A pulmonary function test (PFT) shows how well your lungs move air in and out, how much air they can hold, and how efficiently they transfer oxygen into your bloodstream. These measurements help identify whether a breathing problem is caused by narrowed airways, stiff or scarred lung tissue, or a breakdown in gas exchange. Most PFTs involve three core components: spirometry, lung volume measurement, and diffusion capacity testing.
What Spirometry Measures
Spirometry is the most common part of a pulmonary function test. You breathe into a mouthpiece as hard and fast as you can, and the machine captures two key numbers. The first is your forced vital capacity (FVC), which is the total volume of air you can push out of your lungs in one full exhale. The second is your FEV1, the volume of air you exhale in just the first second of that effort.
The ratio between these two numbers is where the real diagnostic power lies. Your FEV1/FVC ratio tells your doctor what percentage of your total air you can force out in one second. A low ratio, generally below the 5th percentile for your age, sex, and height, signals an obstructive pattern. That means something is narrowing your airways and slowing the flow of air out of your lungs. This is the hallmark of conditions like asthma, COPD, and chronic bronchitis.
If both your FEV1 and FVC are reduced but the ratio between them stays normal, that points toward a restrictive pattern instead, meaning your lungs can’t fully expand. This can happen with pulmonary fibrosis, chest wall deformities, or neuromuscular conditions that weaken the breathing muscles.
Lung Volume and Total Capacity
Spirometry only captures the air you can actively move. A full PFT also measures the air that stays trapped in your lungs after you exhale completely, called the residual volume. To measure this, you typically sit inside a clear, phone-booth-sized chamber called a body plethysmograph. The chamber detects tiny pressure changes as you breathe, which allows the technician to calculate your total lung capacity: the full amount of air your lungs can hold at maximum inflation.
These volume measurements are essential for confirming restrictive lung disease. A total lung capacity below the lower limit of normal confirms that your lungs genuinely can’t hold as much air as they should, rather than simply appearing small on spirometry. In obstructive diseases like emphysema, the opposite often happens. Air gets trapped because damaged airways collapse during exhalation, so residual volume and total lung capacity actually increase. This hyperinflation is why people with advanced emphysema often feel like they can’t get a satisfying breath, even though their lungs are technically overfull.
Gas Exchange and Diffusion Capacity
The third major component, called a diffusion capacity test (DLCO), measures how well oxygen crosses from your lungs into your blood. During this test, you inhale a gas mixture containing a tiny, harmless amount of carbon monoxide along with a tracer gas. You hold your breath for about 10 seconds, then exhale. The machine measures how much carbon monoxide was absorbed. Because carbon monoxide transfers across lung tissue in a way that closely mimics oxygen, this gives a reliable picture of your lungs’ gas exchange efficiency.
Several factors determine your diffusion capacity: the total surface area of the tiny air sacs (alveoli) in your lungs, the thickness of the membrane separating those air sacs from your blood vessels, the amount of blood flowing through your lung capillaries, and your hemoglobin levels. A low DLCO with an obstructive pattern on spirometry points toward emphysema, where the walls between air sacs have been destroyed and surface area is lost. A low DLCO with a restrictive pattern suggests interstitial lung disease or pulmonary fibrosis, where scarring thickens the membrane and blocks gas transfer. A normal DLCO alongside obstruction is more typical of asthma or early COPD, where the air sacs themselves are still intact.
The Bronchodilator Response
If your spirometry shows an obstructive pattern, you’ll often be given an inhaled bronchodilator (a medication that relaxes airway muscles) and then asked to repeat the breathing test 15 to 20 minutes later. A positive response is defined as an improvement in FEV1 or FVC of at least 12% and at least 200 milliliters compared to your baseline. This reversibility is a hallmark of asthma, where the airway narrowing is largely caused by muscle spasm and inflammation that can be relieved with medication. In COPD, the airways are structurally damaged and respond less, so improvement after the bronchodilator is typically smaller or absent.
What the Results Look Like
Your results are compared to predicted values based on your age, sex, height, and ancestry. Current guidelines from the European Respiratory Society and American Thoracic Society recommend using standardized reference equations from the Global Lung Function Initiative rather than older fixed cutoffs like “80% of predicted.” Results are expressed as z-scores, which show how far your values fall from what’s expected for someone like you. A z-score between negative 1.65 and negative 2.5 indicates mild impairment. Moderate impairment falls between negative 2.51 and negative 4.0, and anything below negative 4.1 is severe.
Notably, the old practice of applying fixed racial adjustment factors to predicted values has been formally discouraged. When a person’s ancestral background is uncertain, standardized reference equations for mixed or unspecified populations are used instead.
How to Prepare for the Test
Getting accurate results depends heavily on preparation. On the day of your test, do not smoke at all. Avoid heavy exercise for at least 30 minutes beforehand, skip alcohol for at least four hours, and don’t eat a large meal within two hours of your appointment. Wear loose, comfortable clothing that won’t restrict your ability to take a deep breath.
If you use inhalers, you’ll likely need to stop them before the test. Short-acting rescue inhalers like albuterol should be withheld for at least 6 hours. Short-acting anticholinergic inhalers need a 12-hour hold. Long-acting inhalers require 24 to 36 hours, depending on the specific medication. Bring a list of all your medications, including doses, so the technician knows what you’ve been taking.
If you’re on supplemental oxygen, you’ll typically be asked to come off it for a few minutes before the diffusion and plethysmography portions of the test. The full battery of tests usually takes 30 to 60 minutes, and while the forced breathing maneuvers can feel tiring, they aren’t painful. You’ll likely repeat each test several times so the technician can ensure the readings are consistent.
Conditions PFTs Help Identify
No single PFT number diagnoses a disease on its own, but the pattern across all three components narrows the possibilities considerably. An obstructive pattern with a positive bronchodilator response and normal diffusion capacity is classic for asthma. Obstruction with poor reversibility and low diffusion capacity fits emphysema. A restrictive pattern with low diffusion capacity is characteristic of interstitial lung diseases like pulmonary fibrosis or hypersensitivity pneumonitis. Restriction with normal diffusion capacity may point to a problem outside the lungs themselves, such as obesity, chest wall stiffness, or neuromuscular weakness limiting how fully the lungs can expand.
PFTs also play a role in tracking disease progression over time. Repeating the tests at regular intervals shows whether lung function is stable, declining, or improving with treatment. For people with chronic lung conditions, this trend line is often more informative than any single snapshot.