Spirometry: the lung function test explained

What is spirometry?

Spirometry is the most common lung function test. It measures how much air a person can blow out and how quickly. Spirometry is the main test for detecting airflow obstruction, while other lung function tests measure lung volumes and gas exchange.

What happens during the test?

The person wears a nose clip so air goes through the mouth, and breathes through a mouthpiece into the lung function machine. They breathe normally, take a deep breath in, and then blow out as hard and fast as possible. Each blow may be repeated several times, with rests in between. One NHS trust says the test normally takes around 15 minutes.

Why is spirometry done?

The NHS says spirometry measures how much air a person can breathe out in one breath, and it is also called a lung function test. It is used to find out whether symptoms come from a lung condition such as asthma, COPD or pulmonary fibrosis (scarring of the lungs), to monitor an existing condition, or to check fitness before surgery. NHS England guidance says NICE recommends quality-assured spirometry as one of several objective tests for diagnosing asthma and COPD. The same guidance says normal spirometry does not rule out asthma.

How do people prepare?

One NHS practice suggests avoiding vigorous exercise for 30 minutes before the test. The same guidance suggests wearing loose clothing. It also notes that a recent chest infection, or recent antibiotics or steroids, can affect the results. Preparation varies, and patients are told whether they need to stop any inhalers or medicines beforehand.

How is the test done, according to the NHS?

NHS trust leaflets say staff record the person's height and weight, which are entered into the machine for the calculations. The person uses a nose clip, breathes in deeply, seals their lips around the tube and breathes out as hard and fast as possible. The NHS says this is repeated two more times, and that staff may ask the person to use an inhaler and repeat the test. The NHS says the appointment should take no more than an hour, and that dizziness during or just after the test is possible but should be brief. The NHS says the test should not be done during a chest infection or soon after a heart attack or stroke. Trust and practice pages say the results are reviewed and then sent to the doctor who requested the test, who discusses them with the person.

What is the reliever step?

The test is sometimes repeated after a reliever inhaler to check whether it helps the airways. The person takes the test, uses the inhaler, waits a short time and repeats the test. The comparison shows how much the airways respond to the medicine, and the test is sometimes used when asthma is being investigated.

What do the results show?

Results are compared with predicted values for someone of the same height, age and sex. A typical report shows the measured values alongside the predicted values. In obstructive conditions such as asthma and chronic obstructive pulmonary disease (COPD), the airways narrow, making it harder for air to flow out of the lungs. Restrictive conditions are different, because they decrease the amount of air the lungs can hold. One NHS leaflet says results are discussed at the next clinic appointment with the consultant or GP.

Are there any risks?

NHS leaflets describe the test as painless and non-invasive, with dizziness or fainting the most common side effects and more serious complications very rare. The test needs a good effort, which can make some people feel dizzy or faint. Some people feel dizzy or faint, and coughing is common.