Breathing and lung function tests

Pulse oximetry test

pulse oximetry test

What is it?

This measures how much oxygen your blood is carrying. This is called the oxygen saturation and is a percentage (scored out of 100). It’s a simple, painless test using a sensor on your skin.

What’s it used for?

If you’re living with a lung condition, your blood oxygen level may be lower than normal. This is important to know because when your oxygen level is low, the cells in your body can have a hard time working properly. Having a very low blood oxygen level can also put a strain on your heart and brain.

The test can be done as a one-off spot measurement. It can also be used to see if your oxygen levels drop during exercise – for example walking – or when you are asleep.

What happens during the test?

You will have a small device clipped to your finger or ear lobe, called an oximeter. This gadget shines light through your fingertip or earlobe and measures how red your blood is (blood cells with haemoglobin carrying oxygen are red – if they’re not, they are bluer).

If you wear nail varnish, this can block the light and affect the reading, so you’ll be asked to take it off – on one finger only.

What will the results look like?

The oximeter will be attached to a monitor that will show the percentage of oxygen in your blood. For someone who’s healthy, the normal blood oxygen saturation level will be around 94–99%.

If the oxygen level is below this, it may mean that you need additional oxygen therapy or other treatment. Your health care professional will discuss this with you. Low levels can put a strain on the heart. Usually you will have a blood gas test before oxygen is prescribed.

Current guidelines recommend that people with a resting stable oxygen saturation of 92% or less should be referred for a blood gas assessment to see if oxygen therapy is appropriate.

A change in your oxygen level may mean that your lung condition has got worse. In people with pneumonia and in children with lung problems oxygen saturation can help to decide if someone needs to be looked after in hospital.

If you have concerns or need advice, call our helpline on 03000 030 555 between 9am and 5pm on a weekday or email them.

We'll take good care of your personal info and you can update the way we contact you at any time - check out our privacy policy at to find out more.

Download this information (294KB, PDF)

Last medically reviewed: January 2017. Due for review: January 2020

This information uses the best available medical evidence and was produced with the support of people living with lung conditions. Find out how we produce our information. If you’d like to see our references get in touch.