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Oxygen Saturation, Arterial (SaO2)

How much of the hemoglobin in your arterial blood is carrying oxygen, measured directly from a blood sample. Finger pulse oximeters estimate this same number.

AI-verified against cited sources. Awaiting expert reviewAlso called SaO2LOINC 2708-6

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What it is

Hemoglobin in your red blood cells picks up oxygen in your lungs and carries it to the rest of your body. Arterial oxygen saturation (SaO2) is the percent of hemoglobin that is holding oxygen in blood taken from an artery. A blood gas machine with a built-in co-oximeter measures it with light. Some labs estimate it from the oxygen pressure instead, and the report may say which was used.

Why it matters

Oxygen saturation shows how well your lungs are moving oxygen into your blood. Doctors read it with the oxygen pressure (PaO2), carbon dioxide, pH and your symptoms to judge breathing and guide oxygen treatment. It is the blood-based standard that finger pulse oximeters are checked against. A saturation in the usual range doesn't prove your tissues are getting enough oxygen, because anemia or poor blood flow can limit delivery even when the percent is in the usual range.

If it’s low

A lower result means less of your hemoglobin is carrying oxygen. It can happen when the lungs can't move enough oxygen into the blood, when breathing is slowed, at high altitude, or when some blood bypasses the lungs. The percent alone can't show the cause, so doctors read it with your symptoms and other results. A low result is worth talking about with your care team promptly.

Common causes

  • Lung conditions such as asthma, COPD, pneumonia or cystic fibrosis
  • Weak breathing muscles or nerve and muscle disorders, or medicines that slow breathing
  • Being at high altitude, where the air has less oxygen
  • Heart or lung problems that let blood bypass the lungs (a shunt)

What can help

  • Ask your care team what is causing it, because treatment depends on the cause
  • Ask whether you need oxygen therapy or a visit with a breathing specialist
  • Ask whether a home pulse oximeter would be useful for you and how to read it

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.

For adults: 95–100 %. Source: MedlinePlus (U.S. National Library of Medicine), Breathe 2015;11:194-201, Alberta Precision Laboratories (Alberta Health Services).

What can affect your result

  • Oxygen therapy: a result on extra oxygen is not the same as one on room air
  • Altitude: the air has less oxygen at high altitude
  • How the number is obtained: measured by a co-oximeter, or estimated from the oxygen pressure
  • Carbon monoxide or methemoglobin in the blood, which can make estimated and finger-clip readings look fine
  • Anemia does not change the percent, even though it lowers the oxygen your blood can carry
  • Oxygen therapy changes the result. If you are on oxygen, your care team may switch it off for about 20 minutes before the test, if you can breathe without it.
  • Some analyzers estimate saturation from the oxygen pressure instead of measuring it. An estimate (or a finger pulse oximeter) can look normal when carbon monoxide or methemoglobin is in the blood, so a direct co-oximeter measurement is more reliable in those cases.
  • Typical values are for people at sea level. Altitude changes what is expected.
  • Poor circulation, nail polish, skin pigmentation, carbon monoxide and methemoglobin mainly affect finger pulse oximeter readings (SpO2) rather than a saturation measured directly from a blood sample.

It reflects the moment the blood was drawn and can change within minutes with breathing or oxygen therapy. Providers may switch oxygen off for about 20 minutes before a room-air test, if you can breathe without it.

When to talk to a clinician

Call 911 or your local emergency number for severe shortness of breath, blue or gray lips or fingertips, confusion, or chest pain. A result below 95% is under the usual lab range, but how you feel and your own baseline matter. If your lab flagged the result as critical, call the clinician who ordered the test today.

Questions you could ask

  • Was my saturation measured directly or estimated from the oxygen pressure?
  • How does this compare with my finger pulse oximeter readings?
  • What do you think is behind my lower oxygen saturation?
  • Do I need follow-up testing, such as a lung function test, or oxygen therapy?

Questions people ask

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

A finger pulse oximeter (SpO2) estimates saturation by shining light through your finger. SaO2 is measured from a blood sample. They usually agree within about 2 percentage points, but the finger device can be less reliable with poor circulation, nail polish, darker skin, carbon monoxide or methemoglobin.

