Pulmonary function testing (PFT) is a group of breathing tests that measure how well your lungs take in air, hold it, and move oxygen into your bloodstream. A full PFT typically takes 15 to 45 minutes and involves three core components: spirometry (which measures airflow), lung volume testing (which measures how much air your lungs can hold), and diffusion capacity testing (which measures how efficiently oxygen crosses from your lungs into your blood). Together, these tests help identify conditions like asthma, COPD, and pulmonary fibrosis, and they track how well treatments are working over time.
The Three Parts of a Full PFT
Each component of a pulmonary function test answers a different question about your lung health. Some appointments include all three, while others use only spirometry as a screening tool. Your doctor decides which combination you need based on your symptoms and medical history.
Spirometry
Spirometry is the most common lung function test and the one most people picture when they hear “breathing test.” You take the deepest breath you can, seal your lips around a mouthpiece, and blow out as hard and fast as possible until your lungs are completely empty. The machine records two key numbers: how much total air you forced out (called FVC, or forced vital capacity) and how much of that air came out in the first second (called FEV1). The ratio between these two numbers is the primary tool for detecting obstructive lung diseases like asthma and COPD. A ratio below 70% after using a bronchodilator is consistent with COPD.
The test usually requires several attempts. You’ll repeat the maneuver at least three times so the technician can confirm consistent results. It can feel physically demanding, especially if you’re already short of breath, but each effort only lasts a few seconds.
Lung Volume Testing
Spirometry only measures air you can blow out. Lung volume testing goes further by measuring all the air in your lungs, including the air that stays trapped after you exhale as hard as you can (called residual volume) and your total lung capacity after the biggest breath possible. These measurements matter because some diseases cause your lungs to overinflate (as in emphysema) while others shrink them (as in pulmonary fibrosis).
The most precise method is body plethysmography. You sit inside an airtight, clear plastic booth about the size of a phone booth. As you breathe through a mouthpiece, sensors in both the booth and the mouthpiece track pressure changes. When your chest expands, the air pressure inside the booth shifts slightly, and the equipment uses that relationship between pressure and volume to calculate exactly how much air your lungs contain. It looks more intimidating than it is. The booth door can be opened at any time, and the test itself involves normal breathing with brief pauses.
Diffusion Capacity (DLCO)
This test measures how effectively oxygen passes from the tiny air sacs in your lungs into your bloodstream. You breathe in a gas mixture that includes a very small, harmless amount of carbon monoxide plus a tracer gas. You hold your breath for about 10 seconds, then exhale into the machine. The machine compares how much carbon monoxide you breathed in versus how much you breathed out. The difference tells your doctor how much crossed into your blood.
A healthy result falls within a predicted range based on your age, height, and sex. Results between 60% and 75% of your predicted value are considered mildly reduced. Below 40% is severely reduced, which signals that your lungs are struggling to get oxygen into your blood. Low diffusion capacity can point to lung scarring, emphysema, or even heart problems affecting blood flow through the lungs.
How Results Are Interpreted
Your PFT results aren’t compared to a single universal standard. Reference equations factor in your age, height, and sex to generate predicted values specific to you. The most widely used reference set, developed by the Global Lung Initiative, covers ages 5 through 80 using data from 11 countries. Your results are expressed as a percentage of your predicted value, and doctors look at the pattern across all three tests to classify what’s going on.
Two broad patterns emerge. An obstructive pattern means air has trouble getting out of your lungs, which happens in asthma, COPD, and chronic bronchitis. The hallmark is a reduced FEV1 relative to your total exhaled volume. A restrictive pattern means your lungs can’t fill with enough air in the first place, which happens in pulmonary fibrosis, chest wall disorders, or neuromuscular conditions. Lung volumes will be low, but the ratio of air exhaled in the first second compared to the total may look normal or even high. Some conditions produce a mixed pattern with features of both.
Conditions PFTs Help Diagnose and Monitor
PFTs aren’t a one-time diagnostic tool. They’re used repeatedly over months or years to track disease progression and treatment response. For asthma and COPD, spirometry alone is often sufficient for ongoing monitoring. People with COPD who smoke may lose 45 to 70 milliliters of lung capacity per year, though there’s wide variability. Tracking that decline helps doctors adjust treatment before symptoms worsen significantly.
For interstitial lung diseases like pulmonary fibrosis, doctors watch for specific thresholds that signal the disease is progressing. A decline of more than 5% in forced vital capacity within a year, or a drop of more than 10% in diffusion capacity, suggests worsening disease. These numbers can trigger changes in treatment even if you don’t feel noticeably different yet.
PFTs also play a role before surgery. If you’re having a procedure that could affect lung function, especially lung or heart surgery, your surgeon may order PFTs to assess whether your lungs can handle the stress of the operation and recovery.
How to Prepare for PFT Testing
Preparation mostly involves timing your medications correctly. If your test includes a bronchodilator comparison (a “pre and post” spirometry), you’ll need to stop certain inhalers beforehand so the test captures your baseline lung function. Short-acting rescue inhalers like albuterol should be held for 6 hours before the test. Long-acting inhalers need a longer pause, typically 24 hours, and ultra-long-acting ones require 36 hours. Your doctor’s office should give you specific instructions based on which medications you take.
Beyond medications, the practical advice is straightforward. Avoid smoking for at least 4 to 6 hours before the test. Don’t eat a heavy meal right beforehand, since a full stomach can restrict how deeply you breathe. Wear loose, comfortable clothing that doesn’t squeeze your chest or abdomen. Skip the tight waistband.
During the test itself, the technician will coach you through each maneuver. The hardest part for most people is the spirometry portion, which requires maximum effort. You may feel lightheaded or tired after repeated forced exhalations, but this passes quickly. The lung volume and diffusion capacity portions involve more relaxed breathing and feel less physically taxing. The entire session, including all three components, wraps up in 15 to 45 minutes depending on how many tests are ordered and how many acceptable efforts you produce.
What Happens After the Test
Results are usually available within a day or two. A pulmonologist or your ordering physician reviews the numbers in context with your symptoms, imaging, and medical history. PFT results alone rarely produce a final diagnosis. They narrow the possibilities and quantify how much your lung function is affected, which guides next steps like additional imaging, medication adjustments, or referral to a specialist.
If your results show a new abnormality, your doctor may order a follow-up PFT in a few months to see if the pattern is stable or worsening. For chronic conditions, expect to repeat PFTs at regular intervals, often every 6 to 12 months, to track your trajectory over time.