What Is a Pulmonary Function Test and What Does It Measure?
If your doctor has recommended a pulmonary function test, you might be wondering what exactly it involves and what the results will tell you. A pulmonary function test is one of the most informative tools available for evaluating how well your lungs are working, and it is far more detailed than a routine physical exam can provide.
Understanding what a pulmonary function test measures and why it matters is the first step toward taking your lung health seriously. Whether you have been experiencing shortness of breath, a persistent cough, reduced stamina, or you simply want a baseline picture of your respiratory health, this guide covers everything you need to know before your appointment.
What Is a Pulmonary Function Test?
A pulmonary function test, commonly referred to as a PFT, is a group of non-invasive breathing tests that measure how well your lungs take in air, hold air, and move air in and out. The results give your provider a detailed picture of your lung capacity, airflow efficiency, and overall respiratory function.
Unlike a chest X-ray, which shows the physical structure of the lungs, a pulmonary function test measures performance. It tells your provider not just what your lungs look like, but how well they are actually doing their job of delivering oxygen to your body.
PFTs are used to diagnose respiratory conditions, monitor existing lung disease, assess the impact of occupational exposures, and evaluate lung health before certain medical procedures. They are also used as a baseline measurement so that changes in lung function can be tracked over time.
What Does a Pulmonary Function Test Measure?
A complete pulmonary function test evaluates several distinct aspects of lung performance. Each measurement tells a different part of the story.
Spirometry — The Most Common PFT
Spirometry is the foundational component of most pulmonary function evaluations. During this test, you breathe into a mouthpiece connected to a device called a spirometer, which records the speed and volume of air you move with each breath.
The two most important values spirometry measures are:
- FEV1 (Forced Expiratory Volume in 1 second). This measures how much air you can forcefully exhale in the first second of a breath out. It is one of the most reliable indicators of airway obstruction.
- FVC (Forced Vital Capacity). This measures the total amount of air you can exhale after taking the deepest breath possible.
The ratio of FEV1 to FVC is particularly meaningful. A lower ratio suggests obstructive lung disease, such as asthma or COPD, where air is trapped and cannot move out efficiently. A normal ratio with reduced overall volumes may suggest a restrictive pattern, where the lungs cannot fully expand.
Lung Volume Tests
Spirometry measures how much air moves in and out, but lung volume tests go further by measuring the total amount of air the lungs can hold and the amount that remains after you exhale completely. Key measurements include:
- Total Lung Capacity (TLC). The total volume of air in the lungs after a maximum inhalation.
- Residual Volume (RV). The amount of air that remains in the lungs after a full exhalation. This air cannot be breathed out and is always present.
- Functional Residual Capacity (FRC). The amount of air remaining after a normal, relaxed exhalation.
These measurements help identify restrictive lung conditions, where the lungs cannot fully expand due to scarring, muscle weakness, or structural changes.
Diffusion Capacity (DLCO)
This test measures how efficiently oxygen crosses from the air sacs in your lungs into your bloodstream. It involves inhaling a small amount of a tracer gas, holding your breath briefly, then exhaling while the device analyzes how much of the gas was absorbed.
A reduced diffusion capacity can indicate conditions such as pulmonary fibrosis, emphysema, or pulmonary hypertension, where the membrane between the air sacs and blood vessels is thickened or damaged.
Peak Flow Measurement
Peak flow measures the maximum speed at which you can exhale air. It is a simpler, faster test often used to monitor asthma and assess how well airways are opening and closing in response to treatment or environmental triggers.
What Conditions Can a Pulmonary Function Test Detect?
A pulmonary function test provides the clinical data needed to diagnose or rule out a range of respiratory conditions, including:
- Asthma. Characterized by variable airflow obstruction, often showing a reduced FEV1 that improves after bronchodilator medication is administered during testing.
- Chronic Obstructive Pulmonary Disease (COPD). Identified by a persistently reduced FEV1/FVC ratio, indicating irreversible airflow limitation. According to the American Lung Association, COPD affects more than 16 million Americans, and many cases go undiagnosed for years.
- Pulmonary fibrosis. A restrictive condition where lung tissue becomes scarred and stiff, reducing total lung capacity and diffusion capacity.
- Occupational lung disease. Conditions caused by long-term exposure to dust, smoke, chemicals, or other airborne hazards, which are particularly relevant for firefighters, construction workers, and other high-exposure professions.
- Pulmonary hypertension. Elevated pressure in the lung’s blood vessels, which can reduce diffusion capacity and exercise tolerance.
PFTs do not diagnose these conditions on their own. Results are always interpreted alongside symptoms, medical history, and other diagnostic findings.
What Happens During a Pulmonary Function Test?
Knowing what to expect can make the appointment feel much less uncertain. The process is straightforward and causes no pain or discomfort.
