Pulmonary function tests (PFTs)

If the care team orders a pulmonary function test, or PFT, it’s important to understand what the test involves and how to prepare.

What is a pulmonary function test?

Pulmonary function tests are also called lung function tests. They're a group of tests that measure the amount of air patients' lungs can hold, how well they can empty their lungs of air and how well the lungs can get oxygen from the air they breathe into the bloodstream.

Lung function 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.

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

PFTs may also be 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 the patient's lungs are healthy enough for general anesthesia if he or she needs surgery. PFTs are very important to determine how much of the lung can be resected, if lung resection is required.

Pulmonary function test types

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

Spirometry

Spirometry measures the rate of air flowing in and out of the lungs. It also gives a rough estimate of the size of the patient's lungs. To do this test, the patient will place a mouthpiece in his or her mouth and a clip on his or her nose, then breathe into a tube connected to a special device, called a spirometer. The patient will be asked to breathe normally and also to breathe as deeply as he or she can.

Spirometry is the most basic and useful PFT. It can tell whether the patient has a breathing problem caused by an obstruction of air that's left in his or her lungs or a restriction of air entering the lungs. However, it doesn’t give an accurate measurement of the patient's total lung capacity.

Lung volume testing

Lung volume testing—also called plethysmography—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 the patient's lungs can hold.

This test is important for diagnosing obstructive, air-trapping diseases like COPD.

Lung diffusion capacity

Lung diffusion capacity tests measure how well oxygen gets into the patient's blood. During the test, the patient will breathe in and out through a tube, like the other tests, but the breathing instructions will be different. With this test, the patient may also have a blood test to measure his or her hemoglobin level.

Pulse oximetry

Pulse oximetry can also be used as part of PFTs. It’s a simple test that measures the oxygen level in the blood through a sensor placed on the patient's finger.

Arterial blood gas testing

Another test that may be part of PFTs is an arterial blood gas test, a blood test taken from an artery, usually in the patient's wrist. This blood test measures oxygen and carbon dioxide.

How to prepare for a pulmonary function test

Different PFT testing sites have different instructions for preparation, so it’s important to ask the health care provider about instructions before the pulmonary function 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.
• The patient may be instructed not to use any breathing medications before the test.
• Ask the care team if there are any medications that should be stopped before the test.

What happens during PFTs?

Pulmonary function tests aren’t typically painful and are usually performed by a pulmonary function technician. The patient may have one or more types of PFTs. Possible side effects include feeling tired, light-headed or dizzy during or after a test. Patients who have claustrophobia and are undergoing plethysmography testing may feel anxious, so tell the technician about any history of claustrophobia.

What happens during testing depends on the specific test. What to expect with each common test:

During spirometry:

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

During plethysmography:

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

During lung diffusion testing:

• The patient will be asked to empty his or her lungs by breathing out as much as possible.
• The patient will then take a quick deep breath and hold it for 10 seconds. Other breathing instructions may follow.
• This test takes about 30 minutes.

After the pulmonary function test, the patient may rest for a bit if he or she is tired or light-headed before resuming normal activities.

Pulmonary function test results

According to a Concise Review for Clinicians review, spirometry testing creates a graph called a flow-volume curve. The results may indicate whether the patient has an obstruction or restriction in breathing, as well as how much air is left in the patient's lungs after he or she exhales.

PFT results may reveal a variety of measurements:

• Peak expiratory flow rate (PEFR) measures the fastest rate the patient can force air out of his or her lungs.
• Vital capacity (VC) measures the total volume of air the patient can exhale after inhaling deeply.
• Forced vital capacity (FVC) measures the amount of air the patient can quickly and forcefully exhale after inhaling deeply.
• Tidal volume (VT) measures how much air is inhaled or exhaled when the patient is breathing normally.
• Minute volume (MV) measures how much air the patient can exhale per minute.
• Functional residual capacity (FRC) measures how much air remains in the patient’s lungs following normal exhalation.
• Total lung capacity (TLC) measures the total lung volume when the lungs are filled with the maximum amount of air.
• Residual volume (RV) measures how much air remains in the patient’s lungs after exhaling as much as possible.
• Forced expiratory volume (FEV) measures how much air is expired during the FVC test’s first, second and third seconds.
• Forced expiratory flow (FEF) measures the average flow rate during the FVC test’s middle half.

Since everyone’s lungs are different, the patient's test results are compared to normal PFT results of people who are of the same age, sex and height. These results are called predicted values. If the patient's results match the predicted values, the testing is normal. If not, the care team will discuss the results and what they may mean for the patient.