Lung function tests (also called pulmonary function tests, or PFTs) check how well your lungs work. The tests determine how much air your lungs can hold, how quickly you can move air in and out of your lungs, and how well your lungs put oxygen into and remove carbon dioxide from your blood. The tests can diagnose lung diseases, measure the severity of lung problems, and check to see how well treatment for a lung disease is working.

Other tests—such as residual volume, gas diffusion tests, body plethysmography, inhalation challenge tests, and exercise stress tests—may also be done to determine lung function.

Spirometry is the first and most commonly done lung function test. It measures how much and how quickly you can move air out of your lungs. For this test, you breathe into a mouthpiece attached to a recording device (spirometer). The information collected by the spirometer may be printed out on a chart called a spirogram.

The more common lung function values measured with spirometry are:

• Forced vital capacity (FVC). This measures the amount of air you can exhale with force after you inhale as deeply as possible.
• Forced expiratory volume (FEV). This measures the amount of air you can exhale with force in one breath. The amount of air you exhale may be measured at 1 second (FEV1), 2 seconds (FEV2), or 3 seconds (FEV3). FEV1 divided by FVC can also be determined.
• Forced expiratory flow 25% to 75%. This measures the air flow halfway through an exhale.
• Peak expiratory flow (PEF). This measures how quickly you can exhale. It is usually measured at the same time as your forced vital capacity (FVC).
• Maximum voluntary ventilation (MVV). This measures the greatest amount of air you can breathe in and out during one minute.
• Slow vital capacity (SVC). This measures the amount of air you can slowly exhale after you inhale as deeply as possible.
• Total lung capacity (TLC). This measures the amount of air in your lungs after you inhale as deeply as possible.
• Functional residual capacity (FRC). This measures the amount of air in your lungs at the end of a normal exhaled breath.
• Residual volume (RV). This measures the amount of air in your lungs after you have exhaled completely. It can be done by breathing in helium or nitrogen gas and seeing how much is exhaled.
• Expiratory reserve volume (ERV). This measures the difference between the amount of air in your lungs after a normal exhale (FRC) and the amount after you exhale with force (RV).

Gas diffusion tests

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

• Arterial blood gases, which determine the amount of oxygen and carbon dioxide in your bloodstream.
• Carbon monoxide diffusion capacity (also called DLCO), which measures how well your lungs transfer a small amount of carbon monoxide (CO) into the blood. Two different methods are used for this test. If the single-breath or breath-holding method is used, you will take a breath of air containing a very small amount of carbon monoxide from a container while measurements are taken. In the steady-state method, you will breathe air containing a very small amount of carbon monoxide from a container. The amount of carbon monoxide in the breath you exhale is then measured. Diffusing capacity provides an estimate of how well a gas is able to move from your lungs into your blood.

Body plethysmography

Body plethysmography may be used to measure:

• Total lung capacity (TLC), which is the total amount of air your lungs can hold. For this test, you sit inside a small airtight room called a plethysmograph booth and breathe through a mouthpiece while pressure and air flow measurements are collected.
• Residual volume (RV), which is the amount of air that remains in your lungs after you exhale as completely as possible. For this test, you sit inside the plethysmograph 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 the response of your airways to substances that may be causing asthma or wheezing. These tests are also called provocation studies.

During inhalation testing, increasing amounts of a substance are inhaled through a nebulizer, a device that uses a face mask or mouthpiece to deliver the substance in a fine mist (aerosol). Sometimes, increasing amounts of methacholine or mannitol may be inhaled through the nebulizer. Spirometry readings are taken to evaluate lung function before, during, and after inhaling the substance.

In rare cases, a bronchospasm can occur with inhalation challenge testing. You will be closely monitored during and after the test.

Exercise stress tests

Exercise stress tests evaluate the effect of exercise on lung function tests. Spirometry readings are done after exercise and then again at rest.

Multiple-breath washout test

The multiple-breath washout test is done to check lung function in people with cystic fibrosis. For this test, you breathe air that contains a tracer gas through a tube. 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.

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).