A pulmonary function test (PFT) is a series of breathing exercises performed while you sit in a chair or inside a clear plastic booth, blowing into a mouthpiece connected to a machine. There are no needles, no scans, and no pain involved. The whole appointment typically lasts 45 minutes to an hour, though a basic spirometry test can be done in under 10 minutes.
The Equipment and Room Setup
The most common version of this test uses a device called a spirometer, which is essentially a mouthpiece attached to a small machine that measures airflow. You sit in a regular chair, and a technician places soft clips on your nose so all of your breathing goes through your mouth. The mouthpiece looks similar to a snorkel mouthpiece, and you press your lips tightly around it to create a seal. If you wear dentures, keep them in, as they help you form that seal.
If your provider orders a more complete set of lung measurements, you may sit inside a body plethysmograph. This is an enclosed, airtight, see-through plastic box, roughly the size of a phone booth. You can see everything around you through the clear walls, and the technician talks to you throughout the test. Sensors inside the box detect tiny changes in air pressure as you breathe, which lets the machine calculate how much air your lungs can hold. If tight spaces make you anxious, mention this ahead of time.
What Happens During Spirometry
Spirometry is the core of most pulmonary function testing. Once the nose clips are on and the mouthpiece is in place, the technician will walk you through each breathing maneuver before you do it. The basic sequence goes like this: you take a few normal breaths, then inhale as deeply as you possibly can, and then blast the air out as hard and fast as you can, continuing to blow until your lungs are completely empty.
The technician will coach you enthusiastically, often raising their voice near the end to keep you blowing. This isn’t because something is wrong. Getting every last bit of air out is critical for accurate results, and encouragement genuinely improves performance. You need to keep exhaling until almost no air is coming out, which can feel like blowing through a very long straw. The start of each blow matters too: there should be no hesitation, just an explosive push from the very first moment.
You’ll repeat this maneuver at least three times to make sure the results are consistent. Between attempts, you rest and breathe normally. In some cases, the technician will give you an inhaled medication that opens your airways and then have you repeat the test. Comparing the before and after results shows whether your airways respond to treatment, which helps distinguish between different lung conditions.
The Diffusion Capacity Test
If your provider wants to know how well oxygen passes from your lungs into your bloodstream, you may also do a diffusion capacity test. For this one, you breathe in air containing a tiny, harmless amount of carbon monoxide and a tracer gas like helium. You hold your breath for about 10 seconds, then blow it all out quickly. The machine analyzes your exhaled air to see how much of the tracer gas was absorbed. The setup feels identical to spirometry: same nose clips, same type of mouthpiece, same seated position.
How to Prepare
Preparation is straightforward but important, because certain substances can temporarily change how your lungs perform and throw off the results. General guidelines include:
- Smoking: Avoid for 12 to 24 hours before the test.
- Inhaler medications: Stop about 6 hours before, or as your provider directs. Don’t stop any medication without checking first.
- Caffeine: Skip coffee, tea, cola, and chocolate beforehand.
- Alcohol: Avoid before testing.
- Food: Eat only a light meal and avoid anything large within two hours of the test.
- Heavy exercise: Skip intense workouts the day of testing.
- Clothing: Wear something loose and comfortable, since tight clothing around your chest can restrict your breathing.
Who Should Not Take the Test
The forceful blowing required during PFTs temporarily raises pressure in your chest, abdomen, and head. That makes the test risky if you’ve recently had eye surgery, chest or abdominal surgery, a heart attack, or an unstable heart condition. A bulging blood vessel (aneurysm) in the chest, abdomen, or brain is also a concern, as is active tuberculosis or a respiratory infection like a cold or flu. Recovery time after surgery used to require a six-week wait, but modern, less invasive techniques have shortened that to under three weeks in many cases.
What the Results Tell You
Your results are compared to predicted values for someone of your age, height, sex, and overall health. The comparison produces a score that shows how far your measurements fall from what’s expected. If your values land within the range that covers 90% of healthy people, they’re considered normal.
Two key numbers drive most of the interpretation. The first is how much air you can force out in one second compared to the total amount you can exhale. When that ratio is low, it suggests an obstructive pattern, meaning your airways are narrowed, as in asthma or COPD. The second is your total lung capacity. When that’s low, it points to a restrictive pattern, meaning your lungs can’t expand fully, which can happen with scarring or chest wall problems. Some people show both patterns at once.
Recent standards have moved toward race-neutral reference values, meaning your results are no longer adjusted based on ethnicity. This change, recommended by the American Thoracic Society in 2023, is designed to reduce missed diagnoses that older race-based formulas sometimes caused. If a bronchodilator was used during your test, your provider will also look at whether your airflow improved after inhaling it, which helps pinpoint the type of lung condition involved.
How It Feels Afterward
The repeated forceful blowing can leave you feeling lightheaded or slightly tired, especially if you did many attempts. This passes quickly. Some people feel a bit of chest tightness or coughing from the effort, but there’s no recovery period. You can drive yourself home and go about your day normally. Your provider will typically review the results with you at a follow-up visit or, in some cases, the same day.