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A pulse oximeter is a small device that clips onto your finger and monitors oxygen levels in the blood and costs around £30. They have been getting attention for their potential to help monitor symptoms of COVID-19 at home, but there are some caveats you should know about before rushing out to buy one.

One of the many strange things about COVID-19 is that some patients with very low blood oxygen levels (which require medical treatment) aren’t always aware of it. They may not feel particularly unwell or have other symptoms of low oxygen levels.

This is where pulse oximeters can be helpful as they allow people to track their blood oxygen levels at home, and are an important preventative tool for certain groups.

They are used commonly in clinical settings, and have helped people with Long Covid (enduring post-viral symptoms), or those who are more vulnerable to the disease monitor themselves at home.

There’s currently a trial going on in England called ‘COVID oximetry @home,’ which supplies people in certain groups (Covid positive, symptomatic and 65 or older, symptomatic and under 65 but clinically vulnerable) with a pulse oximeter and instructs them on how to measure their blood oxygen levels at regular intervals, and communicate the readings to a doctor.

Some doctors are saying everyone should have them just in case, but others are more skeptical of their value for otherwise healthy people.

Used incorrectly, they may give inaccurate or poor readings, and it’s unwise to rely on them without wider diagnostic support from a medical professional.

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What does a pulse oximeter do?

A pulse oximeter measures your pulse and the percentage of oxygen in your blood, by shining a light into your finger.

Pulse oximeters are commonplace medical devices that have been used in some form since the 1970s. They are used most often by people with respiratory problems, and also sometimes by athletes and pilots who have to monitor blood oxygen levels.

They are mostly used for tests and monitoring in clinical settings, such as at the GP surgery or in hospital, but for specific groups of people (like those mentioned above), they are useful for home monitoring too.

According to the British Lung Foundation, the normal blood oxygen saturation level for someone who’s healthy will be around 95–100%. If the oxygen level is below this, it can be an indicator that there is a lung problem.

A level below 92% (or 88% for people with chronic obstructive pulmonary disease – COPD) would suggest someone is seriously ill and may need supplementary oxygen or to be monitored in hospital.

In the COVID oximetry @home scheme, patients reporting 93-94% blood oxygen are instructed to call the GP or 111 , and patients at 92% or below are told to go to A&E or call an ambulance.

Can a pulse oximeter diagnose COVID-19?

There has been some confusion over this, but the answer is no.

A pulse oximeter may signal issues with blood oxygen levels, which could be related to coronavirus, but it’s only one part of a wider diagnostic picture.

Londonwide LMCs, a representative committee of NHS GPs in London, advises that: ‘pulse oximetry can be a useful aid to clinical decision-making but it is not a substitute for a clinical assessment, nor sufficient for diagnosis by itself.’

Should you buy a pulse oximeter?

If you don’t have any underlying respiratory issues and have never used one before, you may not need a pulse oximeter. It’s best to only use one if advised by your doctor.

Professor Chris Hui, clinical assistant professor at Hong Kong University and honorary consultant in respiratory and critical care medicine at the Royal Free Hospital London, says pulse oximeters can be useful monitoring and early-alert devices for self-monitoring at home. This is dependent on them being given to the right groups of patients, such as those who are vulnerable, prone to respiratory failure, suffer from a chronic respiratory condition, or require oxygen therapy at home.

Some doctors have now suggested buying one and testing your oxygen levels as a precaution, but the reading may be less useful without prior guidance from a medical professional, and if you haven’t used one before you may do it incorrectly or misinterpret the results.

Dr Andy Whittamore, clinical lead for Asthma UK and the British Lung Foundation, advised that ‘at this stage, while the use of pulse oximeters in response to COVID-19 is still being examined, it remains really important that before testing at home, people talk to their healthcare professional.’

He adds that ‘any monitoring done at home needs to be part of a clear management plan and is not a substitute for clinical advice. Anyone worried about their symptoms should speak to a doctor.’