Can anemia cause a low oxygen saturation?

No. Saturation is the percent of the hemoglobin you have that is carrying oxygen. With anemia there is less hemoglobin, so less oxygen is carried even when the percent is in the usual range. Your clinician looks at both.

Why do I see 95–100% here, but my report says something else?

Labs set their own limits and may measure on different machines. Use the range on your report first, and ask your clinician how your result fits your situation, such as altitude or lung disease.

Show the science and sources

Units

  • 1 % = 1 %
  • 1 % = 0.01 fraction (0–1)

Open the SaO2 unit converter

For clinicians and researchers

SaO2 is the fraction of hemoglobin binding sites occupied by oxygen, reported as a percent. In healthy people breathing room air at sea level it is about 96% to 98%. Blood gas analyzers measure it directly by multiwavelength co-oximetry. Where only PO2 is available, saturation can be calculated from an oxygen dissociation curve equation such as Severinghaus. In a study of 3,524 routine blood gases, the 2,255 with normal pH were used to compare calculated with measured saturation. The calculated value averaged 0.16 percentage points below the measured value (SD 1.12) and tracked the measured curve well when saturation was above 70%, but it assumes normal pH and normal hemoglobin forms. In carbon monoxide poisoning PaO2 stays normal, and pulse oximetry can read normal because two-wavelength devices read carboxyhemoglobin as oxyhemoglobin. Methemoglobin pushes many pulse oximeters toward about 85%. Instruments can also report fractional oxyhemoglobin (FO2Hb), which uses total hemoglobin including carboxyhemoglobin and methemoglobin as the denominator, so it can be lower than saturation when those forms are present. Percent saturation does not reflect hemoglobin concentration: anemia lowers oxygen content with no change in SaO2. A pulse oximeter (SpO2) typically differs from SaO2 by less than 2 percentage points. In an emergency department study the mean difference was -0.9% with 95% limits of agreement of -6.9 to 5.2%. In 11,018 paired ICU measurements, pulse oximeters overestimated saturation at lower values, and readings were on average 0.6 to 1.5 percentage points higher in people with darker skin tones. In a 2025 ICU study, sO2 stayed broadly stable in arterial samples held 60 minutes at room temperature and then dropped and shaken.

LOINC 2708-6 (primary), 20564-1, 51733-4. Specimen: whole-blood.

Sources

  1. Arterial Blood Gas (ABG) Test, MedlinePlus (U.S. National Library of Medicine)
  2. Relating oxygen partial pressure, saturation and content: the haemoglobin-oxygen dissociation curve, Breathe 2015;11:194-201 (2015)
  3. The impact of skin tone on performance of pulse oximeters used by NHS England COVID Oximetry @home scheme: measurement and diagnostic accuracy study, BMJ 2026;392:e085535 (2026)
  4. Comparison of peripheral venous and arterial blood gas in management of patients with respiratory complaints in the emergency department, PLoS One 2025;20:e0330190 (2025)
  5. Carbon Monoxide Poisoning: From Occupational Health to Emergency Medicine, Journal of Clinical Medicine 2024;13:2466 (2024)
  6. Recommendations for diagnosis and treatment of methemoglobinemia (Joint EHA and EuroBloodNet recommendations), American Journal of Hematology 2021;96:1666-1678 (2021)
  7. Stability of arterial blood gas samples after delayed analysis and mechanical stress, PLoS One 2025;20:e0334710 (2025)
  8. Alkalosis, MedlinePlus Medical Encyclopedia (U.S. National Library of Medicine)
  9. Respiratory acidosis, MedlinePlus Medical Encyclopedia (U.S. National Library of Medicine)
  10. LOINC codes verified with the NLM Clinical Tables LOINC search, U.S. National Library of Medicine
  11. Laboratory Bulletin: Changes to Blood Gas Reference Intervals and Critical Values, Alberta Precision Laboratories (Alberta Health Services) (2021)

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

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