Lung function tests, sometimes called pulmonary function tests (PFTs), measure how well your lungs move air in and out and how effectively oxygen passes from your lungs into your blood. They are commonly used alongside symptoms, scans, blood tests and sputum tests to help assess lung conditions, including aspergillosis.
The tests can be tiring because they need your best effort, but the staff carrying them out will explain what to do and can pause if you need a rest. A result is one part of the overall picture; it should never be interpreted in isolation.
Why might I have lung function tests?
Your clinical team may use them to:
- help investigate breathlessness, cough or wheeze
- identify patterns that suggest narrowed airways or reduced lung volume
- monitor asthma, COPD, bronchiectasis or other lung disease alongside aspergillosis
- assess whether treatment is helping
- provide a baseline before some treatments or surgery
- follow changes over time.
Spirometry: the most common test
Spirometry measures how much air you can blow out and how quickly you can do it. You sit upright, breathe into a mouthpiece and are usually asked to take a full breath in, then blow out as hard and as long as you can. You will normally repeat the manoeuvre several times so the best reliable result can be recorded.
It can feel surprisingly demanding. The encouragement from the physiologist is not a judgement: it helps make sure the result genuinely reflects the best you could do on that day.
FEV1
Forced expiratory volume in one second (FEV1) is the amount of air you can force out in the first second of a full exhalation. It is one useful measure of airflow through the airways.
FVC
Forced vital capacity (FVC) is the total amount of air you can force out after taking the deepest breath possible.
The FEV1/FVC ratio
Comparing these two values helps clinicians look for a pattern of airflow obstruction, which can occur in conditions such as asthma or COPD. A low FEV1 does not, by itself, explain why it is low or diagnose a particular condition.
Other lung function tests
Peak flow
Peak flow measures the fastest speed at which you can blow air out. It is often used to monitor asthma, especially when readings are recorded regularly at home. A single reading is less useful than the pattern over time and how it relates to your symptoms.
Bronchodilator reversibility testing
You may be asked to repeat spirometry after using an inhaler that opens the airways. This can show whether airflow improves after bronchodilator medicine. Follow the appointment instructions carefully: you may be asked to take, or to temporarily withhold, particular inhalers before the test.
Gas-transfer testing
A gas-transfer test, also called diffusion capacity testing, looks at how effectively oxygen moves from the tiny air sacs in your lungs into your bloodstream. You may be asked to breathe in a harmless test gas, hold your breath briefly, then breathe out. This can provide useful information when assessing a range of lung conditions.
Lung volumes
Some people have more detailed tests to measure the total volume of their lungs and the amount of air left after breathing out fully. These tests can help distinguish between different patterns of lung disease and may be arranged in a specialist lung-function laboratory.
What do the results mean?
Your results are compared with reference values for people with similar characteristics, including age, height and sex. They may be shown as a percentage of the predicted value or alongside a reference range.
It is understandable to focus on a number, but trends are usually more useful than one result. Readings can be affected by a recent infection or flare-up, tiredness, cough, technique, whether you have taken an inhaler, and how well you feel on the day.
Your team will interpret the results alongside your symptoms, examination, scans, blood tests and previous lung-function results. Ask what your results mean for you, whether they have changed over time, and whether any action is needed.
Preparing for your test
Your appointment letter should tell you whether to take your usual inhalers and other medicines before the test. Do not stop any prescribed medicine unless you have been specifically asked to do so.
- Wear comfortable clothing that does not restrict your chest or abdomen.
- Arrive in good time so you are not rushed.
- Tell the physiologist if you feel unwell, have recently coughed up blood, have chest pain, or are recovering from an illness or procedure.
- Tell them if you feel dizzy, very breathless or anxious during testing. You can ask to pause.
Key message
Lung function tests are not a pass-or-fail test and they do not measure effort or determination. They give your clinical team useful information about how your lungs are working at that moment, particularly when compared with earlier results and the rest of your clinical picture.
This page provides general information and does not replace advice from your own clinical team.
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