Lung Function Tests

  • Lung function tests (LFTs), also called pulmonary function tests (PFTs), are used to diagnose respiratory conditions such as asthma and COPD (chronic obstructive pulmonary disease).
  • They may also be used to see how well treatments, such as asthma medications, are working.
  • Lung function tests measure the size of your lungs, how well the air can move in (inspiration) and out of the lungs (expiration), and how well your lungs deliver oxygen to your blood.
  • Breathing tests include spirometry, peak flow meter, lung volume measurement, and lung diffusion capacity. Pulse oximetry and arterial blood gas tests are used to measure the oxygen level in your blood.
  • For breathing tests, you will breathe through a tube that's attached to a testing machine. You may be asked to breathe normally, slowly, or rapidly. You also may be asked to inhale and then exhale a small amount of gas.
  • Ask your doctor if you need to stop taking any medications before the test.
  • You can return to your normal activities and restart your medicines after lung function tests. Talk to your doctor about when you'll get the test results.

There are two categories of lung function tests: breathing tests and tests to measure oxygen level.

Breathing tests


Spirometry measures how much (volume) and how fast (flow) you can move air into and out of your lung. A computerized sensor (which is part of the spirometer) calculates and graphs the results. The results demonstrate an person's air flow rates or the volume forced out within the first second. This is the Forced Expiratory Volume in the first second (FEV1).  This indicates whether or not there is airway obstruction. Spirometry also records the total volume of air forced out of the lungs. This is the Forced Vital Capacity (FVC). 

During the test, a technician will ask you to take a deep breath in and then blow as hard as you can into a tube connected to a small machine. Your doctor may have you inhale a medicine that helps open your airways. He or she will want to see whether the medicine changes or improves the test results.

Spirometry is done to look for diseases and conditions that affect how much air you can breathe in, such as sarcoidosis or lung tissue scarring. It's also done to look for diseases that affect how fast you can breathe air out, like asthma and COPD (chronic obstructive pulmonary disease).

Spirometry can show whether you have:

  • Blockage (obstruction) in your airways. This may be a sign of asthma, COPD (chronic obstructive pulmonary disease), or another obstructive lung condition.
  • Smaller than normal lungs (restriction). This may be a sign of heart failure, damage or scarring of the lung tissues, or another restrictive lung condition.

Peak flow meter

A peak flow meter is a small, hand-held device that you blow into. It shows how well air moves out of your lungs. People who have asthma sometimes use this device. It helps them (and their doctors) check their breathing. A peak flow meter can be used at home or in a doctor's office.

Lung volume measurement

This test measures the size of your lungs and how much air you can breathe in and out. During the test, you sit inside a glass booth and breathe into a tube that's hooked to a computer.

Sometimes you breathe in nitrogen or helium gas and then blow it out. The gas you breathe out is then measured to test how much air your lungs can hold.

The test can help diagnose lung tissue scarring or a stiff and/or weak chest wall.

Lung diffusing capacity

This test measures how well oxygen passes from your lungs to your bloodstream. During this test, you breathe in a gas through a tube. You hold your breath for a brief moment and then blow the gas out.

Abnormal test results may suggest loss of lung tissue, emphysema (a type of COPD), very bad scarring, or problems with blood flow through the body's arteries.

These tests may not show what’s causing breathing problems. Other tests, such as a cardiopulmonary exercise test, also may be done. This test measures how well your lungs and heart work while you exercise on a treadmill or bicycle.

Tests to measure oxygen level

Pulse oximetry and arterial blood gas tests show how much oxygen is in your blood. These tests are also called blood oxygen tests.

During pulse oximetry, a small light is placed over your fingertip, earlobe, or toe to measure the oxygen. This test is painless and no needles are used.

During an arterial blood gas test, your doctor inserts a small needle into an artery, usually in your wrist. He or she takes a sample of blood. The oxygen level of the blood is checked in a lab.

After the needle is removed, you may feel mild pressure or throbbing at the needle site. Applying pressure to the area for 5 to 10 minutes should stop the bleeding. You will be given a small bandage to place on the area.

Testing in infants and young children

Spirometry and other measures of lung function usually can be done in children older than 6 years, if they can follow directions well. Spirometry may be tried in children as young as 5 years. However, technicians who have special training with young children may need to do the testing.

Instead of spirometry, a growing number of medical centers measure respiratory system resistance. This is another way to test lung function in young children.

The child wears nose clips and has his or her cheeks supported with an adult's hands. The child breathes in and out quietly on a mouthpiece, while the technician measures changes in pressure at the mouth. During these lung function tests, parents can help comfort their children and encourage them to cooperate.

Very young children (younger than 2 years) may need an infant lung function test. This requires special equipment and medical staff. This type of test is only available at a few centers. The doctor gives the child medicine to help him or her sleep through the test.

A technician places a mask over your child's nose and mouth and a vest around your child's chest. The mask and vest are attached to a lung function machine. The machine gently pushes air into your child's lungs through the mask. As your child exhales, the vest slightly squeezes his or her chest. This helps push more air out of the lungs. The exhaled air is then measured.

In children younger than 5 years, the doctor likely will use signs and symptoms, medical history, and a physical exam to diagnose lung problems.

Pulse oximetry and arterial blood gas tests may be used for children of all ages.

Reference: The National Heart, Blood, and Lung Institute

This information is for general educational uses only. It may not apply to you and your personal medical needs. This information should not be used in place of a visit, call, consultation with or the advice of your physician or health care professional.

Communicate promptly with your physician or other health care professional with any health-related questions or concerns.

Be sure to follow specific instructions given to you by your physician or health care professional.

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