Oxygen level testing

Tests to check the level of oxygen in your blood can be helpful in diagnosing or monitoring lung conditions.

Pulse oximetry

What is pulse oximetry?


What is pulse oximetry used for?

Pulse oximetry is a test to measure the level of oxygen in your blood. This is called your oxygen saturation level. It’s a simple, painless test that uses a sensor placed on your fingertip or earlobe.

Having a lung condition may mean your blood oxygen levels are lower than they should be. Pulse oximetry can help to understand if you might have a lung condition. It can also be used to monitor an existing lung condition.  

The more the lungs are damaged, the more likely there is to be a problem with your oxygen levels.

The test may be done once to help diagnose a lung condition. It can also be used to measure your oxygen levels over time. For example, during exercise like walking or when you are asleep. 

How can I prepare for a pulse oximetry test?

Nail varnish or false nails can block the light and affect the test. You’ll be asked to remove them on one finger only. This will help get an accurate result.

Pulse oximetry results can also be affected by medical conditions including anaemia and Raynaud’s syndrome. Talk to your healthcare professional if you have other medical conditions.

What happens during a pulse oximetry test?

You will have a small device called a pulse oximeter clipped to your finger or earlobe. This shines light through your fingertip or earlobe. It works out how much oxygen is in your blood.

How a pulse oximeter works

A pulse oximeter measures how much light is absorbed by your blood. This shows how much oxygen is in your blood.

The pulse oximeter shines two lights through your fingertip or earlobe: one red light and one infrared light.

Blood containing lots of oxygen absorbs more infrared light. Blood without enough oxygen absorbs more red light. 

Can I do a pulse oximetry test at home?

Home-use pulse oximeters are available to buy online and in pharmacies. However, these can sometimes give poor measurements as they are not as good quality as the ones used in hospitals. 

Pulse oximeter results on darker skin may be less accurate because they can show a higher reading of oxygen level in the blood. If you have darker skin and have been told to use a pulse oximeter at home, you should continue to use the oximeter and monitor any changes in your readings.  

Talk to your healthcare professional if you have concerns about using a pulse oximeter at home. 

What do the results mean?

The pulse oximeter display shows the percentage of oxygen in your blood. For someone who’s healthy, blood oxygen saturation level will be around 95–100%.

If the oxygen level is below this, it can tell you that there is a problem. People with low oxygen levels may need oxygen or other treatment. Your healthcare professional will discuss this with you.

Current guidelines recommend people with an oxygen saturation level of 92% or less should be referred for a blood gas assessment to see if oxygen therapy is needed.  

Usually, you will need to have a few blood gas tests before oxygen is prescribed.

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, pulse oximetry can help to decide if someone needs to be looked after in hospital.

Blood gas test

What is a blood gas test?

A blood gas test is used to accurately measure how much oxygen and carbon dioxide there is in your blood. A sample of your blood will be taken from your wrist or your earlobe.

What is a blood gas test used for?

A blood gas test is used to check how well your lungs are working and whether they’re able to exchange oxygen and carbon dioxide properly.

It can be used to see if you need oxygen therapy.

What happens during a blood gas test?

Your doctor will take a small sample of your blood. They will normally take this by using a needle and a syringe in your wrist. The test can be a little uncomfortable but the doctor will do their best to reduce this. Sometimes, they will use local anaesthetic so that you won’t feel the needle.

Some hospitals check blood gas by taking a blood sample from your earlobe.

  • Your doctor will put a cream on your earlobe that helps increase blood flow. This makes your ear go red and feel hot.
  • The blood vessels in your ear lobe will then contain about the same amount of oxygen as blood taken from your wrist.
  • After a few minutes your doctor can take a sample by pricking the earlobe and catching the blood droplet. This isn’t painful - it’s similar to the way blood sugar levels can be checked from a pinprick on the fingertip.

