What are pulmonary function tests (PFTs)?

Pulmonary function tests (PFTs) are a group of tests that measure how well your lungs work. This includes how well you’re able to breathe and how effective your lungs are able to bring oxygen to the rest of your body.

Your doctor may order these tests:

  • if you’re having symptoms of lung problems
  • if you’re regularly exposed to certain substances in the environment or workplace
  • to monitor the course of chronic lung disease, such as asthma or chronic obstructive pulmonary disease (COPD)
  • to assess how well your lungs are working before you have surgery

PFTs are also known as lung function tests.

Why are these tests done?

Your doctor will order these tests to determine how your lungs are working. If you already have a condition that’s affecting your lungs, your doctor may order this test to see if the condition is progressing or how it’s responding to treatment.

PFTs can help diagnose:

How do I prepare for pulmonary function tests?

If you’re on medications that open your airways, such as those used for asthma or chronic bronchitis, your doctor may ask you to stop taking them before the test. If it isn’t clear whether or not you should take your medication, make sure to ask your doctor. Pain medications may also affect the results of the test. You should tell your doctor about any over-the-counter and prescription pain medications you’re taking.

It’s important that you don’t eat a large meal before testing. A full stomach can prevent your lungs from inhaling fully. You should also avoid food and drinks that contain caffeine, such as chocolate, coffee, and tea, before your test. Caffeine can cause your airways to be more open which could affect the results of your test. You should also avoid smoking at least an hour before the test, as well as strenuous exercise before the test.

Be sure to wear loose-fitting clothing to the test. Tighter clothing may restrict your breathing. You should also avoid wearing jewelry that might affect your breathing. If you wear dentures, wear them to the test to ensure that your mouth can fit tightly around the mouthpiece used for the test.

If you have had recent eye, chest, or abdominal surgery or a recent heart attack, you will likely need to delay the test until you have fully recovered.

What happens during the tests?

Spirometry

Your PFTs may include spirometry, which measures the amount of air you breathe in and out. For this test, you’ll sit in front of a machine and be fitted with a mouthpiece. It’s important that the mouthpiece fits snugly so that all the air you breathe goes into the machine. You’ll also wear a nose clip to keep you from breathing air out through your nose. The respiratory technologist will explain how to breathe for the test.

You may then breathe normally. Your doctor will ask you to breathe in and out as deeply or as quickly as you can for several seconds. They may also ask you to breathe in a medication that opens your airways. You’ll then breathe into the machine again to see if the medication affected your lung function.

Plethysmography test

A plethysmography test measures the volume of gas in your lungs, known as lung volume. For this test, you’ll sit or stand in a small booth and breathe into a mouthpiece. Your doctor can learn about your lung volume by measuring the pressure in the booth.

Diffusion capacity test

This test evaluates how well the small air sacks inside the lungs, called alveoli, work. For this part of a pulmonary function test, you will be asked to breathe in certain gases such as oxygen, helium, or carbon dioxide.

You may also breathe in a “tracer gas” for one breath. The machine can detect when you breathe out this gas. This tests how well your lungs are able to transfer oxygen and carbon dioxide to and from your bloodstream.

What are the risks of pulmonary function tests?

A PFT can cause problems if:

  • you’ve recently had a heart attack
  • you’ve recently had eye surgery
  • you’ve recently had chest surgery
  • you’ve recently had abdominal surgery
  • you have a severe respiratory infection
  • you have unstable heart disease

PFTs are usually safe for most people. However, because the test may require you to breathe in and out quickly, you may feel dizzy and there’s a risk that you may faint. If you feel lightheaded, tell your doctor. If you have asthma, the test may cause you to have an asthma attack. In very rare cases, PFTs may cause a collapsed lung.