Lung function tests

by | Tests and Investigations

Lung function tests, also called pulmonary function tests or breathing tests, are tests that can be done to find out how well your lungs and airways are working. Lung function tests can help in the diagnosis of respiratory (breathing) conditions, work out how severe a condition is, and help monitor the response to treatments.

Lung function tests can be done in adults and in children older than about 7 years. They may make you feel a little breathless, but should not be painful or uncomfortable. They include tests such as spirometry and peak flow measurement.

Lung function tests are often used to help diagnose or assess asthma and chronic obstructive pulmonary disease (COPD). They are also used to assess other conditions that affect breathing, including lung scarring and illnesses that affect the chest or breathing muscles.

What are the different types of lung function tests?

Different types of lung function tests may be recommended by your general practitioner (GP), respiratory physician (specialist in conditions affecting the airways and lungs) or paediatrician (specialist in children’s health), depending on your age and condition.

Lung function tests that measure airflow include:

  • peak expiratory flow (PEF) measurement; and
  • spirometry (which may also include bronchial provocation testing).

Tests that measure gas exchange in the lungs include:

  • arterial blood gas analysis; and
  • gas diffusion tests.

Exercise tests can show whether or how much your breathing problems are impacting on your daily life.

Lung function testing in children

Peak expiratory flow tests and spirometry are the main lung function tests done in children, often to diagnose or assess asthma. Spirometry is also part of the routine care of children with respiratory diseases that get worse with time (such as cystic fibrosis) or progressive neuromuscular conditions (such as Duchenne muscular dystrophy).

Spirometry is a test of lung function that measures the amount of air breathed in and out. Children older than 7 years are usually able to perform spirometry. They may be tested with a spirometer in their doctor’s surgery or local laboratory. Because you need to be able to follow instructions and coordinate your breathing, most experts agree that children cannot properly perform spirometry until they are at least about 6 or 7 years old.

Other tests available at respiratory departments in teaching hospitals include exercise testing, bronchial provocation tests (see below) and lung volume measurements. Exercise testing may be used to assess children older than 8 years who complain of shortness of breath, often associated with exercise. It can help to determine whether the diagnosis may be asthma, restrictive lung disease, anxiety or lack of fitness.

Peak expiratory flow test

Peak expiratory flow (PEF) is the greatest speed that can be reached when you breathe out as hard and fast as possible from the biggest breath in you can manage. It gives an indication of how narrow your airways are. Peak expiratory flow can be measured using a peak flow meter (also called a peak expiratory flow meter) which is a small, hand-held, portable device.

Peak flow measurements are often used to monitor asthma or COPD. People with asthma usually use their own peak flow meter to conduct regular peak flow measurements at home. Peak flow readings should always be done using the same peak flow meter as there is variation between different devices. Your doctor will show you how to do the test and record the results. If you notice your peak flow measurements are getting worse, you may need to see your doctor or adjust your medicines according to your asthma action plan.

Using a peak flow meter requires you to coordinate your breathing and follow instructions. They can be done in children from about the age of 7 years, but results can be unreliable in young children.

Spirometry testing

Spirometry test

Spirometry is a test of lung function that measures the amount of air breathed in and out using a device called a spirometer. It gives a measure of the maximum amount of air you can breathe out (after taking a deep breath in), as well as how much and how quickly you can breathe out.

Spirometry testing gives information about the size of the lungs and the degree of airway narrowing. Spirometry is the most accurate test of lung function in people with asthma, and is also commonly used in people with COPD.

Some additional tests that may be done as part of spirometry testing include:

  • Bronchodilator response test – where spirometry tests are done both before and after taking asthma reliever medicines, to see what effect they have on your results.
  • Exercise challenge test – where spirometry is done before and after exercising (e.g. using an exercise bike, walking or running on a treadmill or climbing stairs), to see if exercise triggers airway narrowing. This test may be used if exercise-induced asthma is suspected.

