The pulse oximeter is one of the most used devices in healthcare: quick, painless and cheap. Oxygen saturation (SpO₂) is part of early warning scores and guides oxygen therapy. But it has important limitations. Treating the number rather than the patient can lead to missed hypoxia or unnecessary treatment.
How it works
The probe shines red and infrared light through a finger or earlobe. Oxygenated and deoxygenated haemoglobin absorb light differently, and the device estimates the percentage of haemoglobin carrying oxygen, using the pulsatile signal of arterial blood.
Target saturations
- For most acutely ill adults, a common target is 94–98%.
- For patients at risk of hypercapnic respiratory failure (e.g. some people with COPD), 88–92% is commonly used, as prescribed.
- Children and newborns have their own targets.
See our oxygen therapy guide.
Causes of inaccurate readings
- Poor perfusion: cold hands, shock, peripheral vascular disease. Check the waveform (pleth) and try an earlobe probe.
- Movement: shivering, seizures, restless patients.
- Nail varnish and artificial nails, especially dark colours.
- Bright ambient light.
- Carbon monoxide poisoning: oximeters can read falsely normal because they can’t distinguish carboxyhaemoglobin from oxyhaemoglobin. Consider in fire victims and people using generators or charcoal indoors.
- Methaemoglobinaemia: readings often stuck around 85%.
- Severe anaemia: SpO₂ may be normal even though oxygen delivery is poor.
- Arrhythmias affecting the pulse signal.
- Poorly fitting or low-quality devices.
The skin tone issue
Research, including studies highlighted during the COVID-19 pandemic, has shown that pulse oximeters can overestimate oxygen saturation in people with darker skin. This means “occult hypoxaemia”, true low oxygen with a reassuring reading, is more likely to be missed in Black and other darker-skinned patients. Regulators including the US FDA and the UK’s independent review on equity in medical devices have called for improvements. Until devices improve:
- Don’t rely on SpO₂ alone. Look at respiratory rate, work of breathing, mental state and the overall picture.
- Pay attention to trends: a falling SpO₂ matters even if still “normal”.
- Have a lower threshold for arterial blood gas measurement if the patient looks unwell. See our ABG guide.
- Check for cyanosis in the lips, tongue and conjunctivae, which are easier to assess across skin tones.
Good practice
- Use the right probe for the site and size (adult, paediatric, neonatal).
- Wait for a stable reading and good waveform.
- Check the pulse rate on the oximeter matches the patient’s pulse.
- Rotate probe sites for continuous monitoring to prevent pressure injuries.
- Document whether the patient was on air or oxygen, and how much.
Related: NEWS2 explained · Culturally competent care
Based on BTS guideline for oxygen use in healthcare, US FDA communications on pulse oximeter accuracy, and the UK Independent Review on Equity in Medical Devices (2024). Educational content.