An Overview of Pulmonary Function Tests

Spirometry, Diffusion Tests, and Plethysmography

In This Article

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.

A doctor looking at a chest x-ray on a tablet
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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 exhale
  • Lung diffusion test: Provides information about you how well the oxygen that you inhale moves into your bloodstream
  • Lung 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:

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).

GOLD Spirometric Criteria for COPD Severity
I. Mild COPD FEV1/FVC < 0.7

FEV1 >/= 80% predicted
At this stage, the patient is probably unaware that lung function is starting to decline.
II. Moderate COPD FEV1/FVC < 0.7


Symptoms during this stage progress, with shortness of breath developing upon exertion.
III. Severe COPD FEV1/FVC < 0.7


Shortness of breath becomes worse at this stage and COPD exacerbations are common.
IV. Very Severe COPD FEV1/FVC < 0.7

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

Quality of life at this stage is gravely impaired. COPD exacerbation can be life-threatening.

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.

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.

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

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.

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:

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.

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.

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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  4. Lung Institute. Lung Plethysmography: what it is and why it’s done. Updated March 20, 2017.

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