A pulmonary function test (PFT) is a series of breathing exercises performed through a mouthpiece while a machine measures how much air your lungs can hold, how fast you can push air out, and how well oxygen moves from your lungs into your blood. The full battery typically takes 45 minutes to an hour, involves no needles or pain, and you can go home right afterward. Here’s what each part involves and how to prepare.
What a PFT Actually Includes
A “pulmonary function test” isn’t one test. It’s an umbrella term for several breathing tests that are often done together in a single appointment. Your doctor may order all of them or just a few, depending on what they’re looking for. The most common components are spirometry, lung volume measurement, and a diffusion capacity test. Some appointments also include a bronchodilator reversibility test, a six-minute walk test, or a challenge test that deliberately triggers mild airway narrowing to check for asthma.
How Spirometry Works
Spirometry is the core of most PFT appointments and the test people are most likely to encounter. You’ll sit upright in a chair with your feet flat on the floor and legs uncrossed. A technician will place a clip on your nose so all your breathing goes through your mouth.
There are two common approaches. In the open circuit method, you take a deep breath in as fast and fully as you can, pause for less than one second, place the mouthpiece between your lips to form a tight seal, then blast the air out as hard and as long as possible until your lungs are completely empty. You then breathe in deeply again through the mouthpiece and remove it. In the closed circuit method, you start with the mouthpiece already in place, breathe normally for two or three breaths, then do the same rapid inhale, brief pause, and explosive exhale.
The machine records three key numbers: the total amount of air you pushed out (forced vital capacity), how much came out in the first second (FEV1), and the ratio between the two. In healthy lungs, roughly 75% to 85% of the total air comes out in that first second. You’ll repeat the maneuver at least two or three times so the technician can confirm consistent results. Expect some coaching, as the technician will encourage you to blow harder or longer.
Lung Volume Measurement in the Booth
If your doctor orders lung volumes, you’ll step into a clear, phone-booth-sized chamber called a body plethysmograph. It’s airtight but has a window, and the technician will be right outside talking to you the whole time. You’ll breathe through a mouthpiece connected to a flow sensor.
At a certain point, a small shutter in the mouthpiece will briefly close, blocking airflow. While the shutter is closed, you’ll pant gently against it at a steady rhythm, roughly one to two pants per second. This panting creates tiny pressure changes that sensors in the booth detect. By comparing the pressure at your mouth with the pressure inside the sealed chamber, the machine calculates how much air remains in your lungs even after you exhale as far as you can (residual volume) and your total lung capacity. The shutter only stays closed for a few seconds at a time, and the whole process is painless.
The Diffusion Capacity Test
This test checks how efficiently gases cross from the air sacs in your lungs into your bloodstream. You’ll sit with a nose clip on and breathe normally through a mouthpiece for a moment. Then you’ll exhale completely, emptying your lungs as much as possible.
Next, you’ll inhale a special gas mixture (containing a tiny, harmless amount of carbon monoxide along with a tracer gas, oxygen, and nitrogen) as rapidly as you can, ideally reaching a full breath within about four seconds. You then hold your breath for 10 seconds and exhale completely. The machine analyzes a small sample of the exhaled air to see how much carbon monoxide your lungs absorbed during that breath-hold. The test is repeated at least once after a minimum four-minute rest between attempts.
Bronchodilator Reversibility Testing
If spirometry shows airflow limitation, the technician may give you an inhaled bronchodilator (a medication that opens the airways) and then repeat spirometry about 15 to 20 minutes later. This helps distinguish between conditions like asthma, where airways open up significantly with medication, and conditions like COPD, where they may not.
A positive response is traditionally defined as an improvement of at least 12% and 200 milliliters in either the total air exhaled or the amount exhaled in the first second. Updated guidelines from the European Respiratory Society and American Thoracic Society now define a significant response as a 10% or greater increase in the percentage of predicted values. Your doctor will use these numbers alongside your symptoms and history to make a diagnosis.
How to Prepare
Preparation matters because several common substances and activities can skew your results. The National Heart, Lung, and Blood Institute recommends the following:
- Smoking: Do not smoke on the day of your test. For diffusion capacity testing, avoid smoking and alcohol entirely that day. For exercise-related tests, stop smoking at least 8 hours before.
- Inhalers: Withholding times depend on the type. Stop short-acting rescue inhalers (like albuterol) at least 6 hours before. Stop short-acting anticholinergic inhalers at least 12 hours before. Stop long-acting inhalers at least 24 hours before, and ultra-long-acting inhalers at least 36 hours before.
- Exercise: Avoid heavy exercise for at least 30 minutes before spirometry and at least 2 hours before diffusion testing.
- Caffeine: If you’re having an exercise or bronchial challenge test, avoid caffeine for 12 hours.
Wear loose, comfortable clothing that doesn’t restrict your chest or abdomen. Eat a light meal beforehand rather than a heavy one. Your testing facility will typically give you specific instructions when you schedule the appointment.
Who Should Avoid Testing
The forceful breathing required by PFTs creates pressure changes in your chest, which can be risky in certain situations. The highest-risk contraindications involve recent cardiovascular events like a heart attack, pulmonary embolism, or an ascending aortic aneurysm. Recent major chest, abdominal, or head surgery also requires a waiting period before testing, though modern surgical techniques have shortened that window from the traditional six weeks to often less than three weeks. Your doctor will evaluate whether testing is safe based on your specific situation.
What It Feels Like
PFTs are noninvasive and generally well tolerated, but the repeated deep breathing and forceful exhalation can leave you feeling dizzy, lightheaded, or tired. Some people cough from blowing hard into the mouthpiece. If you feel dizzy during the test, the technician will pause and let you recover before continuing. These symptoms typically go away within minutes of finishing. You can drive yourself home and return to normal activities right away.
What the Results Tell You
Your results are compared against predicted values based on your age, height, sex, and ethnicity. Two broad patterns emerge. In obstructive lung diseases (like asthma or COPD), air has trouble getting out, so the amount exhaled in the first second drops disproportionately and the FEV1/FVC ratio falls below normal. In restrictive lung diseases (like pulmonary fibrosis or chest wall disorders), the lungs can’t expand fully, so total volumes are reduced but the ratio between first-second and total air stays normal or even rises.
Low diffusion capacity suggests a problem at the level of the lung tissue itself, where oxygen crosses into the blood. Reduced lung volumes on plethysmography confirm restriction. Together, these tests give your doctor a detailed picture of where and how your breathing is impaired, which is information that no single test can provide on its own.