A pulmonary function test (PFT) is a group of breathing tests that measure how well air moves in and out of your lungs and how efficiently oxygen reaches your bloodstream. A full PFT session typically includes three core tests: spirometry, lung volume measurement, and diffusion capacity. The entire visit takes between 15 and 45 minutes depending on which tests your doctor orders.
Spirometry: The Foundation of Every PFT
Spirometry is the most common component and often the first test performed. You’ll take the deepest breath you can, seal your lips around a mouthpiece, then blast all the air out as hard and fast as possible. A technician will coach you through the effort, because the test depends on you giving maximum exertion. You’ll repeat this at least three times, and the best result from those attempts is recorded.
The two key numbers spirometry produces are FVC and FEV1. FVC (forced vital capacity) is the total volume of air you can forcefully exhale after a full breath. FEV1 (forced expiratory volume in one second) is how much of that air comes out in the first second alone. In healthy lungs, roughly 75% to 85% of your total exhaled air comes out in that first second. The ratio between these two numbers is what clinicians use to detect airflow obstruction: a low FEV1/FVC ratio suggests the airways are narrowed, as in asthma or COPD.
Your doctor may also order a “before and after” version of spirometry. You’ll do the test once, use an inhaler that opens the airways, wait a few minutes, then repeat the test. If your numbers improve significantly after the inhaler, it suggests your airway narrowing is reversible, which is a hallmark of asthma.
Lung Volume Measurement
Spirometry only captures air you can forcefully push out, but your lungs always hold some air you can’t exhale, called residual volume. Lung volume testing measures everything, including that leftover air, to determine four main capacities:
- Total lung capacity (TLC): the full amount of air your lungs can hold
- Functional residual capacity (FRC): how much air stays in your lungs at the end of a normal, relaxed breath
- Vital capacity (VC): the difference between a completely full breath and a completely empty one
- Inspiratory capacity (IC): how much air you can inhale starting from a normal resting breath
The most precise way to measure these is body plethysmography. You’ll sit inside a clear, phone booth-sized box with the door closed and breathe through a mouthpiece while sensors track tiny pressure changes inside the chamber. It feels unusual but isn’t painful. This method is especially accurate for people with COPD, where other techniques tend to underestimate how much air is actually trapped in the lungs. Some labs use a simpler gas dilution method instead, where you breathe in a small amount of a harmless tracer gas and the lab calculates your lung volumes based on how diluted the gas becomes.
Diffusion Capacity (DLCO)
This test answers a different question than spirometry or lung volumes. Instead of measuring airflow, it measures how well gas crosses from the air sacs in your lungs into the blood. You’ll take a single deep breath of a gas mixture containing a tiny, harmless amount of carbon monoxide, hold it for about 10 seconds, then exhale. The lab analyzes how much carbon monoxide was absorbed during that breath hold.
Carbon monoxide is used because it binds very strongly to red blood cells. That strong binding means the test is measuring how healthy the membrane between your air sacs and blood vessels is, rather than being thrown off by how fast blood is flowing. Two main factors affect the result: the surface area available for gas exchange (which shrinks in emphysema, for example, as air sac walls break down) and the thickness of the membrane (which increases in conditions that cause scarring or inflammation in the lung tissue). A low diffusion capacity can help explain why someone feels short of breath even when spirometry looks normal.
Additional Tests You Might Encounter
Beyond the three core tests, your doctor might add specialized tests depending on what they’re looking for.
A methacholine challenge test is used when asthma is suspected but spirometry looks normal. You inhale increasing concentrations of methacholine, a substance that triggers airway narrowing in sensitive lungs, with spirometry performed after each dose. If your FEV1 drops by 20% or more, the test is considered positive for airway hyperreactivity. This test isn’t performed on people whose FEV1 is already below 70% of predicted, or those who’ve had a heart attack, unstable heart disease, or stroke within the previous three months.
A six-minute walk test measures how far you can walk on a flat surface in six minutes, tracking your oxygen levels and symptoms along the way. It’s a practical snapshot of how your lungs perform during real activity. Pulse oximetry, the small clip placed on your fingertip, is often used during this test and sometimes at rest to check oxygen saturation levels.
How to Prepare
If you use inhalers, your doctor’s office will likely tell you to stop certain ones before the test. Short-acting rescue inhalers are typically withheld for 4 to 8 hours, and long-acting inhalers for 12 to 48 hours, though the specific instructions vary by medication. You should also avoid smoking, heavy meals, and vigorous exercise in the hours before testing, since all of these can skew results.
Wear loose, comfortable clothing. Tight waistbands or restrictive tops can limit how deeply you breathe. Expect the technician to record your height, weight, age, and sex before testing, because your results are compared against predicted values for someone with your characteristics. The current standard, recommended by the American Thoracic Society and the European Respiratory Society, uses race-neutral reference equations from the Global Lung Function Initiative rather than older equations that adjusted for race and ethnicity.
What the Results Reveal
PFT results generally fall into one of three patterns. An obstructive pattern means air has trouble getting out of the lungs, which shows up as a low FEV1/FVC ratio. This is the pattern seen in asthma, COPD, and chronic bronchitis. A restrictive pattern means the lungs can’t expand fully, so total lung capacity is reduced, but the FEV1/FVC ratio stays normal or even increases. Pulmonary fibrosis, obesity, and conditions affecting the chest wall produce this pattern. Some people show a mixed pattern with features of both.
Your results are reported as a percentage of predicted values for your age, height, and sex. A result of 80% or above is generally considered within normal range for spirometry, though interpretation depends on the full clinical picture. The trend in your results over time can be just as informative as any single test. Comparing your current numbers to previous PFTs helps track whether a lung condition is stable, improving with treatment, or progressing.
Who Should Not Take the Test
Because PFTs require forceful breathing that briefly raises pressure in the chest and abdomen, certain conditions make testing risky. You should wait at least one month after a heart attack. Recent chest or abdominal surgery, a collapsed lung, coughing up blood, pulmonary embolism, and severely elevated blood pressure (above 200/120) are all reasons to postpone. An ascending aortic aneurysm is another concern, since the pressure spikes during forced exhalation could be dangerous. Your doctor will weigh the risks before ordering the test if any of these apply.