What happens during spirometry testing and how to prepare

Spirometry can be performed in your doctor’s office or clinic or hospital laboratory, and usually takes about 10 to 20 minutes to complete.

On the day of the test, you may be asked to not use your puffers and some other medicines. Check with your doctor if you are not sure about this. Your doctor might also advise you to wear loose-fitting clothing for the test, and not to eat a large meal or smoke beforehand.

You will be asked to breathe into a mouthpiece that is connected to the spirometer. It’s important that your lips form a tight seal around the mouthpiece. You will probably need to wear a clip on your nose while you perform the breathing tests.

You should be aware that the amount of effort you put in can influence the accuracy of your lung function tests. Your doctor may get you to repeat the test several times to get accurate readings, or to check the effects of your medication.

Interpreting spirometry tests – what is normal?

A spirometer can measure the following.

  • Forced vital capacity (FVC): This is the total amount (volume) of air expelled from the lungs after a full breath into the lungs (to total lung capacity).
  • Forced expiratory volume in one second (FEV1): This is the amount (volume) of air expelled in the first second after a full breath into the lungs and then breathing out hard and fast, as above, trying to push all the air out of the lungs.
  • Forced expiratory ratio (FEV): FEV1/FVC: this measurement helps work out whether there is an obstructive or restrictive breathing pattern (see below).
  • Peak expiratory flow (PEF): This is the greatest speed that can be reached when you breathe out as hard and fast as possible from the biggest breath in you can manage.

Your spirometry results will be compared with results from other people who are roughly the same age, sex, height and ethnicity as you, which is the predicted value. Your lung function tests may be considered normal (in the normal range) if they are within 80 to 120 per cent of the predicted value.

Abnormal results may show an obstructive or restrictive pattern. Obstructive respiratory conditions include asthma and COPD. Restrictive conditions include interstitial lung disease, respiratory muscle weakness and conditions that restrict movement of the chest wall (such as extreme obesity).

Bronchial provocation test

A bronchial provocation test may be recommended to help diagnose asthma and determine its severity. Bronchial provocation testing is usually done in a specialised respiratory laboratory.

The test takes about half an hour and involves performing spirometry tests after breathing in substances (such as histamine or cold, dry air) that can trigger airway narrowing. The degree of airway hyper-responsiveness (an over-reaction of the airways that causes them to narrow) is measured and reliever medicines (such as a ventolin puffer) are then given to help open up the airways.

The test may be repeated 10-15 minutes after taking the reliever medicine to see how well the airways have responded.

You may be asked to not use your usual asthma reliever medicines and some other medicines for several hours before the test – check with your doctor. Also, if you have recently had a cold or cough, you may need to wait a few weeks before having this test.

Gas diffusion test

A gas diffusion test can be used to assess the severity of lung conditions such as pulmonary fibrosis and emphysema. It is simple and quick to perform, giving information about how easily gases are able to move across the membrane in the lungs and into the bloodstream (and vice-versa).

In a gas diffusion test, you breathe in a special gas, hold your breath for about 10 seconds, and then breathe out into a testing machine. The machine calculates the difference in the amount of special gas breathed in and out, measuring how much has crossed the lung membrane. This measurement gives an indication of how easily oxygen can move from the lungs into the bloodstream.

Exercise testing

Exercise tests such as the 6-minute walk test may be done to see how much a respiratory condition is impacting on your daily activities. The test measures your exercise capacity by determining how far you can walk on a treadmill in 6 minutes, and what impact that has on your oxygen levels and heart rate.

Arterial blood gases

Arterial blood gas analysis measures the amount of oxygen and carbon dioxide in your blood. A tiny needle is used to take a small sample of blood from an artery – usually the artery in your wrist. Local anaesthetic may be given before the needle is inserted.

This test is usually done in hospital to help work out the severity of a lung condition or to determine whether home oxygen therapy is needed.