- Preparation. You will be asked to avoid smoking, heavy exercise, and certain medications such as inhalers in the hours before your test. Your provider will give you specific instructions beforehand.
- Baseline measurements. Your height, weight, age, and sex are recorded because normal lung function values vary based on these factors.
- Spirometry. You will sit upright, wear a nose clip, and breathe through a mouthpiece. You will be asked to inhale as deeply as possible and then exhale as hard and fast as you can for several seconds. This is typically repeated three times to ensure consistent results.
- Additional tests if indicated. Depending on what your provider is evaluating, lung volume testing and diffusion capacity testing may follow. These involve slightly different breathing techniques but are equally non-invasive.
- Bronchodilator testing. If airflow obstruction is detected, you may be given an inhaled bronchodilator medication and retested after 15 to 20 minutes to determine whether the obstruction is reversible, which is a key distinction between asthma and COPD.
The entire appointment typically takes between 30 and 60 minutes.
How Are Pulmonary Function Test Results Interpreted?
Your results are compared against predicted normal values based on your age, height, sex, and ethnicity. These predicted values represent what would be expected in a healthy person with your same characteristics.
Results are typically expressed as a percentage of the predicted value. A result at or above 80 percent of the predicted value is generally considered within the normal range. Results below this threshold may indicate reduced lung function, though the pattern of reduction, whether obstructive, restrictive, or mixed, guides the interpretation.
Your provider will explain your results in the context of your symptoms and health history. A single number in isolation rarely tells the complete story, which is why clinical judgment is always part of the interpretation process.
Who Should Get a Pulmonary Function Test?
A pulmonary function test is appropriate for a wide range of individuals, not just those with obvious breathing problems. Testing is particularly recommended for:
- People experiencing persistent shortness of breath, chronic cough, or unexplained wheezing
- Individuals with a history of smoking or long-term exposure to secondhand smoke
- Workers with occupational exposure to dust, fumes, chemicals, or smoke, including firefighters, construction workers, and industrial laborers
- Anyone with a known or suspected diagnosis of asthma, COPD, or another respiratory condition who needs their current lung function documented
- Individuals with a family history of lung disease
- People preparing for surgery, where pre-operative lung function assessment may be required
- Anyone seeking a baseline measurement of lung health for long-term monitoring purposes
If you fall into any of these categories and have not had a pulmonary function test, it is worth discussing with your provider whether one is appropriate for you.
Frequently Asked Questions
No. A pulmonary function test is entirely non-invasive. The most physically demanding part is breathing as hard and fast as you can during spirometry, which can feel slightly tiring but causes no pain. Some people feel briefly lightheaded after the forced exhalation, which passes quickly.
Your provider will give you specific instructions, but generally you should avoid smoking for at least four to six hours before the test, avoid heavy exercise on the day of testing, and hold any inhaled bronchodilator medications unless your provider instructs otherwise. Wear loose, comfortable clothing that does not restrict your breathing.
It is an important tool in the diagnosis of asthma, but it is typically used alongside a clinical history and physical examination. Asthma is characterized by variable airflow obstruction, so a normal PFT result on one day does not necessarily rule it out. Your provider may recommend additional testing such as a bronchoprovocation challenge if asthma is strongly suspected despite a normal baseline result.
For people with known lung conditions, testing frequency depends on the condition and how well it is controlled. For healthy individuals seeking preventive monitoring, testing every few years is generally sufficient. People with high occupational exposure to respiratory hazards may benefit from more frequent baseline comparisons.
A chest X-ray shows the physical structure of the lungs and surrounding tissues. A pulmonary function test measures how the lungs perform. They provide complementary information, and your provider may order both depending on what they are evaluating.
Conclusion
A pulmonary function test is one of the most valuable tools available for understanding how your lungs are performing, catching problems early, and tracking changes over time. It is non-invasive, takes less than an hour, and provides your provider with detailed, actionable data that a routine examination simply cannot match.
If you have been experiencing breathing changes, have a history of occupational exposure, or simply want to establish a baseline for your respiratory health, a pulmonary function test is a straightforward next step. Learn more about our cardiovascular and pulmonary testing services and what a comprehensive evaluation includes.
For more context on when testing is recommended and what the process involves from start to finish, read our guides on when cardiovascular and pulmonary testing is needed and how cardiovascular and pulmonary testing works step by step.
Disclaimer
This article is intended for informational purposes only and does not constitute medical advice. The content provided here is not a substitute for professional medical evaluation, diagnosis, or treatment. If you are experiencing respiratory symptoms or have concerns about your lung health, consult a qualified healthcare provider before making any decisions about your care. Individual results and experiences may vary.
References
American Lung Association — https://www.lung.org
National Heart, Lung, and Blood Institute — https://www.nhlbi.nih.gov
American Thoracic Society spirometry guidelines — https://www.thoracic.org