The earlobe method normally can’t be used if you need to have the test when you are unwell. For example, if you’re admitted to hospital with a flare-up of COPD symptoms, you won’t have a blood sample taken from your earlobe. 

What do the results mean?

The results will be a set of readings showing:

  • the level of oxygen in your blood
  • the level of carbon dioxide in your blood
  • how acidic or alkaline your blood is (pH).

The results show if your body is getting enough oxygen. They also show if it is getting rid of enough carbon dioxide. If you aren’t getting rid of enough carbon dioxide, your blood will be more acidic.

A high level of carbon dioxide may mean that your breathing is shallow at night, and you may benefit from using a breathing machine, called a ventilator, at home. 

Long-term oxygen therapy (LTOT) assessment

What is a long-term oxygen therapy assessment?

Some people with very low oxygen levels may benefit from using oxygen therapy. A long-term oxygen therapy assessment is a set of tests to measure the levels of oxygen in your blood to see if they are low enough for oxygen therapy to be helpful.

What’s it used for?

A long-term oxygen therapy assessment is used to test oxygen levels in people who have a long-term condition such as COPD, pulmonary fibrosis, pulmonary hypertension or cystic fibrosis.  

The results can help your health care professional decide if you should have oxygen therapy.

What happens during an oxygen assessment?

An oxygen assessment usually happens when your lung condition is stable, and you don’t have a chest infection. Oxygen levels can drop if you have an infection. If your oxygen levels stay low even when an infection has gone, you may need oxygen.  

The assessment involves measuring your blood gases on two occasions, a few weeks apart.
Your oxygen levels will be tested while you’re sitting down. You’ll also have a pulse oximetry test and you may have a lung function test using a spirometer.

Sometimes you’ll be asked to do a walking test to see if your oxygen levels go down as you exercise and, if that happens, whether extra oxygen means you can walk further.

Some people who don’t need to use oxygen all the time can benefit from oxygen when they are exercising. This is called ambulatory oxygen. This is only beneficial in people whose oxygen levels fall significantly when they walk.

What happens if I have low levels of oxygen?

If your oxygen levels are low and you need oxygen therapy, your blood gases will be checked again while you are breathing in extra oxygen. The oxygen is delivered by small, clear tubes known as nasal cannulas or a face mask covering the nose and mouth. The blood gas check will help work out how much oxygen you need to improve the level of oxygen in your blood.


Fitness-to-fly (hypoxic challenge) test

What is the hypoxic challenge test? 

The fitness-to-fly (hypoxic challenge) test is designed to test whether you would need oxygen on an aeroplane or at any high altitude. The test involves breathing in reduced levels of oxygen, just as you would on a plane.

What’s it used for?

It’s used to see if you will need oxygen while you are flying or going somewhere with a high altitude, such as a mountain.

If you live with a lung condition, your oxygen levels may be lower than normal. During a flight, the oxygen level in the cabin air is much lower. This means that during a flight your blood oxygen levels can fall even further. This could result in heart problems or other complications. Using oxygen can help you avoid this.  

If you are planning a flight, ask your healthcare professional if you need this test well in advance. This will help you plan your trip.

What happens during a hypoxic challenge test?

While you’re sitting down, you’ll breathe a low oxygen mixture using a face mask. Your oxygen levels and heart rate will be monitored. At the end of the test, a small blood sample may be taken from your earlobe or your wrist.

If your oxygen levels go down during the test, your healthcare professional will add extra oxygen to see if your oxygen levels go back to normal.

What do the results mean?

At the end of the test, your doctor can decide if you need oxygen during your flight. If your oxygen goes down during the test, even after adding extra oxygen, it may be unsafe for you to fly. If your healthcare professional was able to bring your oxygen to a stable level, they’ll know the amount of extra oxygen you need to be able to fly safely.

If you are already on oxygen, you will definitely need additional oxygen in flight. Ask your health care professional about this.
For more information on flying with oxygen, have a look at our travel pages.

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