A pulmonary function test (PFT) is a group of breathing tests that measure how well your lungs take in air, how much they can hold, and how efficiently they move oxygen into your bloodstream. The most common PFTs are spirometry, diffusion testing, and body plethysmography. Together, these tests help identify conditions like asthma, COPD, and pulmonary fibrosis, and they can also determine whether your lungs are healthy enough to handle surgery.
The Three Main Tests
When your doctor orders pulmonary function testing, you may get one or all three of these, depending on what they’re looking for.
Spirometry is the most commonly ordered test. It measures how much air you can breathe into your lungs and how quickly you can blow it all out. You’ll take the deepest breath you can, then exhale as hard and fast as possible into a mouthpiece connected to a machine. The key numbers it produces are your forced vital capacity (FVC), which is the total volume of air you can push out, and your FEV1, which is the volume you exhale in the first second. The ratio between these two numbers is what helps your doctor tell the difference between obstructive lung diseases (where air has trouble getting out) and restrictive lung diseases (where your lungs can’t fully expand).
Diffusion testing looks at how well oxygen crosses from your lungs into your blood. You’ll wear nose clips and breathe through a mouthpiece, empty your lungs gently, then take one quick, deep breath of air that contains a tiny amount of carbon monoxide. After holding your breath for about 10 seconds, you exhale as instructed. The machine measures how much of that gas was absorbed, which tells your doctor how efficiently the transfer is happening. Several factors affect this result: the surface area inside your lungs, the blood flow through the tiny vessels surrounding your air sacs, your hemoglobin levels, and whether the membranes in your lungs have thickened or been damaged.
Body plethysmography measures the total amount of air your lungs can hold after a full breath in, and how much air stays trapped in your lungs after you’ve breathed out as much as possible. For this test, you sit inside a small, clear booth (similar in size to a phone booth) and breathe through a mouthpiece. The booth detects tiny pressure changes as your chest expands and contracts, which allows precise calculation of your lung volumes. This is especially useful for detecting air trapping, a hallmark of certain lung diseases where air gets stuck in the lungs and can’t be fully exhaled.
What These Tests Diagnose
PFTs are used to diagnose and monitor asthma, COPD, pulmonary fibrosis, breathing muscle weakness, and narrowing of the trachea. Your doctor may also order them if you have unexplained coughing or shortness of breath, if you smoke, or if you’re being evaluated before surgery.
The pattern of your results points toward a specific category of lung problem. A low FEV1/FVC ratio signals an obstructive pattern, meaning something is narrowing your airways and making it hard to push air out. This is the classic finding in asthma and COPD. If the ratio is normal but your total lung capacity (measured by plethysmography) is reduced, that points to a restrictive pattern, where the lungs themselves are stiff or small. Pulmonary fibrosis is a common cause. When both the ratio and total lung capacity are low, that’s called a mixed pattern, meaning both obstruction and restriction are present. A low diffusion result, on its own, suggests damage to the air sacs or the blood vessels around them, even when other numbers look normal.
How Your Results Are Interpreted
Your results aren’t compared to a single universal number. Instead, they’re measured against predicted values calculated from your age, height, sex, and race. The goal is to figure out what “normal” looks like for someone with your specific body.
Current guidelines from the American Thoracic Society and European Respiratory Society recommend using a statistical threshold called the lower limit of normal (the 5th percentile of healthy people) to decide whether a result is abnormally low. Older methods that used fixed cutoffs, like 80% of predicted for FEV1 or a 0.70 ratio for FEV1/FVC, are now strongly discouraged because they can over-diagnose problems in older adults and miss them in younger ones.
Once an abnormality is identified, severity is graded on a three-level scale: mild, moderate, or severe, based on how far your result falls below the expected range.
Bronchodilator Response
If your spirometry shows obstruction, you may be given a bronchodilator, usually an inhaled medication that relaxes the muscles around your airways. After waiting several minutes, you’ll repeat the spirometry. If your FEV1 or FVC improves by more than 10% relative to the predicted value, that’s considered a significant response. This helps distinguish asthma (which typically responds well to bronchodilators) from COPD (which often doesn’t respond as dramatically).
How to Prepare
Preparation mostly involves adjusting your medications. If your doctor wants to test your lungs without the influence of bronchodilators, you’ll need to stop them beforehand. Short-acting inhalers like albuterol should be stopped 6 hours before. Short-acting anticholinergic inhalers need 12 hours. Long-acting inhalers require 24 hours, and ultra-long-acting agents need 36 hours. Your doctor’s office will give you specific instructions, but the general rule is: the longer-acting the medication, the earlier you need to stop it.
Avoid smoking before the test, wear loose clothing that doesn’t restrict your chest, and eat lightly. You’ll need to give maximal effort on several breathing maneuvers, so anything that limits your ability to take a deep breath or blow out hard can affect the accuracy of your results.
When PFTs Shouldn’t Be Done
The forceful breathing required during these tests creates significant pressure changes in your chest, abdomen, and head. That makes them unsafe in certain situations. You should not do PFTs within one week of a heart attack or eye surgery, within four weeks of brain, chest, or abdominal surgery, or if you have an active pneumothorax (collapsed lung). Late-term pregnancy, uncontrolled high blood pressure, significant heart rhythm problems, and uncompensated heart failure are also reasons to postpone.
Active respiratory infections, including tuberculosis, rule out testing both for your safety and to protect other patients and staff. If you’ve had a recent concussion with ongoing symptoms or a history of fainting during coughing or forced exhalation, your doctor will weigh the risks before ordering the tests.
What the Experience Feels Like
The tests are noninvasive and don’t involve needles or sedation. You’ll spend most of your time seated, breathing into a mouthpiece with a nose clip on. Spirometry requires you to blast air out as forcefully as possible, which can feel tiring, especially since you’ll repeat the maneuver several times to get consistent results. The diffusion test involves holding your breath for 10 seconds, which most people manage without difficulty. Body plethysmography requires sitting in the enclosed booth, which can feel confining, but the walls are transparent and the door can be opened at any time.
A full battery of all three tests typically takes 30 to 60 minutes. Spirometry alone is faster, often around 15 minutes. You may feel lightheaded or short of breath between attempts, but this passes quickly. The technician coaching you through it will give you breaks as needed.