Lung Plethysmography Test: Uses and Results

A type of pulmonary function test for measuring residual capacity

Plethysmography is a lung test (pulmonary function test) performed to measure the compliance of the lungs by determining how much air your lungs can hold. It may be used to help in the diagnosis of lung disease, to determine its severity, to see whether treatment is working (or if the condition is getting worse), or to evaluate the lungs before lung surgery. It is most often used along with other lung tests to determine the absolute volume of air in the lungs.

Lung plethysmography is also called pulmonary plethysmography or body plethysmography and differs from impedance plethysmography, a test for blood clots in the legs.

woman taking deep breath of air / Stock Photo / AntonioGuillem


Plethysmography may be ordered for several reasons. Some of these include:

  • In diagnosis to distinguish obstructive versus restrictive lung diseases. (In some cases it can be difficult to determine if lung disease is obstructive or restrictive (there is an overlap in some of the pulmonary function tests) and plethysmography can help make this difficult distinction.
  • To assess your response to treatment. For example, the test may be done to see if your lung disease is getting worse, improving, or staying the same.
  • To determine the severity of COPD
  • To evaluate your lungs to see if you would tolerate lung cancer surgery 

How Does Plethysmography Work?

Plethysmography is based on one of the gas laws called Boyle's law. As long as the temperature is constant, the pressure and volume of a gas are inversely proportional. This will make more sense when we discuss the procedure.


When your doctor recommends this test, she will discuss the preparation, what will happen during the procedure, and when you can expect to receive your results.

Before the Test

No special dietary guidelines are usually given prior to the test, but it's wise not to eat heavily, or at least to the point where it could interfere with your breathing. Wearing loose-fitting, comfortable clothes is important, especially avoiding any attire that could limit your breathing such as tightly fitted shirts or a tight belt. You should not exercise heavily or smoke for several hours prior to the test. It's also important to avoid environmental pollutants, such as indoor or outdoor air pollution prior to the test to ensure the results are as accurate as possible.

Perfumes or perfumed shampoos or other personal care products should be avoided before the procedure both for your own health and for anyone who may have the procedure after you who is sensitive to these products or smells.

You will be able to drive yourself to and from the test, but many people appreciate having a companion with them to keep them company.

During the Test

During plethysmography, you will be asked to sit in a small, airtight room that looks a bit like a telephone booth. If you ordinarily use oxygen, you will not need to receive your oxygen during the test.

A technician will place clips on your nose and give you a mouthpiece to breathe through. Some people feel claustrophobic when the procedure begins, but you can open the door or remove the mouthpiece at any time if you need to (though this can lengthen the procedure). The technician will then take you through various breathing patterns, having you breathe normally, then pant for several breaths, then take a deep breath in and blow it all out.

Risks and Side Effects

Most people tolerate the procedure very well, though some people may feel claustrophobic or become lightheaded during the procedure. Overall, the active part of this test takes around 15 minutes.

What the Test Measures

Plethysmography gives your doctor measurements that can help her understand how well your lungs are functioning. Most pulmonary function tests do not measure residual volume or the amount of air left in your lungs after you exhale as much air as you can. By determining this measurement, plethysmography helps your doctor calculate other numbers as well.

Measurements that can be made using this test include:

  • Functional residual volume: Functional residual volume is the amount of air left in your lungs after you exhale as much air as you can.
  • Functional residual capacity (FRC): Functional residual capacity (FRC) is a measure of how much air is left in your lungs after you have exhaled as much as possible (the expiratory reserve volume) plus the amount of air left in your lungs after you breathe out normally (the residual volume).
  • Total lung capacity (TLC): This is a measure of the total amount of air in your chest after you've taken the deepest breath you possibly can.

Plethysmography vs. Spirometry

Spirometry is another lung test that looks at lung volumes but is unable to determine residual volume.

Interpreting Results 

Depending on whether your functional residual capacity is increased, decreased, or normal, lung diseases may be broken down into different categories.

The results can vary based on age, sex, height, and weight.

Increased Functional Residual Capacity

Obstructive lung diseases often cause increased FRC. In order to picture this, you can imagine how, with conditions such as emphysema, after each breath, the full volume is not exhaled. The elastic recoil is damaged so that extra air is left over. The extra air that is left over that cannot be exhaled is added to the normal volume left over after breathing out.

Conditions that may result in an increased FRC include:

  • Emphysema
  • Cystic fibrosis                                    

Decreased Functional Residual Capacity (FRC)

A decreased functional residual capacity means that there is a decreased amount of airspace present in the lungs. This, in turn, can have several causes. The lungs may be "less elastic" either externally or internally, for example, from the weakness of the chest muscles related to a stroke, or a decreased elasticity of the lungs themselves (decreased compliance) due to a chronic lung condition. This pattern may also be seen if you have had part of a lung removed for lung cancer.

Conditions that may result in decreased FRC include:

These numbers may be abnormal if your airways are narrowed or blocked in some way, if too much air is left in your lungs after you exhale (as in emphysema), or if your lungs are unable to expand completely.

A Word From Verywell

When combined with other pulmonary function tests, lung plethysmography can help differentiate lung diseases or determine the response to treatment and much more. While it can be frustrating taking the time to have these tests done and then waiting for results, having an accurate assessment of your condition can help guide you and your doctor to the treatments that will work best for you personally.

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