Pulmonary Function Test

Medically Reviewed by Minesh Khatri, MD on August 14, 2022
6 min read

Pulmonary function tests (PFTs) are lung tests. They show how well your lungs work. They’re noninvasive, which means that the doctor doesn’t cut you or put any tools inside your body.

Testing your lungs can help doctors diagnose lung diseases such as:

You need a diagnosis to get the right treatment.

Your doctor chooses your tests based on what they need to find out. Each test works a different way.


This is one of the most common pulmonary function tests. Spirometry measures how much air you can breathe in and out. It also measures how fast you can empty the air out of your lungs.

Spirometry helps diagnose breathing problems such as asthma and COPD. If you’re taking an asthma medicine, spirometry can help your doctor learn how well the medicine is working.

During the test, you’ll breathe in as much air as you can. Then, you’ll quickly blow out as much air as you can through a tube connected to a machine called a spirometer.

The test measures two things:

  • The most air you can breathe out after inhaling deeply. The results will let you know if you’re less able to breathe normally.
  • How much air you can exhale in 1 second. The score tells your doctor how severe your breathing problem is.

Body plethysmography

You’ll breathe in deeply for body plethysmography, which measures how much air is in your lungs when you inhale. It also checks how much air stays in your lungs after you breathe out as much as you can.

Plethysmography is used for several reasons:

  • Your doctor might order this test to see how a disease such as COPD or asthma affects your lungs. The test may show that your treatment needs to change.
  • It can help confirm whether you have a lung disease that reduces the amount of space in your lungs. Your doctor might call this a restrictive disease.
  • It can show if your airways have narrowed. If so, the test may help your doctor decide whether lung medications called bronchodilators could help you. Bronchodilators open airways.
  • It can help your doctor figure out how well you’d do if you needed surgery.

The test is painless and takes about 15 minutes. During plethysmography, you sit in a clear plastic box. You wear a nose clip and breathe through your mouth through a special mouthpiece.

Lung diffusion capacity

A lung diffusion capacity test measures how well oxygen moves from your lungs into your blood.

This test is similar to spirometry. You breathe into a tube attached to a machine. The test can help diagnose a disease of the blood vessels between your heart and lungs, and it can show the amount of damage done by a disease such as emphysema, a disease in which your air sacs are gradually destroyed.

Bronchial provocation test

If you have asthma, you know that triggers such as exercise, smoke, and dust can suddenly make it hard to breathe. A bronchial provocation test can help diagnose asthma. Your doctor may also use it to measure how severe your asthma is.

During the test, you inhale a medication that makes your airways narrow. Then, you take a spirometry test. You do this several times. Your doctor will use the readings to learn how much your airways narrow during an asthma attack.

Cardiopulmonary exercise stress test

This test measures lung and heart strength. It’s usually given to people who may have heart disease or lung problems. Sometimes, these conditions show up only during exercise.

During the test, you walk on a treadmill or ride a stationary bicycle. The doctor keeps an eye on your heart rate as your heart beats faster and faster. You’ll breathe into a tube that monitors your lungs as they work harder.

Pulse oximetry test

This painless test measures how much oxygen is in your blood. The doctor clips a probe to your finger, your earlobe, or another part of your skin. The device uses light to measure the level of oxygen in your red blood cells.

Arterial blood gas test

This test measures the levels of gases like oxygen and carbon dioxide in blood taken from one of your arteries.

You’ll typically go to a hospital or your doctor’s office for an arterial blood gas test. A nurse or technician will use a needle to take the blood, probably from your wrist. It might hurt, and you could bleed a bit where the needle went in.

Fractional exhaled nitric oxide test

When you have certain kinds of asthma, you might have high levels of a gas called nitric oxide in your body. The fractional exhaled nitric oxide test measures how much of it is in the air you breathe out.

For this test, you breathe out slowly and steadily into a tube that’s connected to a portable device.

Your doctor will explain the test and what you can expect. Ask them any questions you may have. They may also tell you to wear loose clothing and comfortable shoes.

For the best results:

  • Take your medications before your test unless your doctor tells you not to.
  • Don’t smoke before the test; ask your doctor how long before the test you should stop.
  • If you use a short-acting inhaler, try not to use it for 6 to 8 hours before the test. If you have to use it, let your doctor know before the test.
  • Don’t drink alcohol at least 4 hours before the test.
  • Don’t eat a large meal at least 2 hours before the test.
  • Don’t do any hard exercise at least 30 minutes before the test.

Most tests take 15 to 30 minutes. You may be tired afterward. Your medical team will give you time to rest. Then, you can go back to your normal activities.

Some things can affect how accurate your results are. They include:

  • How well you follow the medical team’s directions
  • Medicines that affect your airways
  • Pain medicines
  • Pregnancy
  • Stomach bloating
  • Severe fatigue

“Normal” results are based on your age, height, and sex. A result that’s not normal may mean you have a lung problem. Your doctor will talk with you about what your results mean and the next steps to take.

Pulmonary function tests are usually safe. Risks include:

  • Dizziness
  • Shortness of breath
  • Coughing
  • An asthma attack

Your doctor probably won’t recommend a PFT if you have:

If you’ve noticed that you’re short of breath, ask your doctor about having a lung function test soon.

Lung function tests are important, whether you’ve been diagnosed with a breathing problem or think you may have one. They can be the first step toward breathing easier.