A pulse oximeter is the most trusted device most people own and the least understood. It shines two wavelengths of light through a fingertip and infers, from how much of each gets through, the percentage of haemoglobin that is carrying oxygen. Everything useful and everything misleading about it follows from that being all it does.
1. Why 94% and 90% are not four points apart
The relationship between saturation and the oxygen actually dissolved in blood is a curve, not a line, and it is flat at the top and steep at the bottom.
| Saturation | Roughly what it corresponds to | What a further 4% drop costs |
|---|---|---|
| 98% | Comfortably on the flat part of the curve | Very little. The reserve is large |
| 94% | Still on the flat part | Still modest |
| 90% | The knee of the curve. Below here it falls away steeply | A great deal — the same 4% now represents a much larger fall in dissolved oxygen |
| 86% | Well down the steep section | Deteriorating fast, and small further falls matter enormously |
So a fall from 98 to 94 and a fall from 92 to 88 are not the same event, even though both are four points. The first is a person using up reserve; the second is a person running out. This is why 90–92% is where thresholds cluster across every guideline, and why a slow drift downward should be acted on before it reaches the interesting part rather than after.
The corollary is the useful one. A single reading of 97% is almost meaningless as a trend, because the curve is flat there and a healthy person and a slightly unwell person both read 97. The direction across several readings tells you more than any one of them, and that is true of every measurement on this site.
2. Saturation is not oxygen content — the two conditions that prove it
This is the part most often left out, and it explains two situations where a perfect reading accompanies a person in serious trouble.
| What the oximeter says | What is actually happening | |
|---|---|---|
| Severe anaemia | Normal. Often 98–100% | The haemoglobin that is there is fully loaded — there just is not much of it. Someone who has lost half their red cells can saturate perfectly while delivering half the oxygen. Major blood loss does not show up on a pulse oximeter, which is worth knowing alongside the bleeding chapter. And because cyanosis requires a certain quantity of desaturated haemoglobin to be visible, a severely anaemic patient may never look blue however hypoxic they are |
| Carbon monoxide poisoning | Falsely high. Can read 99% | Carboxyhaemoglobin absorbs light almost identically to oxyhaemoglobin, so the device counts it as loaded when it is loaded with the wrong thing. The most dangerous false reading these devices produce |
| Methaemoglobinaemia | Sticks near 85% and barely moves with oxygen | Rare, but characteristic: a saturation stuck around 85 that does not respond to oxygen, in someone who may look grey or blue out of proportion to how they feel. Clinically: teething or throat gels containing benzocaine, dapsone and some other antibiotics, poppers and other nitrites, some dyes and mothball-type compounds, and well water high in nitrates in infants |
3. Getting a reading you can use
Warm hand, still hand, right finger
Cold fingers are the commonest reason a probe reads low or refuses to read. Warm the hand, rest it on the chest or a surface, and give it 30 seconds rather than reading the first number that appears.
Clear the nail
Dark varnish and false nails block the light path. Remove it, use another digit, or turn the probe sideways so the light crosses the fingertip rather than the nail.
Look at the waveform, not only the number
Most devices show a trace or a signal-strength bar. A number with a poor trace is not a measurement, and this is the check almost nobody does. If the pulse rate displayed does not match the pulse you can feel, the reading is wrong — and this single cross-check catches most bad readings.
Write down what they were breathing
On air, or on oxygen and how much. 94% on four litres is a completely different situation from 94% on air, and the two get recorded identically.
In children, use a probe sized for them — an adult clip on a small finger gives motion artefact and poor contact. A toe, or a foot with a wrap-around probe, works in infants and toddlers, and a still child asleep gives a better reading than a distressed one being held. As everywhere in paediatrics, the work of breathing tells you more, and tells you sooner: nasal flaring, tracheal tug, grunting and recession all appear before the saturation moves.
