Learn About Pulmonary Function Tests

What Are Pulmonary Function Tests?

Pulmonary function tests (PFTs) are medical measurements that show how well your lungs work. These tests measure how much air your lungs can hold, how quickly you can move air in and out, and how well oxygen moves into your bloodstream. Doctors use these tests to diagnose lung diseases, monitor existing conditions, and track how treatments are working.

Your lungs have two main jobs: bringing oxygen into your body and removing carbon dioxide. When you breathe in, air travels through your windpipe into smaller tubes called bronchi, which branch into even tinier tubes. At the end of these tubes are millions of tiny air sacs called alveoli, where oxygen enters your blood and carbon dioxide leaves. Pulmonary function tests measure different parts of this process.

The term "pulmonary function tests" actually refers to a group of different tests, not just one. Some tests measure how much air your lungs can hold. Others measure how fast you can breathe air in and out. Still others measure how well oxygen moves from your lungs into your blood. Your doctor might order one test or several tests together, depending on what they need to learn about your lung health.

According to the American Thoracic Society, about 28 million Americans have some form of chronic lung disease. Pulmonary function tests play an important role in identifying these conditions early. The tests are safe, non-invasive (meaning nothing enters your body), and can be done in an outpatient clinic or doctor's office. Most people tolerate them well, though they do require effort and cooperation from the person being tested.

Practical Takeaway: Pulmonary function tests are breathing measurements that help doctors understand how well your lungs work and identify potential problems. Understanding what these tests measure can help you prepare and know what to expect when your doctor orders them.

Common Types of Pulmonary Function Tests

Spirometry is the most common type of pulmonary function test. During spirometry, you breathe into a machine called a spirometer. You'll be asked to take a deep breath and then exhale as hard and as fast as you can into a tube. The spirometer measures how much air you can exhale and how quickly you can do it. This test helps diagnose conditions like asthma, chronic obstructive pulmonary disease (COPD), and restrictive lung diseases.

The test produces several important measurements. FVC (forced vital capacity) is the total amount of air you can exhale after taking the deepest breath possible. FEV1 (forced expiratory volume in one second) is how much air you can exhale in just one second. The ratio between these two numbers (FEV1/FVC) tells doctors a lot about your lung function. In healthy people, this ratio is typically 70 percent or higher, meaning you can exhale at least 70 percent of your total lung capacity in the first second.

Lung volume tests measure how much air your lungs can hold. Some of this air stays in your lungs even after you exhale completely—this is called residual volume. Regular spirometry doesn't measure residual volume, so doctors use additional methods. Helium dilution and nitrogen washout are two techniques that measure total lung capacity and residual volume by having you breathe air mixed with special gases.

Diffusion testing, also called DLCO (diffusing capacity of the lungs for carbon monoxide), measures how well oxygen passes from your lungs into your bloodstream. During this test, you inhale a mixture of gases including a small amount of carbon monoxide, hold your breath briefly, then exhale. The machine measures how much carbon monoxide your lungs absorbed. Lower-than-normal results may indicate problems like pulmonary fibrosis, emphysema, or anemia.

Bronchial challenge testing measures how sensitive your airways are. During this test, you inhale gradually increasing amounts of a substance (usually methacholine) while having your spirometry measured. If your airways narrow significantly, it suggests airway hyperresponsiveness, which is common in asthma. This test helps doctors distinguish between different types of lung conditions.

Practical Takeaway: Different pulmonary function tests measure different aspects of lung health. Spirometry is the most common, but your doctor may order additional tests depending on what information they need about your specific situation.

Why Doctors Order Pulmonary Function Tests

Doctors order pulmonary function tests for several important reasons. If you have symptoms like persistent cough, shortness of breath, wheezing, or chest pain, your doctor may order these tests to determine the cause. The results help differentiate between various lung diseases, heart problems, and other conditions that can cause similar symptoms. A persistent cough lasting more than three weeks is one of the most common reasons doctors refer patients for pulmonary function testing.

These tests are also used to diagnose specific lung conditions. Asthma affects about 8 percent of the U.S. population, and pulmonary function tests help confirm the diagnosis. COPD, which includes emphysema and chronic bronchitis, affects approximately 16 million Americans, and these tests are essential for diagnosis and monitoring. Restrictive lung diseases like pulmonary fibrosis, which causes lung tissue to scar and stiffen, also require pulmonary function testing for diagnosis.

Pulmonary function tests help doctors monitor how well treatments are working. If you have asthma or COPD and your doctor prescribes medications like inhalers, repeat testing shows whether the medications are improving your lung function. Your results can be compared to previous tests to see if you're getting better, staying stable, or declining. This information helps doctors adjust your treatment plan.

These tests also screen for lung disease in people at high risk. Workers exposed to dust, chemicals, or other harmful substances may need regular testing to catch occupational lung diseases early. Smokers often have pulmonary function tests to assess lung damage. Before surgery, patients with lung disease may undergo testing to make sure their lungs are healthy enough for the procedure. People with a family history of lung disease or cystic fibrosis may also be screened.

Pulmonary function tests can also evaluate shortness of breath that seems disproportionate to the patient's physical condition or identify whether shortness of breath is coming from the lungs or the heart. Some patients have symptoms that could be caused by several different problems, and these tests help narrow down the possibilities.

Practical Takeaway: Doctors use pulmonary function tests to diagnose lung conditions, monitor treatment effectiveness, and screen for disease in at-risk populations. Understanding why your doctor ordered these tests can help you prepare mentally and physically.

How to Prepare for Pulmonary Function Tests

Proper preparation improves the quality of your test results and helps ensure accurate diagnosis. Your doctor's office should provide specific preparation instructions when they schedule your test. Generally, you should avoid strenuous exercise for 30 minutes before testing, as this can temporarily affect your breathing and test results. You should also avoid heavy meals right before testing, as a full stomach can restrict your ability to take deep breaths.

Certain medications can affect test results. If you use bronchodilators (medications that open airways), your doctor will tell you whether to take them before testing or to skip them that day. Some doctors want you to take your regular medications to see how you function with them, while others want to see your baseline lung function without them. Ask your doctor specifically about each medication you take regularly.

Avoid smoking for at least one hour before your test, and ideally for 24 hours before. Smoking irritates your airways and can affect measurements. Similarly, avoid caffeine before testing, as caffeine can stimulate your nervous system and affect how your body responds during the test. Wear comfortable, loose-fitting clothes that don't restrict your breathing.

During the test itself, following instructions carefully is crucial. The technician will explain each step before it happens. When asked to take a deep breath, breathe in as much as you can. When asked to exhale forcefully, give it maximum effort—this is particularly important for spirometry measurements. Many people don't exhale forcefully enough on their first try, and the test may need to be repeated several times. The technician will give you encouragement and feedback to help you perform your best.

It's normal to feel slightly lightheaded or dizzy after forceful spirometry testing, particularly if you're performing multiple maneuvers. This