Spirometry is a standard test doctors use to measure how well your lungs are functioning. The test works by measuring airflow into and out of your lungs.

To take a spirometry test, you sit and breathe into a small machine called a spirometer. This medical device records the amount of air you breathe in and out and the speed of your breath.

Spirometry tests are used to diagnose these conditions:

  • COPD
  • asthma
  • restrictive lung disease (such as interstitial pulmonary fibrosis)
  • other disorders affecting lung function

They also allow your doctor to monitor chronic lung conditions to check that your current treatment is improving your breathing.

Spirometry is often done as part of a group of tests known as pulmonary function tests.

You shouldn’t smoke one hour before a spirometry test. You’ll also need to avoid alcohol that day as well. Eating too large of a meal could also impact your ability to breathe.

Don’t wear clothing that’s so tight that it could restrict your breathing. Your doctor may also have instructions about whether you should avoid using inhaled breathing medications or other medications prior to your test.

A spirometry test usually takes about 15 minutes and generally happens in your doctor’s office. Here’s what happens during a spirometry procedure:

  1. You’ll be seated in a chair in an exam room at your doctor’s office. Your doctor or a nurse places a clip on your nose to keep both nostrils closed. They also place a cup-like breathing mask around your mouth.
  2. Your doctor or nurse next instructs you to take a deep breath in, hold your breath for a few seconds, and then exhale as hard as you can into the breathing mask.
  3. You’ll repeat this test at least three times to make sure that your results are consistent. Your doctor or nurse may have you repeat the test more times if there is a lot of variation between your test results. They’ll take the highest value from three close test readings and use it as your final result.

If you have evidence of a breathing disorder, your doctor might then give you an inhaled medication known as a bronchodilator to open up your lungs after the first round of tests. They’ll then ask you to wait 15 minutes before doing another set of measurements. Afterward, your doctor will compare the results of the two measurements to see whether the bronchodilator helped increase your airflow.

When used to monitor breathing disorders, a spirometry test is typically done once a year to once every two years to monitor changes in breathing in people with well-controlled COPD or asthma. Those with more severe breathing problems or breathing problems that aren’t well controlled are advised to have more frequent spirometry tests.

Few complications can occur during or after a spirometry test. You may feel a bit dizzy or have some shortness of breath immediately after performing the test. In very rare cases, the test may trigger severe breathing problems.

The test requires some exertion, so it isn’t recommended if you recently had a heart condition or have other heart problems.

Normal results for a spirometry test vary from person to person. They’re based on your age, height, race, and gender. Your doctor calculates the predicted normal value for you before you do the test. Once you’ve done the test, they look at your test score and compare that value to the predicted value. Your result is considered normal if your score is 80 percent or more of the predicted value.

You can get a general idea of your predicted normal value with a spirometry calculator. The Centers for Disease Control and Prevention provides a calculator that lets you enter your specific details. If you know your spirometry results already, you can enter those as well, and the calculator will tell you what percent of the predicted values your results are.

Spirometry measures two key factors: expiratory forced vital capacity (FVC) and forced expiratory volume in one second (FEV1). Your doctor also looks at these as a combined number known as the FEV1/FVC ratio. If you have obstructed airways, the amount of air you’re able to quickly blow out of your lungs will be reduced. This translates to a lower FEV1 and FEV1/FVC ratio.

FVC measurement

One of the primary spirometry measurements is FVC, which is the greatest total amount of air you can forcefully breathe out after breathing in as deeply as possible. If your FVC is lower than normal, something is restricting your breathing.

Normal or abnormal results are evaluated differently between adults and children:

For children ages 5 to 18:

Percentage of predicted FVC valueResult
80% or greaternormal
less than 80%abnormal

For adults:

FVCResult
is greater than or equal to the lower limit of normalnormal
is less than the lower limit of normalabnormal

An abnormal FVC could be due to restrictive or obstructive lung disease, and other types of spirometry measurements are required to determine which type of lung disease is present. An obstructive or restrictive lung disease could be present by itself, but it’s possible to have a mixture of these two types at the same time.

FEV1 measurement

The second key spirometry measurement is forced expiratory volume (FEV1). This is the amount of air you can force out of your lungs in one second. It can help your doctor evaluate the severity of your breathing problems. A lower-than-normal FEV1 reading shows you might have a significant breathing obstruction.

Your doctor will use your FEV1 measurement to grade how severe any abnormalities are. The following chart outlines what’s considered normal and abnormal when it comes to your FEV1 spirometry test results, according to guidelines from the American Thoracic Society:

Percentage of predicted FEV1 valueResult
80% or greaternormal
70%–79%mildly abnormal
60%–69%moderately abnormal
50%–59%moderate to severely abnormal
35%–49%severely abnormal
Less than 35%very severely abnormal

FEV1/FVC ratio

Doctors often analyze the FVC and FEV1 separately, and then calculate your FEV1/FVC ratio. The FEV1/FVC ratio is a number that represents the percentage of your lung capacity you’re able to exhale in one second. The higher the percentage derived from your FEV1/FVC ratio, in the absence of restrictive lung disease that causes a normal or elevated FEV1/FVC ratio, the healthier your lungs are. A low ratio suggests that something is blocking your airways:

AgeLow FEV1/FVC ratio
5 to 18 yearsless than 85%
adultsless than 70%

Spirometry produces a graph that shows your flow of air over time. If your lungs are healthy, your FVC and FEV1 scores are plotted on a graph that could look something like this:

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If your lungs were obstructed in some way, your graph might instead look like this:

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If your doctor finds that your results are abnormal, they’ll likely perform other tests to determine whether or not your impaired breathing is caused by a breathing disorder. These could include chest and sinus X-rays or blood tests.

The primary lung conditions that will cause abnormal spirometry results include obstructive diseases such as asthma and COPD and restrictive diseases such as interstitial pulmonary fibrosis. Your doctor may also screen for conditions that commonly occur together with breathing disorders that can make your symptoms worse. These include heartburn, hay fever, and sinusitis.

Read more: How spirometry results track COPD progression »