How pulmonary function tests work

By Jon Johnson 

Reviewed by Deborah Weatherspoon, PhD, RN, CRNA

What they test for Spirometry Lung volume tests Pulse oximetry test Lung diffusion capacity Exercise test Results Preparation Risks Takeaway

Doctors carry out pulmonary function tests to check how well a person's lungs work and identify any issues. The tests are noninvasive and provide measurements, such as lung capacity, volume, and gas exchange.

These measurements may indicate how well the person is breathing or how well the lungs can bring oxygen to the rest of the body.

Every individual may require a different type of test, and doctors may order one or more pulmonary function tests (PFT), depending on the underlying issue.

What conditions do pulmonary function tests identify?

Man breathing into peak flow meter to test lung capacity as part of pulmonary function tests.

Pulmonary function tests can help a doctor diagnose respiratory conditions, such as asthma or COPD.

There are many reasons for pulmonary function tests.

Doctors may order PFTs to help investigate or diagnose possible health conditions including:

asthma

allergies

chronic bronchitis

emphysema or COPD

lung cancer or pulmonary tumors

lung fibrosis, where scar tissue appears in the lung tissue

respiratory infections

sarcoidosis, which occurs when inflammatory cells begin growing in the lungs

scleroderma, which causes the connective tissues in the lungs to harden and become tight

Doctors may also order PFTs for people who work in hazardous environments or breathe in very fine particles in their workplace. This may include:

sawdust

coal

asbestos

graphite

paint

Doctors may also use PFTs to monitor treatment or test the effectiveness of treatment for a chronic condition, such as asthma, bronchitis, or COPD.

Doctors may also order PFTs to check a person's lung function before they have major surgery. This may be more important in people who are at increased risk, such as those with heart or lung problems and people who smoke.

Most PFTs are simple and quick. The time and processes involved will vary depending on the type of test.

Spirometry

Spirometry is a test that measures the amount of air a person breathes in and out.

Doctors may use this test when they want to know the person's lung size and the rate of airflow as they breathe.

During a spirometry test, the person breathes through a tube attached to a computer. To ensure accurate results, a doctor will fit a mouthpiece and a nose clip on the person to prevent air from leaking.

The person then breathes through their mouth as usual. After a few breaths, the doctor will ask the person to breathe in deeply and then expel the air as fast as possible.

Lung volume tests

Lung volume tests are the best way to tell how much air a person's lungs can hold.

The process is similar to spirometry, where the person first breathes normally and then vigorously. The person will sit in a small, sealed booth with clear walls for the test.

Doctors can very accurately measure the amount of air the lungs displace by measuring the pressure in the booth.

Pulse oximetry test

A pulse oximetry test on a person's finger measuring blood oxygen levels

A pulse oximetry test involves clamping a device onto the body to measure blood oxygen levels.

A pulse oximetry test measures the levels of oxygen in the blood, which can indicate how well the lungs function. There is no breathing involved.

Doctors clamp a small device to an area where the skin is thinner, such as an earlobe or finger. Tiny beams of light then measure how much oxygen is in the blood.

This test can help diagnose conditions, as well as determine how well certain treatments are working.

Lung diffusion capacity

The purpose of a lung diffusion capacity test is to see how well oxygen from the air a person breathes gets into the blood.

For this test, a person sits and breathes normally through a tube. There is no need for vigorous breathing during this test. Doctors may also have the person breathe in various gases and monitor how well the body uses or gets rid of these gases.

Doctors may also take a blood sample to check the levels of hemoglobin in the blood.

Exercise test

When doctors are trying to find the cause of specific symptoms, such as shortness of breath, they might want to know how the lungs respond to exercise.

The test involves breathing into a machine while walking on a treadmill or using a standing bike. The test measures how a person responds to exercise at their own pace.

What the results mean

The average values change for each person.

Doctors will take the results of the test and compare them to typical averages of people of a similar height, age, and sex.

The American Thoracic Society state that a person's lungs grow until they reach their mid-20s when the lung function slowly starts to decline. Height and sex are the other factors. Taller people tend to have larger lungs and males tend to have larger lungs than females.

Values that are abnormal in comparison to these other measurements may be a sign of an issue in the lungs. Doctors may order one or more tests to help make a diagnosis depending on the results.

Individual results vary, and doctors will explain the results of each test with each person.

The American Lung Association note that it is possible for a person with healthy lungs to have abnormal results. This is why doctors may run additional tests to confirm their findings before moving forward with a diagnosis.

How to prepare for a PFT

Man on treadmill with device over mouth to measure lung capacity and breathing during exercise test with woman on computer in background.

A doctor can provide specific advice on how to prepare for PFT tests.

Preparation for a PFT may differ between individuals, but there are some general guidelines.

Doctors frequently ask the person to stop taking certain medications before the test to get an accurate reading. The doctor will give each person specific instructions.

Doctors may also ask the person to avoid certain foods or drinks that may make the results of a PFT inaccurate.

Strenuous exercise may also open the airways and lead to an inaccurate reading. Avoid vigorous activity for at least a couple of hours before the test.

Avoid eating or drinking too much before the test. A full stomach may leave less room for the lungs to expand, which could affect the results of a lung capacity test.

Doctors may also ask the person to refrain from smoking or drinking alcohol before the test.

People should avoid wearing tight clothing that may prevent the lungs from inhaling fully.

Anyone who is taking an exercise test should wear loose fitting, comfortable clothing to take the test.

Risks of the tests

While the tests are generally safe and noninvasive, there are some risks.

Many tests require the person to breathe in and out vigorously. This may make some people feel dizzy, and there is a risk that they may faint. Anyone feeling dizzy or lightheaded during the test should tell the person administering the test.

The tests may trigger asthma attacks in some people, especially if they do not take their medication in preparation for the test.

There is minimal risk of germs spreading from people using the same lung function equipment. Technicians change the mouthpieces and other equipment after each use, and the machines have filters to help kill germs.

The National Heart, Lung, and Blood Institute note that in tests that require doctors to take blood, some people may experience bleeding or infection in the area.

Takeaway

PFTs are noninvasive tools that doctors use to check how well the lungs perform their functions. They are generally safe and are highly useful for diagnosing and monitoring certain conditions in the lungs and airways.

People with specific conditions may have difficulty carrying out some of these tests. Always communicate with a doctor, ask questions, voice any concerns, and work together to find a solution.

SOURCE: MEDICAL NEWS TODAY