Oxygen saturation (SaO₂ / sO₂)

What it measures

Oxygen saturation is the percentage of haemoglobin carrying oxygen. Co-oximetry on a blood gas analyser measures it directly and also detects carboxyhaemoglobin and methaemoglobin, which pulse oximeters miss.

Specimen and preparation

• Specimen: Arterial blood in a heparinised (balanced lithium heparin) syringe
• Tube: Other top
• Preparation: Expel air bubbles, cap immediately, mix well and analyse within 30 minutes (ideally 15) at room temperature in a plastic syringe; do not ice plastic syringes, and use a glass syringe on ice only if longer delay is unavoidable. Record FiO₂ and temperature.

Reference range

Reference ranges differ between laboratories, methods and populations. Always use the range printed on your own lab report.

Causes of high values

• Supplemental oxygen
• Hyperventilation

Causes of low values

• Hypoxaemia from lung disease
• Carbon monoxide poisoning (low FO₂Hb)
• Methaemoglobinaemia
• Cyanotic heart disease
• Anaemia does not lower SaO₂ but reduces oxygen content

Method

Multi-wavelength co-oximetry (measured) or calculated from pO₂

Good to know

Pulse oximetry can overestimate saturation in darker skin and in CO poisoning; confirm with co-oximetry.

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