Pulmonary function tests (PFTs), also called lung function tests, are some of the most important, but least understood, tools healthcare providers use to evaluate lung health. Though many people have heard the term, they may not understand how the tests work or how the results may affect their everyday lives. By completing each test, your provider can get a more complete picture of your respiratory system and whether or not it is working properly.
Meredith McCormack, M.D., MHS, an associate professor of medicine in the Division of Pulmonary and Critical Care at the Johns Hopkins University School of Medicine, has clinical expertise in pulmonary physiology and pulmonary function testing. As she has regularly used these tests to diagnose and manage respiratory conditions, she was uniquely qualified to answer all our questions about pulmonary function testing.
What Are Pulmonary Function Tests?
Pulmonary function tests are a group of noninvasive breathing tests that measure how much air your lungs can hold, how quickly you can move air in and out and how efficiently your lungs transfer oxygen into your bloodstream.
There are several types of PFTs, but the most widely used and accessible test is spirometry. This is often the first step when someone has a respiratory complaint or known risk factors for lung disease.
What Is Spirometry?
Spirometry measures how forcefully and completely you can exhale after taking your deepest breath in. “We have you take your maximal breath in and then blow it out as fast and as hard as you can, like you're blowing out birthday candles,” Dr. McCormack explained.
Two key measurements that spirometry uses are:
- FEV₁ (Forced Expiratory Volume in one second): how much air you can exhale in the first second
- FVC (Forced Vital Capacity): the total amount of air you can exhale during the entire breath
Together, these numbers provide valuable insight into whether airflow is blocked or restricted and whether a lung disease may be present. “It’s a really great place to start and a good piece of information to measure over time.”
Spirometry is recommended for anyone with persistent respiratory symptoms such as ongoing cough, shortness of breath or wheezing—especially when symptoms aren’t clearly tied to a temporary illness like a cold. It’s also commonly used for people with risk factors like a history of smoking or occupational chemical exposures.
What Is a Lung Diffusion Test (DLCO)?
In many cases, spirometry is combined with additional tests to give a more complete picture of lung health. A full pulmonary function test usually includes three components:
- Spirometry – measures airflow
- Lung volume tests – measures the volume of air in the lungs upon the maximum effort of inspiration.
- Diffusing capacity (DLCO) – measures how well gases move from the lungs into the bloodstream
The DLCO test, also called diffusing capacity of carbon monoxide, reflects how well oxygen moves from the lungs air sacs (alveoli) into the bloodstream. “That’s really the primary purpose of the lungs, to breathe in oxygen and exhale carbon monoxide,” Dr. McCormack explained.
The DLCO test can be helpful in distinguishing between different lung conditions. “For example, people with asthma often have a normal DLCO, while those with COPD may have a reduced one,” Dr. McCormack continued. So, if test results don’t match a person’s diagnosis, clinicians may look further for additional or overlapping conditions.
Lung volume measurements can also be revealing, especially when diagnosing or monitoring chronic lung conditions. For instance, patients with lung diseases like pulmonary fibrosis tend to have a reduced lung volume. Alternatively, patients with asthma or COPD may have increased lung volume.
What Is the Test Experience Like?
Pulmonary function tests are noninvasive and have little to no risk. They can all be done by breathing into a mouthpiece connected to specialized equipment. There are no needles, incisions or sedation involved. “The biggest risk is someone feeling a little lightheaded from breathing hard,” Dr. McCormack notes, “but overall, they’re very safe.”
A full PFT appointment typically lasts 45 minutes to an hour, mainly to allow time for coaching and ensuring good technique. For people familiar with spirometry, the test itself can take less than 10 minutes. “We coach patients on timing, sealing their lips tightly around the mouthpiece and exhaling forcefully so that we can get accurate information,” Dr. McCormack said.
Who Needs a Pulmonary Function Test?
Pulmonary function testing is often recommended when someone experiences:
- Shortness of breath during activity or at rest
- Feeling more winded than usual with everyday tasks
- A cough that persists beyond a typical illness
- Wheezing without a clear explanation
“These are symptoms that really should be followed up,” Dr. McCormack says, “especially if they’re new, worsening or unexplained.”
But Dr. McCormack stresses that the most powerful aspect of pulmonary function testing is their ability to track lung health over a lifetime. For people with chronic lung conditions like asthma or COPD, spirometry is often repeated annually to monitor disease progression and treatment effectiveness.
Blog last updated: January 21, 2026
