Blood Gas
Oxygen saturation
Oxygen saturation (sO2) is the percentage of haemoglobin binding sites occupied by oxygen, reflecting how well the blood is loaded with oxygen for delivery to the tissues. At the point of care it is either measured directly by co-oximetry on a blood gas analyser or estimated non-invasively as SpO2 by a pulse oximeter.
Why it is measured
Oxygen saturation is a core indicator of oxygenation and respiratory status, helping to guide oxygen therapy, ventilation and clinical escalation in acutely unwell patients. Co-oximetry on a blood gas analyser can also detect dyshaemoglobins such as carboxyhaemoglobin and methaemoglobin that pulse oximetry may miss.
| Typical range | Indicative adult arterial range approximately 95 to 100 percent. Venous and mixed venous saturations are substantially lower and should not be read against this arterial range. Values vary by measurement method: directly measured by co-oximetry, calculated from pO2, or transcutaneous SpO2 by pulse oximetry. Clinical targets are often individualised, for example a lower 88 to 92 percent target in some patients at risk of hypercapnic respiratory failure. Method and device differences mean ranges should be confirmed locally. |
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| Sample | Arterial (or arterialised or venous) whole blood collected into a heparinised blood gas syringe for co-oximetry, analysed promptly and kept free of air bubbles. Pulse oximetry (SpO2) is non-invasive and transcutaneous and requires no blood sample. |
| Turnaround | Blood gas analyser result typically within about 1 to 2 minutes of sample loading. Pulse oximetry provides continuous, near real-time readings at the bedside. |
Point of care devices that report it
- Radiometer ABL90 FLEX (co-oximetry, measured sO2)
- Radiometer ABL800 FLEX (co-oximetry, measured sO2)
- Siemens Healthineers RAPIDPoint 500 (co-oximetry)
- Siemens Healthineers epoc Blood Analysis System (calculated sO2)
- Abbott i-STAT (CG8+ / EG6+ cartridges, calculated sO2)
- Nova Biomedical Stat Profile Prime (co-oximetry)
- Masimo handheld pulse oximeters (SpO2)
- Nonin handheld pulse oximeters (SpO2)
Questions, answered
What is the difference between SpO2 from a pulse oximeter and sO2 from a blood gas analyser?
SpO2 is a non-invasive estimate derived from how light is absorbed across a finger or earlobe, while co-oximetry on a blood gas analyser measures the actual fractions of haemoglobin species in a blood sample and reports sO2 or SaO2. Co-oximetry can detect carboxyhaemoglobin and methaemoglobin, which may leave a pulse oximeter reading misleadingly normal. This is general educational information and not advice for an individual case.
Why might a calculated oxygen saturation differ from a directly measured one?
Many analysers calculate sO2 from the measured pO2 using an assumed oxyhaemoglobin dissociation curve. Factors such as abnormal pH, temperature, 2,3-DPG levels or dyshaemoglobins shift that curve, so a calculated value can diverge from a directly measured co-oximetry value. Where accurate saturation is critical, a measured co-oximetry result is generally preferred.
Does a normal oxygen saturation confirm adequate oxygenation?
Not on its own. Saturation indicates how loaded haemoglobin is, but not total oxygen content or delivery, which also depend on haemoglobin concentration, perfusion and pO2. It is best interpreted alongside the full blood gas profile and the overall clinical picture rather than in isolation.