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Blood gas specimens and oxygen content

15 min

Skills you'll build4 skills
  • Confirm arterial or venous source and inspired oxygen before interpreting a blood gas
  • Predict how room-air exposure changes a blood-gas specimen
  • Recognize how excess liquid heparin dilutes a blood-gas specimen
  • Calculate oxygen content using hemoglobin concentration and the stated saturation

At the bench: Interpret a blood gas result with its specimen conditions

Try first

Try first

A blood gas syringe arrives from a medical ward with pH 7.35, pCO2 48 mm Hg and pO2 38 mm Hg. The requisition names no specimen source and no inspired oxygen. What do you check before anyone reads these values as the patient's arterial status?

Right. Keep going to follow the reasoning.

Keep going to see the reasoning.

Keep going to see the reasoning.

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

Specimen source and inspired oxygen

A blood gas is interpreted according to the specimen source and the oxygen the patient was breathing. Arterial blood is the specimen for assessing oxygenation and ventilation. Venous pO2 and saturation are far lower than arterial values. Venous pCO2 is several mm Hg higher, and venous pH is a few hundredths lower.1,2 The requisition records the source, the inspired oxygen and the collection time. A gas with no recorded source is not reported until the source is confirmed.2

Air in the syringe

Room air has a pO2 near 150 mm Hg and almost no CO2. A bubble left in the syringe shifts the specimen toward those values.1,2

GasEffect of trapped air
pCO2Falls toward zero
pHRises as CO2 leaves
pO2 below 150 mm HgRises
pO2 above 150 mm Hg, as on high oxygenFalls

The collector expels air at once and caps the syringe.2

The heparin fill

The usual syringe contains dry, calcium-balanced lithium heparin. Liquid heparin adds fluid to the blood. In an underfilled syringe that fluid is a large share of the specimen, which dilutes the specimen and shifts pCO2, bicarbonate and the electrolytes. The draw volume follows the manufacturer's ratio.1,2

Oxygen content

pO2 measures only dissolved oxygen, a small share of what blood carries. Hemoglobin carries almost all of it:1

Oxygen content (mL O2/dL) = 1.39 × hemoglobin (g/dL) × saturation (as a fraction) + 0.00314 × pO2 (mm Hg)

Some laboratories use the constant 1.34, and the procedure names the one in use. A normal pO2 with a low hemoglobin still gives a low content.1

Follow the bench demonstration

Sources for this step
References
  1. Rifai N, Chiu RWK, Young I, Burnham CAD, Wittwer CT, eds. Tietz Textbook of Laboratory Medicine. 7th ed. Elsevier; 2023.
  2. Clinical and Laboratory Standards Institute. Blood Gas and pH Analysis and Related Measurements; Approved Guideline. 2nd ed. CLSI document C46-A2. Clinical and Laboratory Standards Institute; 2009. Accessed September 27, 2026. https://clsi.org/shop/standards/c46/

Watch one

Read each decision and its explanation.

An arterial specimen, collected on room air, gives pO2 95 mm Hg and oxygen saturation 97%. The patient's hemoglobin is 10.0 g/dL. Your procedure uses the constant 1.39. What is the oxygen content?

  1. Convert the saturation: 97% = 0.97.

    The saturation enters the formula as a fraction of the hemoglobin loaded with oxygen.

  2. Bound oxygen = 1.39 × 10.0 g/dL × 0.97 = 13.48 mL O2/dL.

    Hemoglobin carries almost all of the oxygen, so the bound oxygen is calculated first.

  3. Dissolved oxygen = 0.00314 × 95 mm Hg = 0.30 mL O2/dL.

    Dissolved oxygen is the only part pO₂ measures.

  4. Add them: 13.48 + 0.30 = 13.78 mL O2/dL.

    Content is the sum of the two parts.

  5. Compare: at hemoglobin 15.0 g/dL and the same saturation, the content would be about 20.5 mL O2/dL. The low hemoglobin lowers the oxygen content by about a third.

    Comparing with a hemoglobin of 15.0 g/dL shows the deficit that a normal pO₂ does not reveal.

The oxygen content is 13.8 mL O2/dL, low because of the hemoglobin, with a pO2 inside the reference interval.

Practice this skill

Your turn

Problem 1 of 3 · Hints available

A blood-gas specimen remains exposed to room air before analysis. Which gas-change mechanism is expected?

Correct. Atmospheric pCO₂ is much lower than blood pCO₂, so air exposure allows CO₂ loss and raises pH.

Incorrect. Room air contains much less CO₂ than blood. A pCO₂ rise with falling pH is more consistent with ongoing cellular metabolism during delay.

Incorrect. Air drives pO₂ toward the air value. A low starting pO₂ can rise, and a sufficiently high starting pO₂ can fall.

Hint
  1. Compare the CO2 in room air with the CO2 in blood.
  2. Gas moves from where its pressure is higher to where it is lower.
  3. Ask what losing CO2 does to the balance that sets pH.

