Pulmonary function tests (PFTs) are a series of different breathing tests led by a trained pulmonary function technologist, usually done at a hospital or clinic. There are global standards and guidelines that help make sure that everyone does and interprets pulmonary function tests in the same way.

To learn about your lung health, your doctor may want you to have several pulmonary function tests done including spirometry, lung volumes, diffusing capacity, and airway resistance.

Most of these breathing tests are done by blowing into a mouthpiece while sitting in a chair. Your doctor may also order a blood test called arterial blood gas as an additional test.

Before you have PFTs, you may get specific instructions on how to get ready for the tests, such as:

• Wear loose clothing which will not restrict your ability to breathe deeply.
• Avoid large meals prior to your test time which will make it more comfortable for you to breathe deeply.
• You may be asked to not use your inhalers on the day of the test, if possible. Check with your physician or the pulmonary function testing site about specific instructions for your breathing medications prior to the testing.

At your appointment:

• The equipment will be set up for you after being cleaned and disinfected, including a clean mouthpiece.
• The technologist will tell you what to do before each test. Listen carefully and follow the coaching from the technologist. If you don’t understand what to do, ask him or her to tell you in a different way.

Spirometry

Spirometry is a very common test to help you and your doctor understand your asthma better, and check how it is improving with treatment. The NHLBI guidelines recommend that all persons with asthma have spirometry done in the beginning of care, again as treatments are started, and at least every year for continuing care.

Spirometry measures how much air you can inhale (breathe in) and exhale (breathe out) as well as how fast you can exhale. For this test, you may be asked to breathe quickly, forcefully, or slowly. The test is always repeated at least three times and often more to be sure that the test is reliable.

Your doctor may order a bronchodilator to be given as part of spirometry. A bronchodilator is an inhaled medication that may dilate, or open up, your airways. Spirometry is often done before and after the bronchodilator to show any response to the medicine. Your response may help your doctor find out what kind and how much, if any, airway disease you may have, and whether you need medication to improve your breathing.

Spirometry measures many different volumes (how much) and flow rates (how fast the air moves). Some of the more common measurements done by spirometry include a test called forced vital capacity.

Forced vital capacity (FVC) – measures the amount of air exhaled from full inspiration to full expiration (empty). You will be asked to breathe in as deeply as you can and immediately blow out as hard and fast as you can until you feel you cannot blow any longer. With the help of a computer, the FVC effort will make a graph called a “flow volume curve” or “flow volume loop.” This graph will look different for everyone. The measurements taken from this test are key in helping your doctor diagnose asthma.

• Forced Expiratory Volume in the first second (FEV1) – measures the amount of air you can forcefully blow out in the first second of the FVC. If this number is lower than what is considered normal, it may mean asthma.
• The ratio between your FEV1 and FVC known as FEV1/FVC can also help diagnose asthma. If your FEV1/FVC is lower than normal, it can mean asthma. It is normal for this number to go down somewhat with age.
• Peak Expiratory Flow (PEF) or Peak Flow (PF) – the fastest flow rate reached at any time during a FVC. This depends a lot on your effort during the test.

Lung Volumes

The doctor may also order tests that measure your lung volumes. There are eight separate volumes of air that can be measured during the lung volumes test. The three most commonly used ways of measuring lung volumes are:

• Body Box – also known as plethysmography, is done while sitting in an enclosed clear chamber while asked to perform a series of very small panting breaths. This is the most accurate way to measure lung volumes
• Nitrogen Washout – done by normal breathing of pure oxygen while exhaled gas is collected and analyzed
• Helium dilution – done by normal breathing of gas mixture of helium and oxygen

People with asthma may show changes in their lung volumes. This can help the doctor diagnose and treat asthma.

Airway Resistance

Airway resistance is a test that measures how easy it is for the air to move in and out of your lungs. It is an easy test to do by plethysmography. You will either be asked to just breathe normally through a mouthpiece or breathe slightly faster through a mouthpiece. Asthma often causes the airways to narrow so this test helps the doctor understand the impact of asthma on the narrowing of your airways.

Nitric Oxide Test (FENO)

Nitric oxide is produced throughout the body, including in the lungs, to fight inflammation and relax tight muscles. This quick test measures the amount of nitric oxide gas that is exhaled, by having you breathe into a mouthpiece attached to a tube that leads to an electronic measurement device. High levels of exhaled nitric oxide in your breath can mean that your airways are inflamed, which can help the doctor diagnose asthma. Nitric oxide testing is also done to help predict whether or not steroid medications, which decrease inflammation, are likely to be helpful for your asthma.

Bronchial Provocation or Challenge Tests

• Methacholine challenge: Methacholine is a known asthma trigger that, when inhaled, will cause mild constriction of your airways. If you react to the methacholine, you likely have asthma. This test may be used even if your initial lung function test is normal.
• Provocative testing for exercise and cold-induced asthma: In these tests, your doctor measures your airway obstruction before and after you perform vigorous physical activity or take several breaths of cold air.

Diffusing Capacity (DLCO)

Diffusing capacity of the lungs measures how well gases such as oxygen move from the lungs into the blood. There are several ways to measure this, but the most common way is the ten second single breath-hold technique. Results of this test can tell your doctor about the amount of damage or abnormality that is present where the air and the blood meet. While this test does not specifically test for asthma, it may help your doctor to diagnose you correctly.

Arterial Blood Gases (ABG)

This is a blood test that may be ordered with your PFTs to give your doctor even more information about your lung health. ABGs can show how well your lungs are getting oxygen into your blood and carbon dioxide out of your blood. For this test, a sample of blood is drawn from your artery, from your wrist or elbow area.

What we can learn from PFTs

A doctor will look over the results of your PFTs and see how you are doing by comparing them to predicted values normal for a person your age, height, sex and ethnicity. Height is important because taller people may have bigger lungs. There are many things that can change the results in PFTs. These include not only the health of your lungs but also the skill of the person testing you, your effort, differences in equipment, and differences in hospital or clinic procedures. A medical diagnosis is not likely to be made from PFTs alone.

Most lung diseases are labeled either as restrictive or obstructive. They are not the names of actual lung diseases, but the labels help group types of lung diseases together. Asthma is an obstructive disease, which means that it causes people to have trouble breathing out.

Words like mild, moderate, or severe may be used to describe how severe the problem is. Ask your doctor to explain the results so that you know what they mean for you.

A PFT may be repeated as often as your doctor thinks it is needed. Lung problems can be checked for change by regular pulmonary tests. Check with your insurance company to see how often PFTs are covered for you – the NHLBI guidelines recommend that all persons with asthma have spirometry done in the beginning of care, again as treatments are started, and at least every year for ongoing care.

Well controlled asthma means maintaining the best lung function possible, but keep in mind that quality of life is not found in the results of your PFTs. Each person is unlike any other, and many people live full lives with limited lung function. The key is to look for ways to keep or improve your quality of life, including exercise, breathing techniques, proper medications, equipment aids and social or emotional support. Exercising and doing breathing exercises can help people with asthma get the most from their lungs. If exercise or exertion is a trigger for your asthma, talk to your doctor about ways to prevent symptoms. Learn more about exercise and asthma.

Adapted from “Understanding PFTs” by the Alpha-1 Association. Updated July 2017.