Pulse Oximeters: What They Actually Tell You
A pulse oximeter is the simplest medical device most people own: clip it on a fingertip, wait ten seconds, get two numbers. Because it is simple, it is also widely misread — as a general health score, as a sleep apnea test, as a reason to stay home instead of seeking care.
What it actually measures is the ratio of oxygen-carrying to oxygen-carrying-capable haemoglobin in arterial blood, estimated optically. It does not measure breathing, lung function, or how you feel. Those are separate questions, and the device is not a substitute for any of them.
How the reading is produced
Two LEDs shine red and infrared light through the fingertip. Oxygenated and deoxygenated haemoglobin absorb those wavelengths differently, and the device calculates a ratio from the light that gets through. Because it depends on light passing through tissue, anything that interferes with that path — cold fingers, nail polish, motion, poor perfusion — interferes with the number.
That is why the reading has a confidence indicator. A weak signal produces a number anyway, and the number looks authoritative. Always check that the pulse waveform or signal bars are strong before believing the saturation figure.
Where it is genuinely useful
Monitoring a known condition. If you have asthma, COPD, or a confirmed respiratory infection and a clinician has told you to watch saturation, a home oximeter gives you a trend between appointments. Trending 96% down to 93% over two days is meaningful.
Altitude. Above roughly 2,500 metres, saturation normally falls. Knowing your baseline at that altitude helps distinguish ordinary acclimatisation from something worse.
Post-operative or post-illness monitoring, when a clinician has asked for it specifically.
Spot-checking during a respiratory illness for people at higher risk, as an adjunct to symptoms — not a replacement for them.
Where it misleads
Low saturation is a late sign. In respiratory illness, people can deteriorate while saturation stays in the mid-90s, because the body compensates. Waiting for the number to fall before seeking care is a known and dangerous pattern. Symptoms — breathlessness at rest, inability to finish a sentence, lips or fingertips going grey-blue, confusion — matter more than the percentage.
Skin pigmentation shifts the reading. The optical method was developed and calibrated largely on lighter skin, and pigmentation absorbs some of the light the device relies on. The FDA issued a safety communication in 2021 warning that pulse oximeters can read falsely high in people with darker skin, and that this can delay treatment. Occlusion, cold, and low perfusion compound it. If you have darker skin, treat a reading near the low end of normal as more concerning, not less.
It is not a sleep apnea test. Consumer overnight SpO2 from a watch or ring is a convenience signal, not a diagnostic one. Sleep apnea is diagnosed with a sleep study that measures airflow, effort and arousals alongside oxygen. A normal overnight SpO2 trace does not rule out apnea. Our guide to sleep trackers covers what those devices are and are not good for.
It does not measure breathing rate, and it is not a fever check. Some devices add those features; treat them as convenience functions with their own accuracy limits.
What the numbers mean at rest, at sea level
| SpO2 | Interpretation for a healthy adult at rest |
|---|---|
| 95–100% | Normal range |
| 90–94% | Below normal; repeat after warming hands, and seek advice if it persists or symptoms are present |
| Below 90% | Seek urgent medical advice, especially with breathlessness or confusion |
These thresholds are for healthy adults at sea level. If you have a chronic lung condition, your clinician may have given you a different target — use theirs.
What to look for in a device
- FDA clearance, which puts it in the regulated device category rather than the general consumer one
- A visible pulse waveform or perfusion indicator, so you can tell a real reading from a bad signal
- Stated accuracy range — most credible devices specify accuracy in the 70–100% range with a tolerance of about ±2–3 percentage points
- Fingertip fit for your hand; a loose clip lets ambient light in and produces nonsense
Do not pay for Bluetooth. The value of a home oximeter is the reading in front of you, not a chart. If you need a chart, you are in a monitoring programme and should be using whatever device that programme specifies.
How this piece was put together
This is an explainer rather than a product ranking. It draws on FDA device guidance and safety communications, published validation literature on oximeter accuracy across skin tones, and clinical guidance on home monitoring. We have not run our own accuracy testing and do not claim to. See our editorial policy and medical disclaimer.
FAQ
Can I use an oximeter to decide whether to go to hospital?
No. Use it as one input alongside symptoms. Breathlessness at rest, chest pain, confusion, or lips turning blue are reasons to seek care regardless of the number.
Does nail polish affect the reading?
It can. Dark polish, gel and acrylics reduce light transmission. Remove it from the measuring finger, or use a different finger.
Why does my reading jump around?
Motion, cold hands and poor perfusion all cause it. Warm your hands, sit still, and wait for a stable waveform before recording. A single erratic reading is not information.
Are phone camera oximeters reliable?
They are not cleared devices in most cases and their accuracy is not established to the same standard. For anything that matters, use a fingertip device.
If you are trying to understand overnight oxygen patterns, read why you wake up at 3 AM first — it explains why breathing events disturb sleep and when that pattern deserves a clinical assessment rather than a gadget.
