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Arterial pO2

How much oxygen is dissolved in your arterial blood. It shows how well your lungs move oxygen into your blood.

AI-verified against cited sources. Awaiting expert reviewAlso called PaO2LOINC 2703-7

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What it is

pO2 is the pressure of oxygen dissolved in your blood, and in arterial blood it is often written PaO2. Most oxygen travels attached to hemoglobin in red blood cells, but the dissolved part is what pushes oxygen onto hemoglobin. The number depends a lot on what you were breathing. Room air is about 21% oxygen. A result only makes sense next to the share of oxygen you were getting and the altitude where you were tested.

Why it matters

A low pO2, called hypoxemia, means less oxygen is entering your blood. That can leave your tissues short of oxygen and can make you feel breathless, tired or confused. An arterial blood gas measures oxygen more precisely than a finger pulse oximeter, so it is used when an exact value matters. On extra oxygen, a pO2 above the range is expected.

If it’s low

A pO2 below the usual range means less oxygen than expected is getting into your blood. It is read against the oxygen you were breathing, your age and your altitude. Your clinician looks at pCO2 and the other results to see whether the problem is the lungs, the heart or something else.

Common causes

  • High altitude, where there is less oxygen in the air
  • Older age, since pO2 gradually falls as people get older
  • Higher body weight, which is linked to a lower pO2 in healthy people
  • A venous sample mislabeled as arterial, or a delay before testing
  • Lung conditions such as asthma, COPD, pneumonia or fluid in the lungs
  • A blood clot in the lung
  • Heart or circulation conditions

What can help

  • Ask your clinician what is behind the change and whether oxygen therapy is right for you
  • If you live or travel at high altitude, ask how that affects your numbers
  • If you smoke, ask about help quitting, since lung health affects oxygen levels

If it’s high

A pO2 above 100 mmHg usually means you were breathing extra oxygen. It can also be normal variation, or an air bubble in the syringe. A high value on its own doesn't point to a lung problem, though care teams try not to give more oxygen than needed.

Common causes

  • Breathing extra oxygen through a mask, nasal tube or ventilator
  • An air bubble in the syringe
  • Normal variation, which is more common in younger people at sea level

What can help

  • Ask your clinician whether the amount of oxygen you receive is still the right amount for you

Usual ranges

What labs print as the reference range, by who was tested. Ranges differ a little between labs, so the range on your own report is the one your result is judged against.

See Arterial pO2 by age and sex, with the U.S. survey numbers where they exist.

What can affect your result

  • The oxygen you were breathing (room air or extra oxygen)
  • Altitude: less oxygen in the air means a lower pO2
  • Age: pO2 gradually falls with age in healthy adults
  • Body weight: a higher BMI is linked to a lower pO2
  • Sample handling: air bubbles raise it, and a delay lowers it
  • Lung and heart conditions
  • Air bubbles in the syringe raise the measured pO2 toward the level in room air. A delay before analysis lets blood cells use up oxygen and lower it.
  • A venous or mixed sample mislabeled as arterial reads much lower.
  • The oxygen you are breathing changes the result. The report should say whether it was room air or extra oxygen.
  • Altitude lowers pO2 in healthy people.

Minutes. After a change in oxygen or ventilator settings, care teams wait for the level to settle before drawing a sample.

When to talk to a clinician

An arterial blood gas is usually done in a clinic or hospital, where your care team sees the result right away. Many US hospitals treat an arterial pO2 below about 40 mmHg (5.3 kPa) as needing prompt attention and call the ordering clinician. If you are not in a care setting and have severe shortness of breath, confusion, chest pain, or bluish or gray lips, call 911 or your local emergency number.

Questions you could ask

  • Was I breathing room air or extra oxygen at the time of the test?
  • How does this compare with my pulse oximeter readings?
  • Was I on extra oxygen when this was taken, and is that amount still right for me?
  • Is this low pO2 related to my lungs, my heart, my age or my altitude?
  • Do I need oxygen or other treatment, and when should we recheck?

Questions people ask

How is pO2 different from the oxygen number on my finger pulse oximeter?

A finger sensor estimates the percent of your hemoglobin that is carrying oxygen (SpO2). pO2 is the pressure of oxygen dissolved in blood taken from an artery. They are related but not the same, and the blood gas is the more exact of the two.

Does my result depend on altitude and on extra oxygen?

Yes, both. Thinner air at altitude lowers pO2 even in healthy people, and breathing extra oxygen raises it. That is why your report should say what you were breathing, and why the same number can mean different things in different settings.

My report shows kPa. How do I convert to mmHg?

Multiply kPa by 7.5. For example, 12 kPa is about 90 mmHg. To go the other way, multiply mmHg by 0.133.

Show the science and sources

Units

  • 1 mmHg = 1 mmHg
  • 1 mmHg = 0.1333 kPa

Open the Arterial pO2 unit converter

For clinicians and researchers

PaO2 is the oxygen tension of arterial plasma, measured with an electrode at 37 °C. Oxygen saturation is either calculated from pO2 by assuming normal hemoglobin, or measured on a co-oximeter (SaO2); the two can disagree when hemoglobin is altered, as in carbon monoxide exposure or methemoglobinemia. Interpretation needs the inspired oxygen fraction (FiO2). A quick expected PaO2 is about five times the inspired oxygen percentage, so roughly 100 mmHg on room air, and one review gives an age-adjusted room-air estimate of 100 minus 0.3 x age in years. Published reference work shows PaO2 falling with age: in 194 healthy nonsmokers, PaO2 (mmHg) for ages 40-74 fitted 143.6 - 0.39 x age - 0.56 x BMI - 0.57 x PaCO2 (SEE 7.48), and for ages 75 and older the mean was 83.4 mmHg (SD 9.15) with a 5th percentile of 68.4. Altitude lowers PaO2: the median in 46 healthy residents at 3,164 m was about 70 mmHg. Air bubbles raise PaO2, and delayed analysis lets leukocytes and platelets consume oxygen; one review advises analyzing room-temperature samples within 15 minutes and iced samples within 1 hour. For someone breathing on their own, the 1992 AARC sampling guideline advises waiting at least 20 to 30 minutes after an oxygen change before drawing the sample. The PaO2/FiO2 ratio and the alveolar-arterial gradient put PaO2 in context. The 40 mmHg limit is the median of hospital notification lists, not a clinical standard.

LOINC 2703-7 (primary), 11556-8, 2704-5. Specimen: whole-blood.

Sources

  1. Arterial Blood Gas (ABG) Test, MedlinePlus, National Library of Medicine
  2. Mastering blood gas interpretation: A practical guide for primary care providers, South African Family Practice (2025)
  3. Blood gas analysis: Clinical applications, interpretation and future directions (Review), Medicine International (2026)
  4. Arterial blood analysis of healthy residents in Huamachuco, Peru (3,164 m): a cross-sectional study, F1000Research (2023)
  5. Reference values of arterial oxygen tension in the middle-aged and elderly, American Journal of Respiratory and Critical Care Medicine (1995)
  6. Visualization of Critical Limits and Critical Values Facilitates Interpretation, Diagnostics (MDPI) (2025)
  7. Critical Decision Thresholds for Urgent Physician Notification of Point-of-Care Testing Results, Diagnostics (MDPI) (2026)
  8. Blood Gas, Arterial, Penn State Health
  9. Blood Gas Arterial, Northwell Health Laboratories
  10. Blood Gases, Comprehensive, Oregon Health & Science University (OHSU) Laboratory

AI-verified against cited sources. Awaiting expert review. Last reviewed 2026-10-03. How we build and check these pages.

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