Lung Function Tests

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Test Overview

Lung function tests check 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 determine all of the lung function values, so more than one type of test may be done. Some of the tests may be repeated after you inhale medicine that enlarges your airways (bronchodilator).

Spirometry

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.

Spirometry 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 evaluate 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 are not 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

Tell your doctor if you:

  • Have had recent chest pains or a heart attack.
  • Take medicine for a lung problem such as asthma. You may need to stop taking some medicines before testing.
  • Are allergic to any medicines.
  • Have had recent surgery on your eyes, chest, or belly, or if you have had a collapsed lung.

Do not eat a heavy meal just before this test. A full stomach may keep your lungs from fully expanding. You should not 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. The test is usually done by a specially trained respiratory therapist or technician.

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 possible and then to exhale as fast and as hard as possible. You also may be asked to breathe in and out as deeply and rapidly 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 similar to a telephone 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.

The testing may take from 5 to 30 minutes. It depends on how many tests are done.

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.

Risks

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

Lung function tests check 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.

Most test results are available right away.

Normal

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

Abnormal

Test results are outside of the normal range for a person with healthy lungs. This may be a sign of some kind of lung disease. There are two main types of lung disease that can be found with lung function tests: obstructive and restrictive.

Obstructive lung conditions

Obstructive lung conditions cause the airways to get narrower.

Examples include emphysema, bronchitis, asthma, and infection that produces inflammation.

Lung function values in obstructive conditions
Lung function test What the test measures Result as predicted for age, height, sex, weight, or race

Forced vital capacity (FVC)

How much air you can exhale with force after you inhale as deeply as possible.

Normal or lower than predicted value

Forced expiratory volume (FEV1)

How much air you can exhale with force in one breath.

Lower

FEV1 divided by FVC

See the first two tests above.

Lower

Forced expiratory flow 25% to 75%

How much air you can breathe out halfway through an exhale.

Lower

Peak expiratory flow (PEF)

How much air you can exhale when you try your hardest.

Lower

Maximum voluntary ventilation (MVV)

The greatest amount of air you can breathe in and out during 1 minute.

Lower

Slow vital capacity (SVC)

How much air you can slowly exhale after you inhale as deeply as possible.

Normal or lower

Total lung capacity (TLC) (VT)

The amount of air in your lungs after you inhale as deeply as possible.

Normal or higher

Functional residual capacity (FRC)

The amount of air in your lungs at the end of a normal exhaled breath.

Higher

Residual volume (RV)

The amount of air in your lungs after you have exhaled completely.

Higher

Expiratory reserve volume (ERV)

The difference between the amount of air in your lungs after a normal exhale (FRC) and the amount after you exhale with force (RV).

Normal or lower

RV divided by TLC ratio

See the test above.

Higher

Restrictive lung conditions

Restrictive lung conditions cause a loss of lung tissue, a decrease in the lungs' ability to expand, or a decrease in how well the lungs can transfer oxygen or carbon dioxide in or out of the blood.

Examples include scleroderma, pulmonary fibrosis, and sarcoidosis. Other restrictive conditions include some chest injuries, being very overweight (obesity), pregnancy, and loss of lung tissue due to surgery.

Lung function values in restrictive conditions
Lung function test What the test measures Result as predicted for age, height, sex, weight, or race

Forced vital capacity (FVC)

How much air you can exhale with force after you inhale as deeply as possible.

Lower than predicted value

Forced expiratory volume (FEV1)

How much air you can exhale with force in one breath.

Normal or lower

FEV1 divided by FVC

See the first two tests above.

Normal or higher

Forced expiratory flow 25% to 75%

How much air you can breathe out halfway through an exhale.

Normal or lower

Peak expiratory flow (PEF)

How much air you can exhale when you try your hardest.

Normal or lower

Maximum voluntary ventilation (MVV)

The greatest amount of air you can breathe in and out during 1 minute.

Normal or lower

Slow vital capacity (SVC)

How much air you can slowly exhale after you inhale as deeply as possible.

Lower

Total lung capacity (TLC) (VT)

The amount of air in your lungs after you inhale as deeply as possible.

Lower

Functional residual capacity (FRC)

The amount of air in your lungs at the end of a normal exhaled breath.

Normal or lower

Residual volume (RV)

The amount of air in your lungs after you have exhaled completely.

Normal, lower, or higher

Expiratory reserve volume (ERV)

The difference between the amount of air in your lungs after a normal exhale (FRC) and the amount after you exhale with force (RV).

Normal or lower

RV divided by TLC ratio

See the test above.

Normal or higher

What Affects the Test

You may not be able to have the test, or the results may not be helpful, if:

  • You use medicine that expands the lungs' airways (such as albuterol) within 4 hours of the test.
  • You use sedatives before the test.
  • You eat or drink something that contains caffeine before the test.
  • You are not able to breathe normally because of pain.
  • You are pregnant, are very overweight, or have an enlarged stomach (after a large meal, for example).
  • You are not able to follow instructions or make an effort during the tests.

What To Think About

References

Other Works Consulted

  • Chernecky CC, Berger BJ (2013). Laboratory Tests and Diagnostic Procedures, 6th ed. St. Louis: Saunders.
  • Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
  • Gustafsson PM, et al. (2008). Multiple-breath inert gas washout and spirometry versus structural lung disease in cystic fibrosis. Thorax, 63(2): 129–134.
  • Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby.

Credits

ByHealthwise Staff
Primary Medical ReviewerE. Gregory Thompson, MD - Internal Medicine
Specialist Medical ReviewerMark A. Rasmus, MD - Pulmonology, Critical Care Medicine, Sleep Medicine

Current as ofFebruary 4, 2015