Pulse oximetry and blood gases
Also known as: oxygen saturation, sats, oxygen level
A clip on the fingertip shines red and infrared light through the tissue and reads the colour of pulsing blood to estimate how saturated haemoglobin is with oxygen. When more precision or a carbon dioxide level is needed, an arterial blood gas takes blood directly from an artery.
- Abbreviations
- SpO2
- Kind
- Physiological measurement
- Categories
- Respiratory, Vital signs & physiological measurements, Point-of-care & home testing
- Specimen
- No specimen
- Method
- Pulse oximetry
- Terminology
- LOINC 59408-5
What it measures
| Measure | What it tells you |
|---|---|
| Oxygen saturation (SpO2) | 95-100% normal in healthy adults; 88-92% is the target in people with COPD who retain carbon dioxide. |
| Arterial oxygen (PaO2) | The pressure of dissolved oxygen; below 8 kPa on air defines respiratory failure. |
| Arterial carbon dioxide (PaCO2) | High when ventilation is inadequate (type 2 respiratory failure); low with hyperventilation. |
| pH and bicarbonate | Acid-base status: respiratory or metabolic acidosis and alkalosis. |
| Lactate | Rises when tissues are starved of oxygen; a key sepsis marker. |
Why it is ordered
- Any breathless or unwell patient (oximetry is a vital sign)
- Pneumonia, COVID-19, asthma attacks, COPD flares
- Monitoring oxygen therapy and ventilation
- Suspected sepsis, diabetic ketoacidosis, poisoning (blood gas)
- Sleep apnoea screening overnight
How it is done
The probe reads within seconds; a blood gas needs a needle in the radial artery at the wrist, which stings briefly, with pressure afterwards.
Preparation. Warm hands and remove nail varnish for oximetry.
Reading the result
| Finding | Usual meaning |
|---|---|
| SpO2 below 92% on air | Hypoxaemia; oxygen and urgent assessment |
| High PaCO2 with low pH | Acute type 2 respiratory failure; may need ventilatory support |
| Low pH, low bicarbonate, high lactate | Metabolic acidosis from sepsis or poor perfusion |
Reference ranges vary by laboratory, method, age, sex and clinical context; a result is read against the person, not a table.
Limitations
Oximetry misreads with cold hands, poor circulation, nail varnish, movement, carbon monoxide poisoning and, to a degree, darker skin; it says nothing about carbon dioxide. A normal saturation does not mean breathing is adequate.
Catalogued concepts this page covers
Concepts of the master test taxonomy that resolve to this page; each links to its category.