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COPD Diagnostic Tests: Pulmonary Function, Spirometry, and More

Lung Volumes

Beyond simple spirometry testing, there are other lung volume measurements that can be important to COPD patients. These measurements can be obtained by one of several different kinds of tests.

The three most commonly used methods for measuring lung volumes are:

  • Nitrogen Washout: You breathe pure oxygen, and the air you breathe out is collected and analyzed for nitrogen content.
  • Helium Dilution: You breathe a gas mixture of helium and oxygen.
  • Body Box: This is the most accurate of these types of tests. You sit in an enclosed chamber, made of glass or clear plastic and perform a series of small panting breaths. While very accurate, the equipment requires specially-trained technicians to operate it.

The following two tests provide the most important information concerning your lung volume:

  • Total Lung Capacity (TLC) is the most amount of air your lungs can hold at the top of breathing in.
  • Residual Volume (RV) is the amount of air remaining in your lungs after you breathe out all the air you possibly can. When the RV is up it indicates air trapping or "hyperinflation". This means too much air is left in the lungs.

Diffusing Capacity (DLCO)

A third type of lung function test that your health care provider may order is the "diffusing capacity" test. This test shows how well oxygen moves from your lungs into your blood.

Three major factors that determine diffusing capacity are:

  1. Amount of air-containing lung tissue that comes in contact with blood
  2. Thickness of air sac walls
  3. The amount of red blood cells

Results of this test can tell your doctor about the amount of air sac damage that may be present, such as in emphysema. In individuals with obstructive lung disease, a decreased DLCO is usually a sign that some or all of the blockage is due to emphysema.

Understanding Your Test Results

A doctor will interpret the results of your PFTs by comparing them to normal values. To describe how serious your COPD is, the categories of mild, moderate, severe and very severe are used. (Normal values are taken from research studies of large numbers of people with healthy lungs.)

Your lung function is compared to what is normal for a person of your age, size and sex. For example, your height is an important factor to consider when reviewing your test results. Taller people generally have larger lungs. Your sex can affect your test results as men tend to have larger lungs than women of the same height. Your age is an important factor because as we get older, it is normal for lung function to decrease. Errors in any of these factors (such as an incorrect height) can alter the results of your tests. This can lead to an incorrect understanding of your lung function. Reviewing changes in FEV1 and DLCO over time can tell how well your treatment is doing. It can also track the progression of your lung disease.

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