Pulmonary Function Tests: Spirometry, Diffusion Tests, and Plethysmography

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Pulmonary function tests (PFTs) are used to assess various aspects of your lung function. These tests are used in COPD diagnosis, to determine how your treatment is working, and to follow the progression of your pulmonary condition. You may also need to have PFTs done before or after surgery.

There are several types of PFTs, all of which require that you breath according to certain instructions while measurements are taken. These tests involve your cooperation but they do not typically require substantially more effort than the effort you make when you regularly inhale (breath in) and exhale (breath out)—and they are completely safe.

Types of PFTs

The different types of PFTs can help your medical team evaluate a number of features of your respiratory function. They are all non-invasive tests that do not require needles, injections, or placement of anything in your airways.

Three types of PFTs are used in the diagnosis and evaluation of COPD and other lung diseases:

  • Spirometry: Measures the amount of air volume that you breathe in and out
  • 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. One number does not typically provide enough information to reach a diagnosis of a specific pulmonary condition.

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. This way your medical team can assess whether your spirometry test readings improve with treatment—improvement could predict that your day-to-day symptoms might be alleviated with treatment.

Spirometry Testing

Spirometry is the most common PFT. A spirometer is a hand-held device that you can easily use with the assistance of a technician.

When you have a spirometry test, you will exhale into a mouthpiece, which is attached to a short tube that leads to a lightweight box-like device. The spirometer immediately reads the volume of air that you exhale with each breath. You may be asked to breathe as you normally do, or to inhale or exhale deeply or 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 and 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.
  • FEV1/FVC Ratio: This value is a calculation of 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

* 50%

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

* 30%

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 would 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 of carbon monoxide.

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 DLCO (diffusion capacity of the lungs for carbon monoxide).

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 80% of the normal value suggests that you could have trouble absorbing oxygen. And a value below 60% of the normal value would 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.

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.

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