A pulmonary function test measures how well your lungs work by checking how much air you can breathe in and out, and how efficiently your lungs move oxygen into your blood.

The test is also called spirometry or PFT. It takes 15 to 45 minutes, happens in an outpatient clinic or hospital lab, and involves breathing into a machine called a spirometer. You will be asked to take normal breaths, then deep breaths, then blow out as hard and fast as you can. The machine records the volume and speed of air moving through your airways.

A pulmonary function test does not hurt and does not use radiation or needles. It is one of the most common ways doctors check for lung disease, measure how severe it is, or track whether a lung condition is getting better or worse over time.

Key Takeaways

  • A pulmonary function test measures lung capacity and airflow using a machine you breathe into; it takes 15 to 45 minutes and causes no pain.
  • The test produces numbers that show how much air your lungs hold, how fast you can empty them, and what percentage of normal your lung function is.
  • Your doctor may order this test if you have shortness of breath, a chronic cough, asthma, COPD, or a family history of lung disease.
  • You should avoid heavy exercise, large meals, and certain medications for a few hours before the test; your doctor will tell you what to stop or pause.
  • Results come back within a few days, and your doctor will explain what the numbers mean and whether further testing or treatment is needed.

Why your doctor orders a pulmonary function test

Doctors order this test when they need to understand why you are short of breath, or to check whether a known lung condition is stable or worsening. Common reasons include a chronic cough that has lasted more than a few weeks, asthma symptoms that are not controlled by your current inhaler, or shortness of breath during normal activity.

The test is also used to screen people before surgery, to measure lung damage from smoking or occupational exposure, and to monitor conditions like COPD, pulmonary fibrosis, or cystic fibrosis over months or years. If you have a family history of emphysema or alpha-1 antitrypsin deficiency, your doctor may order one to catch problems early.

What the numbers mean

The spirometer produces several measurements. FVC (forced vital capacity) is the total amount of air you can blow out after taking the deepest breath possible. FEV1 (forced expiratory volume in one second) is how much of that air you can push out in the first second. FEV1/FVC ratio compares these two numbers and shows whether your airways are narrowed.

The machine also calculates what percentage of predicted normal your results are. If your FEV1 is 75% of predicted, your lungs are moving air at about three-quarters the rate expected for someone your age, height, and sex. A result below 80% of predicted often signals airway obstruction; below 50% suggests severe obstruction.

Your doctor will explain which numbers matter most for your situation. A low FVC might mean your lungs are stiff or your chest muscles are weak. A low FEV1 with a normal FVC suggests your airways are blocked. The pattern of results helps your doctor narrow down whether you have asthma, COPD, restrictive lung disease, or something else.

How to prepare for the test

Tell your doctor about all medications you take, especially inhalers, bronchodilators, or steroids. Some of these can affect your results, and your doctor may ask you to skip a dose or two before the test. Do not stop any medication without permission.

Avoid heavy exercise for at least one hour before the test. Do not eat a large meal right before; a light snack is fine. Wear loose, comfortable clothing that does not restrict your chest or belly. If you use an inhaler, bring it with you in case you need it after the test.

On the day of the test, arrive a few minutes early so you can check in and relax. The technician will explain each step before you do it. Let them know if you feel dizzy, short of breath, or unwell at any point; they can pause or stop the test.

What happens during the test

You will sit in a small room with the spirometer machine. The technician will place a clip on your nose so all air goes through your mouth into the mouthpiece. You will practice breathing normally into the machine first, then the technician will give you instructions for each maneuver.

For the main test, you will take the deepest breath you can, then blow out as hard and fast as possible until your lungs feel empty. You may be asked to repeat this two or three times to make sure the results are consistent. Some tests include a bronchodilator challenge: you inhale a medication that relaxes your airways, wait 15 minutes, and repeat the test to see if your numbers improve.

The whole process is tiring because you are working your lungs hard, but it is not painful. Some people feel lightheaded or cough a little afterward. This is normal and passes quickly.

After the test and what comes next

You can go back to your normal activities right away. If you felt lightheaded, sit for a few minutes before driving. Your doctor will have the results within a few days.

If your results are normal, your doctor will tell you that your lungs are working well for your age and size. If results show a problem, your doctor will explain what it means and what the next step is. That might be a second test to confirm the finding, imaging like a chest X-ray or CT scan, or a referral to a lung specialist called a pulmonologist.

If you have asthma or COPD, you may have this test done once a year or every few years to track whether your condition is stable or changing. If you are starting a new medication for a lung condition, your doctor may order a repeat test in a few weeks to see whether the medication is helping.

Limitations and when other tests are needed

A pulmonary function test shows how your lungs move air, but it does not show what is causing a problem. If your results are abnormal, you will usually need other tests to find out why. A chest X-ray or high-resolution CT scan can show scarring, inflammation, tumors, or other structural changes. A blood test can check for infections or autoimmune diseases that affect the lungs.

The test also depends on your effort and cooperation. If you do not blow hard enough or do not follow instructions, the results may look worse than they are. The technician will ask you to repeat the test if the results do not look reliable. If you have severe shortness of breath, chest pain, or a recent heart attack, your doctor may postpone the test until you are more stable.

Frequently Asked Questions

Can I take my regular medications before the test?

Ask your doctor. Some medications like inhalers or bronchodilators can change your results, so your doctor may ask you to skip a dose or two beforehand. Never stop a medication on your own without permission. Bring a list of everything you take so the technician knows what to account for.

What if I feel dizzy or short of breath during the test?

Tell the technician when ready. They can pause or stop the test at any time. Lightheadedness is not uncommon because you are breathing hard, but it passes quickly once you rest. The technician is trained to handle this and will not push you beyond what is safe.

How long does it take to get results?

Results are usually ready within a few days. Your doctor's office will call you or send a message, or you may see them at your next appointment. If results are urgent or abnormal, your doctor may call sooner. Do not assume no news is good news; follow up if you have not heard within a week.

Do I need to repeat the test if my results were normal?

If you have no lung symptoms and no known lung disease, you probably will not need this test again unless something changes. If you have asthma, COPD, or another chronic lung condition, your doctor may order it every one to three years to track your progress or check whether your treatment is working.

What is the difference between this test and a chest X-ray?

A pulmonary function test measures how well your lungs work—the movement of air. A chest X-ray is a picture that shows the structure of your lungs and can reveal infections, scarring, or tumors. Your doctor may order both to get a complete picture of your lung health.