4. The four ways it lies
| The problem | What happens | What to do |
|---|---|---|
| Falsely high in carbon monoxide poisoning | Carboxyhaemoglobin reads as oxyhaemoglobin. Someone dying of CO can read 99% | House fire, faulty boiler, generator, several people in one building with headache and nausea — ignore the oximeter entirely and treat as CO |
| Falsely high in people with darker skin | Pigmentation biases the reading upward, so true hypoxaemia is missed — "occult hypoxaemia". Regulators on both sides of the Atlantic have issued safety communications | Weight the clinical picture more heavily, and treat a borderline reading as lower than it says |
| Falsely low, or absent, with poor perfusion | Cold hands, shock, vasoconstriction, low blood pressure, tremor, movement. In shock the finger is the last place with blood in it | Warm the hand, try an earlobe if the device allows — and note that failure to obtain a reading is itself a finding |
| Nail varnish, false nails, dirt, bright light | Dark varnish in particular blocks the light path | Remove it, change digit, or turn the probe sideways |
5. What a reading means in context
| Situation | What the saturation is telling you |
|---|---|
| Asthma attack | Falls late. A child working visibly hard with a normal saturation is a sick child; the effort of breathing is the better guide |
| Anaphylaxis | Frequently normal until very late, including in reactions that go on to kill. Never let it delay adrenaline |
| Panic or hyperventilation | Normal or high. Genuinely useful here — it excludes hypoxia and gives the person something concrete to be reassured by |
| Shock or major bleeding | Often unreadable, and normal even when it does read, because saturation is not content. A bleeding patient with a good trace and a 99% reading is still bleeding — see catastrophic bleeding, where the earlier signs are rising pulse and poor perfusion rather than anything an oximeter shows |
| Carbon monoxide | Actively misleading. Ignore it |
| Opioid overdose or sedation | Can stay normal while breathing fails. Count the rate |
| After a seizure | Commonly dips briefly and recovers. Recheck rather than reacting to the first number |
| Known severe COPD | Their usual may be 88–92%. See the targets below |
6. Targets, and the exception
| Who | Target |
|---|---|
| Most adults and children | 94–98% |
| Known chronic CO₂ retention, some people with severe COPD | 88–92% as a level to titrate oxygen to |
Two guards on 88–92, because it is the most misusable number here
- It is a titration target and never a reason to delay 999. It tells you where to aim the oxygen in someone with an established diagnosis. It does not tell you that a deteriorating person sitting at 89% is fine.
- It is a floor as well as a ceiling. Below 88% is under target, and is treated as an emergency reading on the vital signs page for that reason. The band is the space between not enough and too much, not a licence to tolerate anything below it.
- If you do not know whether someone is in that group, treat the hypoxia. The group is small and carries alert cards; the harm of under-oxygenating someone who is not in it is immediate.
7. What to do about a low reading
Check it is real before you act on it
Warm hand, good trace, pulse rate matching the pulse you feel, different digit. Perhaps a third of alarming readings are artefact — but check quickly, and never at the cost of treating someone who plainly looks unwell.
Sit them upright
Almost always improves things, and costs nothing. Never lie a breathless person flat.
Treat the cause if you know it
Their inhaler for asthma. Adrenaline for anaphylaxis, which comes before any reading. Naloxone and breaths for an opioid overdose.
Oxygen if you have it and are trained to use it
Aim for 94–98%, or 88–92% in known retention. Oxygen is a treatment for hypoxia and not for breathlessness — a breathless person with a normal saturation does not generally need it.
Call 999 for anything under 92% that does not correct immediately
And for any new cyanosis at any reading. Cyanosis is not visible until roughly 85%, so a blue patient with a reassuring number means the number is wrong.
The overall posture. Pulse oximetry is genuinely useful and it is not a diagnosis. It is at its best tracking one person over time and at its worst producing a single number that overrides what someone can see in front of them. A reading that disagrees with how the person looks is more likely to be wrong than the impression is.