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Blood gases
Arterial oxygen saturation (SaO2)
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The percentage of haemoglobin binding sites occupied by oxygen. It relates to PaO2 through the sigmoid oxyhaemoglobin dissociation curve and says nothing about oxygen content when haemoglobin is low.
Reference interval
| Conventional (%) | SI (fraction) | |
|---|---|---|
| Adults | 95–100 % | 0.95–1 fraction |
Specimen: Arterial whole blood (co-oximetry) or pulse oximetry (SpO2) · Conventional × 0.01 = SI
Causes of high values
Values of 100 % are normal at high PaO2 and give no information about excess oxygen delivery. Saturation cannot exceed 100 %.
Causes of low values
Hypoxaemia from any cause of low PaO2, and a rightward shift of the dissociation curve (acidosis, fever, raised 2,3-DPG) lowers saturation at a given PaO2. Carboxyhaemoglobin and methaemoglobin reduce functional saturation.
Pre-analytical and method notes
Pulse oximetry (SpO2) usually agrees with SaO2 within a few percent but over-reads in carbon monoxide poisoning, is unreliable in poor perfusion, nail polish or darker skin pigmentation, and cannot detect hypercapnia. Normal SaO2 is about 95 to 100 %, and targets are lower in selected chronic lung disease according to local guidelines. Reference intervals vary by laboratory, method and population; use the interval printed on the report.
Related calculators
References
- Murray and Nadel's Textbook of Respiratory Medicine. Elsevier, 7th edition (2021) — Tier 3 (textbook)
- Rifai N, Horvath AR, Wittwer CT. Tietz Textbook of Clinical Chemistry and Molecular Diagnostics. Elsevier, 6th edition (2018) — Tier 3 (textbook)
- Hall JE, Hall ME. Guyton and Hall Textbook of Medical Physiology. Elsevier, 14th edition (2020) — Tier 3 (textbook)
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