Review Specimen collection and quality assurance

Problem 2 of 3 · Hints available

A 3 mL blood gas syringe prepared with liquid heparin arrives holding about 0.5 mL of blood. Which change do you expect in the results?

Liquid heparin is fluid. In a small blood volume it makes up a large share of the specimen, and every measured value is diluted.

Ignored heparin volume in an underfilled syringe

An underfilled syringe with liquid heparin leaves a large share of anticoagulant in a small blood volume, diluting the specimen and shifting gas and electrolyte results. Reporting results without checking the fill releases values distorted by the heparin-to-blood ratio.

The heparin solution contains almost no CO2. Mixing it with blood lowers pCO2 and bicarbonate.

The large share of heparin solution dilutes the blood. pCO2, the calculated bicarbonate and the electrolytes read low, and the specimen is recollected with the correct fill.

Hint
  1. Ask how much of the specimen is now heparin solution.
  2. The heparin solution contains almost no CO2 and no bicarbonate.

Review Specimen collection and quality assurance

Problem 3 of 3 · On your own

An arterial specimen gives pO2 60 mm Hg and oxygen saturation 88%. Hemoglobin is 14.0 g/dL. Using the constant 1.39 and 0.00314 mL/dL per mm Hg for dissolved oxygen, what is the oxygen content in mL O2/dL?

Show the answer

17.3 mL/dL

Bound oxygen = 1.39 × 14.0 g/dL × 0.88 = 17.12 mL O2/dL. Dissolved oxygen = 0.00314 × 60 mm Hg = 0.19 mL O2/dL. Content = 17.12 + 0.19 = 17.3 mL O2/dL.

Review Oxygen capacity and content

Use it

Read the case evidence, then make each decision. The step is complete without it.

  • Ruben Pipkin, 64, MRN 3308142, is in intensive care. A respiratory therapist sends his blood gas syringe.
  • QC for the run is acceptable.
TestResultPreviousReference intervalFlag
pH7.51 High6 hours ago: 7.407.406 hours ago7.35–7.457.35–7.45High
pCO224 mm Hg Low6 hours ago: 41 mm Hg41 mm Hg6 hours ago35–45 mm Hg35–45Low
pO2168 mm Hg High6 hours ago: 235 mm Hg235 mm Hg6 hours ago75–100 mm Hg75–100High
Bicarbonate19 mmol/L Low6 hours ago: 25 mmol/L25 mmol/L6 hours ago22–28 mmol/L22–28Low

Specimen: Not measured on this analyzer. Requisition: arterial, 60% inspired oxygen; about 0.6 mL in a 3 mL liquid-heparin syringe; large air bubble at the cap

Decision 1 of 3

The patient's pO2 was 235 mm Hg six hours ago on the same oxygen. What does the air bubble do to a pO2 this high?

Air shifts the specimen toward room-air values. A pO2 above 150 mm Hg falls toward it, which fits the drop from 235 to 168 mm Hg.

Air raises a pO2 that starts below 150 mm Hg. A patient on 60% oxygen starts above that, so the bubble lowers the value.

Assumed trapped air can only raise pO₂

Air drives specimen pO₂ toward the 150 mm Hg of room air, so a low pO₂ rises and a pO₂ above 150 mm Hg, as during oxygen therapy, falls. Meanwhile pCO₂ falls toward zero and pH rises. Expecting only a rise misreads the direction of the error in a hyperoxic specimen.

Air exchanges both gases. pO2 moves toward 150 mm Hg and pCO2 falls toward zero.

Review Specimen collection and quality assurance

Decision 2 of 3

Which specimen problems explain the low pCO2 and bicarbonate?

The patient's gas six hours ago was near the middle of the reference interval, and this syringe has two known faults. Each fault lowers pCO2.

The bubble lowers pCO2. The syringe also contains only 0.6 mL of blood in liquid heparin, and that dilution lowers pCO2 and bicarbonate as well.

Ignored heparin volume in an underfilled syringe

An underfilled syringe with liquid heparin leaves a large share of anticoagulant in a small blood volume, diluting the specimen and shifting gas and electrolyte results. Reporting results without checking the fill releases values distorted by the heparin-to-blood ratio.

Air lets CO2 escape, and the large share of liquid heparin dilutes the blood. Both lower pCO2 and bicarbonate. The CO2 lost to the bubble raises the pH.

Review Specimen collection and quality assurance

Decision 3 of 3

What do you do with these results?

A comment cannot tell the care team how far each value changed, and the numbers would still be reported.

A repeat measures the same diluted, air-exposed blood and gives the same shifted values.

The pH changed with the pCO2. No value from this syringe reflects the patient's true status.

Both faults happened in the syringe, so a correct new collection avoids them. Record the reason, and tell the therapist the fill volume and the bubble.

Review Specimen collection and quality assurance

Two faults in the syringe explain these results:

  • 0.6 mL of blood in liquid heparin dilutes every value.
  • A bubble shifts pO2 toward 150 mm Hg and pCO2 toward zero.

Every result changed in the direction those two faults predict, so the specimen is rejected and redrawn.

For reference

Sources checked