Pulse Oximetry for Nurses: Normal SpO2, Target Ranges, Skin Tone Bias and When the Reading Is Wrong

The pulse oximeter is clipped on to almost every patient, and SpO2 feeds into early warning scores, oxygen prescribing and decisions to escalate. Because the number appears so reliably, it is easy to forget that it is an estimate, and one that can be wrong in predictable ways.

What a pulse oximeter measures

A pulse oximeter shines red and infrared light through tissue, usually a fingertip or earlobe, and estimates the percentage of haemoglobin carrying oxygen in arterial blood. That estimate is SpO2. The “true” value, measured from an arterial blood gas, is SaO2. Most of the time they are close. Sometimes they are not.

Normal SpO2 and target ranges

• Most healthy adults: about 95–100% on room air.
• Usual target for acutely ill adults: 94–98%, as recommended in British Thoracic Society oxygen guidance.
• Patients at risk of hypercapnic (type 2) respiratory failure, such as many people with COPD: often 88–92%, unless a different target is prescribed.

Always follow the target prescribed for your patient. On the NEWS2 chart, there are separate SpO2 scales for these two groups, so make sure the right one is in use. For more on delivering oxygen, see our guide to oxygen therapy devices.

Skin tone and pulse oximeter accuracy

Evidence shows that pulse oximeters can overestimate oxygen levels in people with darker skin. In a widely cited 2020 study in the New England Journal of Medicine, “occult hypoxaemia”, meaning a dangerously low arterial oxygen level that the oximeter missed, was found about three times as often in Black patients as in White patients.

Regulators have responded. The US Food and Drug Administration issued draft guidance in 2025 calling for pulse oximeters to be tested on people across a wider range of skin tones, and a UK government-commissioned independent review on equity in medical devices, published in 2024, made pulse oximetry a priority.

What this means at the bedside:

• Do not let a “normal” SpO2 override clinical signs of deterioration: rising respiratory rate, increased work of breathing, confusion, new agitation or cyanosis.
• Look at the trend, not just a single reading.
• Have a low threshold for escalating and for an arterial blood gas when the picture does not fit the number.

Other reasons the reading may be wrong

• Poor perfusion: cold hands, shock, vasoconstriction or low blood pressure. The waveform will often be flat or irregular.
• Movement: shivering, tremor or a restless patient.
• Nail polish and false nails, especially dark colours. Remove them or use an earlobe probe.
• Carbon monoxide poisoning: most standard oximeters cannot tell carboxyhaemoglobin from oxyhaemoglobin, so SpO2 can look normal in a patient who is severely poisoned.
• Bright ambient light on the probe.
• Arrhythmias such as fast atrial fibrillation, which can make the pulse signal unreliable.
• A probe on the same arm as a blood pressure cuff, which briefly cuts off flow during each reading.

Normal SpO2 does not mean the patient is well

SpO2 tells you about oxygen saturation, not everything about breathing or oxygen delivery:

• A patient with severe anaemia can have a normal SpO2 but carry very little oxygen to the tissues.
• A patient on high-flow oxygen can have a normal SpO2 while their carbon dioxide rises dangerously. SpO2 does not measure CO2.
• A patient working very hard to breathe may hold a normal SpO2 until they suddenly tire.

How to get a reliable reading

1. Choose a warm, well-perfused finger, or an earlobe.
2. Remove nail polish or false nails if possible.
3. Keep the hand still and at heart level.
4. Wait for a stable, good-quality waveform before recording.
5. Check that the pulse rate on the oximeter matches a manual pulse.
6. Record whether the patient is on air or oxygen, and how much.

The bottom line

Treat SpO2 as one piece of the picture alongside respiratory rate, effort, skin colour, mental state and the patient’s own sense of how they feel. When the number and the patient disagree, believe the patient and escalate. Use SBAR to explain your concern clearly.

This article is for education and does not replace your local policies or clinical judgement.