Pulmonary function tests (PFTs) are used to assess various aspects of your lung function. These noninvasive tests—spirometry, the lung diffusion test, and lung plethysmography—are used to help diagnose lung diseases like COPD, as well as to determine how treatment is working and if such a condition is progressing. You may also need to have PFTs done before or after surgery.

The different types of PFTs can help your medical team evaluate a number of features of your respiratory function:

  • Spirometry: Measures the amount of air volume that you exhaleLung diffusion test: Provides information about you how well the oxygen that you inhale moves into your bloodstreamLung plethysmography: Determines how much air is present in your lungs when you exhale

Your PFT results may be interpreted together, as one does not typically provide enough information to reach a diagnosis or evaluation of a specific pulmonary condition.

Spirometry Testing

Spirometry is the most common PFT. You exhale into a mouthpiece, which is attached to a short tube that leads to a lightweight box-like device. This, the spirometer, immediately reads the volume of air that you exhale with each breath.

You may be asked to breathe as you normally do, to inhale or exhale deeply, or to breathe at a fast or slow pace.

A spirometer can measure a number of different values, including:

  • Vital capacity (VC) and forced vital capacity (FVC): VC is the volume of air that you can exhale after a full inhalation. FCV is the volume of air that you can forcibly exhale after taking the deepest breath possible. These numbers are usually similar.
  • Forced expiratory volume in one second (FEV1): This is the volume of air that you can forcibly exhale in the first second of a forced exhalation.

Using this information, a calculation called the FEV1/FVC ratio may then be done to determine the total amount of air you can exhale from your lungs during the first second of forced exhalation.

Although there are a number of systems to choose from when it comes to interpreting the readings from your spirometry test, the table below is the method recommended by the Global Initiative for Obstructive Lung Disease (GOLD).

Lung Diffusion Test

A lung diffusion test is an evaluation of the function of your alveoli (tiny air sacs) and the capillaries (ting blood vessels) that surround them. Oxygen and carbon dioxide normally diffuse (flow) through your alveoli and alveolar capillaries.

50%

30%

FEV1 < 30% predicted or FEV1 < 50% predicted with chronic respiratory failure

During a lung diffusion test, you inhale carbon monoxide that is bound to a tracer molecule (such as helium). While carbon monoxide is a dangerous gas, this test is completely safe because it only involves a small amount.

As you inhale the gas through a mouthpiece, you will be instructed to hold your breath for a few seconds and then exhale into the mouthpiece. Your exhaled carbon monoxide concentration will be compared to the inhaled concentration of carbon monoxide and the result is used to calculate your diffusion capacity of the lungs for carbon monoxide (DLCO).

If the concentration of exhaled carbon monoxide is higher than the normal predicted value, this suggests that your lungs do not efficiently absorb oxygen (your DLCO would be low). A DLCO below 55% of the normal value suggests that you could have trouble absorbing oxygen, or could indicate severe lung disease.

Lung Plethysmography

A lung plethysmography test is used to measure how much air your lungs can hold. Unlike spirometry, which measures how much air you can exhale, this test measures the air inside your lungs.

A low DLCO can occur in pulmonary conditions like COPD, in which your airways and/or air sacs become thickened, making it hard for you to absorb the oxygen that you breathe.

Lung plethysmography is especially helpful in helping your medical team distinguish between obstructive and restrictive lung diseases. Restrictive lung diseases prevent you from inhaling adequately, while obstructive lung diseases prevent you from exhaling adequately.

Values that are measured with lung plethysmography include:

  • Functional residual capacity (FRC): FRC is a measure of how much air is left in your lungs after you exhale normally.
  • Expiratory reserve volume: ERV is the additional volume of air you can forcibly expire after taking a normal expiration.
  • Total lung capacity (TLC): TLC is a measure of the total volume of air in your lungs after you’ve taken the deepest breath you possibly can.

What to Expect

Each of these pulmonary function tests requires that you breathe according to certain instructions while measurements are taken. The tests involve your cooperation, but they do not typically require substantially more effort than the effort you make when you regularly breathe in and out. Furthermore, they pose no safety concerns.

The volume of air that is left in your lungs after you fully exhale can be higher than expected with obstructive lung diseases and less than expected when you have restrictive lung disease.

These tests may be measured twice in one clinic visit. For example, you might have your spirometry test before and after you use a bronchodilator. If spirometry readings improve after treatment, this indicates to your medical team that using such a drug may help alleviate day-to-day symptoms as well.

A Word From Verywell

The various available PFTs are useful for assessing different aspects of your lung function. You may need to have more than one type of PFT, and you may also need some tests repeated as your medical team monitors your progress over time. Work closely with your healthcare team to figure out which type of testing will best help diagnose, treat, and follow the progression of your COPD.

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By Deborah Leader, RN

 Deborah Leader RN, PHN, is a registered nurse and medical writer who focuses on COPD.