Pulmonary function tests (PFTs)

Pulmonary function tests, or PFTs, are also called lung function tests. They are a group of tests that measure the amount of air your lungs can hold, how well you can empty your lungs of air and how well your lungs can get oxygen from the air you breathe into your bloodstream.

These tests may be done to look for early signs of a lung disease like chronic obstructive pulmonary disease (COPD) or asthma. When used in this way, they are called screening tests.

If you’ve been diagnosed with a lung disease, you may have PFTs to follow the progression of your disease and your response to treatment. These tests can also be done to see how a disease outside your lungs—like heart disease, a nervous system disease or a muscle disease—is affecting your breathing. PFTs are also used to diagnose a lung condition if you have a symptom like shortness of breath, called dyspnea.

PFTs are also used to diagnose or help treat lung diseases like cystic fibrosis, or lung damage due to workplace or home exposure. Another common reason is to make sure your lungs are healthy enough for general anesthesia if you need surgery. PFTs are very important to determine how much of your lung can be resected, if lung resection is required.

What are the types of PFTs?

The three main PFTs are spirometry, lung volume testing and lung diffusion capacity.

Spirometry measures the rate of air flowing in and out of your lungs. It also gives a rough estimate of the size of your lungs. To do this test, you place a mouthpiece in your mouth, a clip on your nose and breathe into a tube connected to a special device, called a spirometer. You will be asked to breathe normally and also to breathe as deeply as you can.

Lung volume testing is similar to spirometry, but it’s done in a small, air-tight booth with clear walls. This is the best test to measure how much air your lungs can hold.

Lung diffusion capacity measures how well oxygen gets into your blood. You’ll breathe in and out through a tube, like the other tests, but your breathing instructions will be different. With this test, you may also have a blood test to measure your hemoglobin level.

Spirometry is the most basic and useful PFT. It can tell if you have a breathing problem caused by an obstruction of air leaving your lungs or a restriction of air entering your lungs. However, it doesn’t give an accurate measurement of your total lung capacity. Lung volume testing—also called plethysmography—can measure your maximum lung capacity. This is important for diagnosing obstructive, air-trapping diseases like COPD.

Pulse oximetry can also be part of PFTs. It’s a simple test that measures the oxygen level in your blood through a sensor placed on your finger. Another test that may be part of PFTs is an arterial blood gases test, a blood test taken from an artery, usually in your wrist. This blood test measures oxygen and carbon dioxide.

How do you prepare for PFTs?

Different PFT testing sites have different instructions for preparation, so it’s important to ask your health care provider about instructions before your test.

General guidelines to follow include:

• Don’t smoke for at least one hour before a PFT.
• Don’t drink alcohol for at least four hours before a PFT.
• Don’t exercise for at least 30 minutes before a PFT.
• Don’t wear tight clothing to the test. This may interfere with breathing.
• Don’t eat a large meal in the two hours before the test.
• You may be instructed not to use any medicines you use for breathing before the test.
• Ask your health care providers if there are any medications you should stop before the test.

What happens during PFTs?

PFTs aren’t typically painful and are usually performed by a pulmonary function technician. You may have one or more types of PFTs. Possible side effects include feeling tired, light-headed or dizzy during or after a test. If you have plethysmography and you have claustrophobia, you may feel anxious, so tell your technician if you have a history of claustrophobia.

What happens during your testing depends on what tests you have. What to expect with each common test:

During spirometry:

• While seated, you’ll take a deep breath through your mouthpiece and then blow out as much air as you can as quickly as you can.
• You may need to repeat this breathing three times to get the best result.
• You may be asked to inhale a bronchodilator medicine and repeat the test.
• The test takes about 30 minutes.

During plethysmography:

• You’ll sit inside a booth while doing the breathing test.
• You’ll take short, shallow breaths, and sometimes, the mouthpiece will make it harder to inhale.
• This test takes about 15 minutes.

During lung diffusion testing:

• You’ll be asked to empty your lungs by breathing out as much as you can.
• You’ll then take a quick deep breath and hold it for 10 seconds. Other breathing instructions may follow.
• This test takes about 30 minutes.

After your PFT, you may rest for a bit if you’re tired or light-headed before resuming normal activities.

Your test results from PFTs

According to a Concise Review for Clinicians review, spirometry testing creates a graph called a flow-volume curve. The results may indicate if you have an obstruction or restriction in breathing. Volume testing gives two important numbers called total lung capacity (TLC) and residual volume (RV), which is how much air is left in your lungs after breathing out.

Since everyone’s lungs are different, your test results are compared to normal PFT results of people your own age, sex and height. These results are called predicted values. If your results match the predicted values, the testing is normal. If not, your doctor will discuss the results and what they may mean for you.