Reviewed by: HU Medical Review Board | Last reviewed: October 2021

Spirometry is a common test to see how well your lungs work. It measures how much air you inhale and exhale over a specific amount of time. Doctors often use it to diagnose asthma and other breathing conditions.

Results from spirometry can help doctors monitor your asthma. They can also help see if asthma drugs are helping you breathe better.

What is spirometry?

Spirometry is the recommended test to diagnose asthma and other breathing conditions. It is a type of lung function test. It helps doctors measure your breathing to diagnose and monitor asthma.1

Spirometry can give different types of information to help doctors diagnose and treat asthma. For example, spirometry can be done:2

  • To confirm an asthma diagnosis
  • To see if asthma drugs are working
  • To see how lung function changes after exercise, allergen exposure, or taking methacholine (an inhaled drug that causes narrowing of the airways in the lungs)
  • To see how your lung function has changed over time

Before the test, a nurse or doctor will give you instructions. Ask them if you do not understand anything. During the test, you will sit and breathe into a mouthpiece connected to a spirometer. A clip on your nose will keep your nostrils closed. You will then take a deep breath before exhaling as hard and quickly as possible.3,4

You may repeat this several times to get an accurate measurement. The whole process usually takes less than 15 minutes. Your doctor may then give you a bronchodilator (an inhaled drug to open your airways) and repeat the test after 15 minutes.3

How does spirometry help diagnose and monitor asthma?

Spirometry provides 3 main results that help diagnose and monitor asthma:5

  • Forced expiratory volume in 1 second (FEV1) – How much air you can forcefully exhale in 1 second
  • Forced vital capacity (FVC) – The maximum amount of air you can forcefully exhale
  • FEV1/FVC ratio – How much of your total air capacity you can forcefully exhale in 1 second

The results are given in percentages compared to normal values for someone your age, height, weight, gender, and race. For example, an FEV1 or FVC above 80 percent is normal. If your results are normal, you may do spirometry again after a challenge test. This can diagnose exercise-induced or allergen-induced asthma.2,6

A low FEV1 or FEV1/FVC means you have difficulty getting air out of the lungs. This is a sign of an obstructed airway, which could result from asthma or chronic obstructive pulmonary disease (COPD). If your FEV1 increases after taking a bronchodilator (usually albuterol), then your airway blockage is reversible. This is a sign of asthma.3,5

The percentages can also tell you how severe or well-controlled your asthma is. An FEV1 of less than 60 percent often means your asthma is severe or poorly controlled. Based on the results, your doctor can suggest whether you should increase or decrease your asthma drugs.2,6

What are the possible side effects of spirometry?

Spirometry is a safe and non-invasive procedure. However, the test can be tiring and uncomfortable. Ask your doctor for a break between repetitions if you need rest. Some potential side effects of the test include:4

  • Shortness of breath
  • Dizziness

Other things to know about spirometry

Your doctor will tell you how to prepare for the test. They may ask you to:4

  • Wear comfortable and loose clothing
  • Avoid vigorous exercise and large meals before the test
  • Avoid smoking and alcohol before the test
  • Stop taking certain medicines before the test

Before taking a spirometry test, tell your doctor if you have any heart conditions.2,5

You may need to do spirometry tests at least every 1 to 2 years. This can help see how lung function changes over time. Your doctor may also have you do spirometry whenever you start a new treatment or symptoms worsen. Pregnant women with asthma may do a spirometry test at every prenatal visit.2

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