A pulmonary function test (PFT) is a series of breathing exercises performed in a clinic or hospital lab, where you breathe into a mouthpiece connected to a machine that measures how much air your lungs can hold, how quickly you can move air in and out, and how efficiently oxygen passes into your bloodstream. The full battery of tests typically takes 30 to 90 minutes depending on which components your doctor has ordered. Here’s what to expect at each stage.
How to Prepare Before the Test
The most important preparation involves your medications. Because certain inhalers temporarily open the airways, using them before the test can mask the problem your doctor is trying to measure. The withholding windows vary by medication type:
- Short-acting rescue inhalers (like albuterol): stop 6 hours before
- Short-acting anticholinergic inhalers (like ipratropium): stop 12 hours before
- Long-acting inhalers (like formoterol or salmeterol): stop 24 hours before
- Ultra long-acting inhalers (like tiotropium): stop 36 hours before
If you’re being tested for exercise-induced breathing problems, you’ll also need to skip caffeine for 12 hours. Beyond medications, you should avoid smoking for at least 4 hours, skip heavy meals beforehand, and wear loose, comfortable clothing that doesn’t restrict your chest or abdomen. Your lab will likely give you specific instructions when you schedule the appointment.
What Happens During Spirometry
Spirometry is the most common PFT and usually the first test performed. You sit upright, a technician clips a soft clamp over your nose to prevent air from escaping, and you place your lips tightly around a mouthpiece attached to a device called a spirometer.
The maneuver itself has three distinct phases. First, you take the deepest breath you possibly can, filling your lungs completely. Then you blast the air out as hard and as fast as possible, like you’re trying to blow out birthday candles across a room. Finally, you keep blowing steadily until your lungs are completely empty. That last part is harder than it sounds. To count as a valid effort, you need to keep exhaling for at least 6 seconds, and the machine needs to detect that less than 50 milliliters of air came out in the final 2 seconds, confirming you’ve truly emptied your lungs.
The start of the blow matters too. The technician is looking for an explosive, immediate blast with no hesitation. If you ease into it gradually, that attempt won’t count and you’ll be asked to repeat it. The goal for each session is to get at least three good-quality attempts, with at least two producing nearly identical results. This confirms the readings are reliable and not affected by inconsistent effort. Expect to repeat the maneuver several times, and don’t be surprised if the technician coaches you enthusiastically throughout.
Measuring Total Lung Volume
Spirometry measures air you can forcefully blow out, but it can’t measure the air that stays trapped in your lungs after a full exhale. To get that number, you may be asked to sit inside a body plethysmograph, which looks like a clear phone booth with a door that seals shut.
Once the door is closed, the booth becomes airtight. You breathe through a mouthpiece, and at a certain point a small shutter briefly closes the mouthpiece while you gently pant against it. When you try to breathe against the closed shutter, the air in your lungs compresses and decompresses slightly. Because the booth is sealed, every tiny change in your lung volume causes an equal and opposite pressure change in the air around you inside the booth. The machine detects these pressure shifts and uses them to calculate exactly how much air is sitting in your lungs at rest.
The booth feels unusual but isn’t painful. Each panting maneuver lasts only a few seconds. If you’re claustrophobic, let your technician know beforehand. The clear walls help, and the door can be opened quickly between measurements.
Testing Gas Exchange (Diffusion Capacity)
This test measures how well oxygen crosses from the tiny air sacs in your lungs into your bloodstream. Instead of measuring oxygen directly, the test uses a small, harmless amount of carbon monoxide mixed with a tracer gas (usually helium or methane). Carbon monoxide behaves similarly to oxygen when crossing into the blood, making it a useful stand-in.
The procedure is straightforward. You breathe out fully, then take one deep breath of the test gas mixture. You hold that breath for about 10 seconds, then exhale steadily. The machine analyzes the exhaled air to see how much carbon monoxide was absorbed during that brief breath-hold. The less carbon monoxide that comes back out, the better your lungs are at transferring gas into the blood. The tracer gas concentration in the exhaled sample also helps the machine calculate how much of your lung was actually participating in the exchange.
This test is particularly useful for detecting conditions that damage the thin membrane between the air sacs and blood vessels, such as emphysema or pulmonary fibrosis.
The Bronchodilator Challenge
If your initial spirometry results show airflow limitation, the technician will often perform a bronchodilator reversibility test. You inhale a dose of a fast-acting airway-opening medication through an inhaler or nebulizer, wait about 15 minutes for it to take full effect, and then repeat the spirometry maneuvers.
The purpose is to see whether your airway narrowing is reversible. A significant improvement in airflow after the medication suggests asthma, where the airways are inflamed but can still open up. If the numbers barely change, it points more toward a fixed obstruction like COPD. This single comparison often plays a pivotal role in determining your diagnosis.
How Results Are Interpreted
The two numbers that matter most from spirometry are your FVC (the total volume of air you can forcefully exhale) and your FEV1 (how much of that air comes out in the first second). The ratio between these two values tells your doctor which category of lung problem you might have.
An FEV1/FVC ratio below 70% in adults signals an obstructive pattern, meaning something is blocking or narrowing the airways and slowing airflow. This is the hallmark of conditions like asthma, COPD, and chronic bronchitis. If the ratio is normal but your total FVC is low, that points to a restrictive pattern, meaning the lungs can’t fully expand. This is seen in conditions like pulmonary fibrosis, chest wall deformities, or neuromuscular diseases. When both the ratio and FVC are low, it suggests a mixed defect with elements of both obstruction and restriction.
Your raw numbers are compared against predicted values for someone of your age, height, sex, and ethnic background. The Global Lung Function Initiative has developed standardized reference equations used worldwide for this comparison. Recently, both the American Thoracic Society and European Respiratory Society have recommended race-neutral approaches to interpreting lung function, moving away from older equations that used separate calculations for different racial groups.
Who Should Postpone Testing
Because PFTs require forceful breathing that increases pressure in your chest, abdomen, and eyes, certain recent medical events mean you should wait before testing. Recent thoracic or abdominal surgery and recent eye surgery (such as cataract removal) are the most common reasons to delay. Older guidelines suggested a 6-week recovery period, but more recent evidence indicates that some of these situations may be safe after less than 3 weeks. Your ordering physician will determine the right timing based on your specific surgery and recovery.
Other situations that call for caution include active chest pain, a recent heart attack, a large untreated aneurysm, or any condition where the sudden pressure changes from forceful breathing could cause harm. The test itself is noninvasive and painless for the vast majority of people, though the repeated deep breaths can occasionally cause temporary lightheadedness or mild fatigue.