute myeloid leukemia (WBC 180 × 10^3/µL [180 × 10^9/L]) is comfortable, with pulse oximetry 97% on room air. His arterial sample, analyzed 40 minutes after collection, shows pO2 48 mm Hg [6.4 kPa]. What is the most likely cause?

An arterial sample will not reach the analyzer for about 60 minutes. Following CLSI guidance, the sample should be collected and transported: