Blood gas specimens and oxygen content

15 min

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

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?

Whether the syringe held arterial or venous blood, and what oxygen the patient was breathing

Right. The next section explains why.

Nothing, since the analyzer's QC passed

The next section explains it.

The hemoglobin, to calculate oxygen content

The next section explains it.

I don't know yet

The next section explains it.

Get the idea

Know the source and the oxygen

A blood gas is read against where the blood came from and what the patient was breathing. Arterial blood is the specimen for judging oxygenation and ventilation. Venous pO₂ and saturation run far lower than arterial values. Venous pCO₂ runs several mm Hg higher, and venous pH runs a few hundredths lower.^1,2 The requisition records the source, the inspired oxygen and the collection time. A gas with no source recorded waits until the source is confirmed.²

Air moves the gases toward room air

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

Gas	Effect of trapped air
pCO₂	Falls toward zero
pH	Rises as CO₂ leaves
pO₂ below 150 mm Hg	Rises
pO₂ above 150 mm Hg, as on high oxygen	Falls

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

The heparin fill

The usual syringe holds dry, calcium-balanced lithium heparin. Liquid heparin adds fluid to the blood. A syringe filled with too little blood leaves a large share of that fluid, which dilutes the specimen and shifts pCO₂, bicarbonate and the electrolytes. The draw volume follows the manufacturer's ratio.^1,2

Oxygen content comes from hemoglobin

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

References

Rifai N, Chiu RWK, Young I, Burnham CAD, Wittwer CT, eds. Tietz Textbook of Laboratory Medicine. 7th ed. Elsevier; 2023.
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

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?

Convert the saturation: 97% = 0.97.
The saturation enters the formula as a fraction of the hemoglobin loaded with oxygen.
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 part comes first.
Dissolved oxygen = 0.00314 × 95 mm Hg = 0.30 mL O₂/dL.
Dissolved oxygen is the only part pO₂ measures.
Add them: 13.48 + 0.30 = 13.78 mL O₂/dL.
Content is the sum of the two parts.
Compare: at hemoglobin 15.0 g/dL and the same saturation, the content would be about 20.5 mL O₂/dL. The low hemoglobin removes about a third of the oxygen the blood carries.
Comparing with a full hemoglobin shows what the normal pO₂ hides.

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

Your turn

Problem 1 of 3

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

pCO₂ falls as CO₂ escapes, tending to raise pH

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

pCO₂ rises from air entry, tending to lower 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.

pO₂ always falls, regardless of its starting value

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

Hint

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

Review Specimen collection and quality assurance

Problem 2 of 3

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?

None, since heparin only prevents clotting

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.

A rise in pCO₂ and bicarbonate

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

Falsely low pCO₂, bicarbonate and electrolytes from dilution

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

Ask how much of the specimen is now heparin solution.
The heparin solution carries almost no CO2 and no bicarbonate.

Review Specimen collection and quality assurance

Problem 3 of 3

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

Ruben Okafor, 64, MRN 3308142, is in intensive care on 60% inspired oxygen.
A respiratory therapist sends an arterial syringe prepared with liquid heparin.
The syringe holds about 0.6 mL of blood in a 3 mL barrel.
A large air bubble sits against the cap.
The requisition records arterial source and 60% oxygen.
QC for the run is acceptable.

Test	Result	Previous	Reference interval	Flag
pH	7.51	7.406 hours ago	7.35–7.45	High
pCO2	24 mm Hg	41 mm Hg6 hours ago	35–45 mm Hg	Low
pO2	168 mm Hg	235 mm Hg6 hours ago	85–105 mm Hg	High
Bicarbonate	19 mmol/L	25 mmol/L6 hours ago	22–29 mmol/L	Low

Specimen: Not measured on this analyzer. Arterial, 60% inspired oxygen; 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?

It pulls the pO₂ down toward about 150 mm Hg

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

It raises the pO₂, as air always does

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

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.

It leaves pO₂ alone and changes only pCO₂

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?

A real respiratory alkalosis

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 on its own.

The air bubble alone

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

Ignored heparin volume in an underfilled syringe

Both the air bubble and the heparin dilution

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

Review Specimen collection and quality assurance

Decision 3 of 3

What do you do with these results?

Release them with a comment about the bubble

A comment cannot tell the care team how far each value moved. The results would still reach the chart as numbers.

Repeat on the same syringe

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

Release the pH only

The pH moved with the pCO2. No value from this syringe describes the patient.

Reject the specimen and ask for a new arterial draw with the correct fill and no air

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

Review Specimen collection and quality assurance

The clue that settles this case is the syringe itself:

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

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

Keep

Sources checked September 27, 2026

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Blood gas specimens and oxygen content

Try first

Get the idea

Know the source and the oxygen

Air moves the gases toward room air

The heparin fill

Oxygen content comes from hemoglobin

Watch one

Your turn

Use it

Results

Keep

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