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Lung Function Tests

Test Overview

Lung function tests check to see how well your lungs work. The tests can find lung problems, measure how serious they are, and check to see how well treatment for a lung disease is working.

The tests look at:

  • How much air your lungs can hold.
  • How quickly you can move air in and out of your lungs.
  • How well your lungs put oxygen into and remove carbon dioxide from your blood.

Types of lung function tests include:

  • Spirometry.
  • Gas diffusion.
  • Body plethysmography.
  • Inhalation challenge test.
  • Exercise stress test.

You may also hear the tests called pulmonary function tests, or PFTs.

Lung function results are measured directly in some tests and are calculated in others.

No single test can check for all of the lung function values, so more than one type of test may be done. Some tests may be repeated after you inhale medicine that enlarges your airways (bronchodilator).

There are several different types of lung function tests.


Spirometry is the most common lung function test. It measures how much and how quickly you can move air out of your lungs. You breathe into a mouthpiece attached to a machine called a spirometer. The machine records your results.

This test can measure many different things about the way you breathe. These include how much air you can exhale, how much air you can breathe in and out in 1 minute, and the amount of air left in your lungs after a normal exhale.

Gas diffusion tests

Gas diffusion tests measure the amount of oxygen and other gases that move through the lungs' air sacs (alveoli) per minute. These tests let you know how well gases are being absorbed into your blood from your lungs. Gas diffusion tests include:

  • Arterial blood gases. This test shows the amount of oxygen and carbon dioxide in your bloodstream.
  • Carbon monoxide diffusion capacity (also called DLCO). This test measures how well your lungs transfer a small amount of carbon monoxide (CO) into the blood. Two different methods are used for this test.
    • Single-breath or breath-holding method: You take a breath of air from a container. The air contains a very small amount of carbon monoxide. Measurements are taken as you breathe in.
    • Steady-state method: You do the same thing, but measurements are taken as you breathe out.

Body plethysmography

Body plethysmography may be used to measure:

  • Total lung capacity (TLC). This is the total amount of air your lungs can hold. For this test, you sit inside a small airtight room. You breathe through a mouthpiece while pressure and air flow measurements are collected.
  • Residual volume (RV). This is the amount of air that remains in your lungs after you exhale as much as you can. For this test, you sit inside the booth and breathe while the pressure of the booth is monitored. You may need to breathe through a mouthpiece while you are in the booth.

Inhalation challenge tests

Inhalation challenge tests are done to measure how your airways respond to substances that may be causing asthma or wheezing. These tests are also called provocation studies.

During the test, you inhale increasing amounts of a substance through a nebulizer. This is a device that uses a face mask or a mouthpiece to deliver the substance in a fine mist (aerosol). Spirometry readings are taken to look at lung function before, during, and after you inhale the substance.

Exercise stress tests

Exercise stress tests look at how exercise affects your lungs. Spirometry readings are done after exercise and then again at rest.

Multiple-breath washout test

The multiple-breath washout test is done to check people who have cystic fibrosis. For this test, you breathe through a tube. First you breathe air that contains a tracer gas. Then you breathe regular air while the amount of tracer gas you exhale is monitored. Test results are reported as a lung clearance index (LCI). A high LCI value means that the lungs aren't working well.

Why It Is Done

Lung function tests are done to:

  • Find the cause of breathing problems.
  • Find certain lung diseases, such as asthma or chronic obstructive pulmonary disease (COPD).
  • Check a person's lung function before surgery.
  • Check the lungs of someone who is regularly exposed to chemicals or other things that can damage the lungs.
  • Check how well treatments for lung diseases are working.

How To Prepare

  • Let your doctor know if you take medicines for a lung problem. You may need to stop some of them before the tests.
  • Do not eat a heavy meal just before this test. A full stomach may keep your lungs from fully expanding.
  • Don't smoke or do intense exercise for 6 hours before the test.
  • For the test, wear loose clothing that doesn't restrict your breathing in any way.
  • Avoid food or drinks with caffeine. Caffeine can cause your airways to relax and allow more air than usual to pass through.
  • If you have dentures, wear them during the test. They help you form a tight seal around the mouthpiece of the machine.

How It Is Done

Lung function tests are usually done in special rooms that have all of the right equipment.

For most of the tests, you'll wear a nose clip to keep air from leaking through your nose. Then you'll breathe into a mouthpiece connected to a recording device.

The exact steps depend on which test you have. For example, you may be asked to inhale as deeply as you can and then to exhale as fast and as hard as you can. You also may be asked to breathe in and out as deeply and as fast as you can for 15 seconds.

Some tests may be repeated after you have inhaled a spray containing medicine that expands the airways in your lungs. You may be asked to breathe a special mixture of gases, such as 100% oxygen, a mixture of helium and air, or a mixture of carbon monoxide and air.

Sometimes a sample of blood may be taken from an artery in your wrist to measure blood gases.

If you have body plethysmography, you will be asked to sit inside a small enclosure. It's like a phone booth, with windows that allow you to see out. The booth measures small changes in pressure that occur as you breathe.

The accuracy of the tests depends on how well you can follow all of the instructions. The therapist may ask you to breathe deeply during some of the tests to get the best results.

How long the test takes

The testing may take from 5 to 30 minutes, depending on how many tests you have.

How It Feels

If you have an arterial blood gas test, you may feel some pain from the needle used to collect the blood. The other lung function tests are usually painless. Some of the tests may be tiring for people who have a lung disease.

You may cough or feel lightheaded after breathing in or out rapidly, but you will be given a chance to rest between tests. It may not be comfortable to wear the nose clip or to breathe through the mouthpiece.

If you have body plethysmography, you may feel uncomfortable in the airtight booth. But the therapist will be nearby to open the door if you feel too uncomfortable.

If you are given breathing medicine, it may cause you to shake or may increase your heart rate. If you feel any chest pain or discomfort, tell the therapist right away.


For a healthy person, there's little or no risk in taking these tests. If you have a serious heart or lung condition, discuss your risks with your doctor.



Results are in the normal range for a person with healthy lungs.

The normal range is just a guide. Some test values vary by things like age and height. Your doctor will also look at your results based on your symptoms, health, and other factors. A value that isn't in the normal range may still be normal for you.


Test results are outside of the normal range for a person with healthy lungs. This may be a sign of lung disease. Many health conditions can change how well your lungs are working. And exposure to certain things, such as smoke or chemical fumes, can also affect how well your lungs are working. Your doctor will talk with you about any abnormal results that may be related to your symptoms and past health.


Current as of: August 6, 2023

Author: Healthwise Staff
Clinical Review Board
All Healthwise education is reviewed by a team that includes physicians, nurses, advanced practitioners, registered dieticians, and other healthcare professionals.