As with many coronavirus-related products, demand for pulse oximeters has risen due to the publicity they have received, so be wary of unscrupulous sellers selling these devices online for inflated prices, or without proper safety certificates (approved medical devices in the UK must display a CE mark).

Pulse oximetry and racial bias

The independent NHS Race and and Health Observatory has published a rapid review into pulse oximeters, saying that readings for Black and Minority Ethnic people could be misleading.

The leader of the review, Olamide Dada, said:

“As this review clearly shows, more evidence is needed to examine the effectiveness of pulse oximeters for all patients. It is essential that clinicians and carers have full knowledge of diverse clinical signs when observing ethnic minority patients at risk of Covid-19, and particularly those using home devices.

More detailed analysis is needed to help determine the reliance on, and future development of, pulse oximetry devices going forward.”

A study in the New England Journal of Medicine found that pulse oximeters were more likely to record false readings for Black patients – recording a value in the ‘normal’ range when the person actually had lowered blood oxygen levels.

The report found incorrect readings in about 12% of all cases, happening to Black people about three times as much as white people.

This doesn’t mean that pulse oximeters are always useless for people with darker skin, but that more research is needed, and it’s important to read the results in context of symptoms and other diagnostic metrics.

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Using a pulse oximeter correctly

Professor Hui says that, although these devices are usually accurate, they need to be used correctly to yield the best information.

How To Use Pulse Oximeter

• They need to be held steady in place for 30 seconds – if there is a lot of movement, poor placement or insufficient time for the computer to calibrate, the readings may be difficult to interpret or inaccurate.
• Avoid nail varnish and false nails – these can block the light from the device getting to your blood and can affect the reading.
• Medical conditions – some, such as anaemia and Raynaud’s syndrome (where fingers are unusually cold) can affect the accuracy of readings.

Professor Hui adds that the quality of the tech inside oximeters – the quality of the LED diodes, photo detectors and quality of electronics – can also affect accuracy of readings.

Accuracy of the oximeter aside, the key is that it doesn’t tell you everything, so relying on it in place of medical advice could give you false reassurance or cause anxiety.

Can your fitness tracker or smartwatch double up as a pulse oximeter?

Some fitness trackers and smartwatches have a pulse oximetry function (often referred to as the SpO2 sensor), but you shouldn’t rely on this reading, as it’s intended for recreational rather than medical purposes.

Popular smartwatch brand Garmin says that knowing your oxygen saturation can help you determine how your body is acclimatising to high altitudes (for alpine sports and expeditions), and can also be useful for flagging signs of sleep apnoea, or if you’re overexerting yourself during exercise.

But it does state clearly that the data is not intended to be used for medical purposes, or to diagnose, treat, cure or prevent any disease or condition.

What Is The Best Pulse Oximeter

Accuracy may be compromised by the location of the device: fitness trackers take blood oxygen measurements from the wrist rather than the fingertip. Here they are more likely to move around, and there is a lower concentration of blood at the surface level.

We recently tested this feature on a couple of popular wearables – the Huawei GT2, Huawei GT2e, Garmin Venu, Garmin Vivoactive 4 and Honor Band 5.

We compared the results to those taken by a reference medical device on 10 healthy people, all within the normal range of SpO2 sensors. Some were easy to use and accurate, giving readings that were close to those of the reference device, while others were more capricious and required us to take a couple of readings.

Notably, some struggled on darker skin, due to the technology involved in taking a reading. One model we looked at is pretty useless unless you have pale white skin.

Even for those devices that worked well on everyone, we’d recommend taking two readings to confirm the values.

And, it’s worth reiterating that fitness trackers and smart watches aren’t medical devices, so you shouldn’t rely on them to detect a problem. However, if you do notice something unusual, then it’s worth following up with your doctor.

This story was originally published on 5 May 2020, but has since been updated to reflect the latest developments.