A complete pulmonary function test (PFT) is a battery of breathing tests that measures how well your lungs take in air, how much air they can hold, and how efficiently they move oxygen into your bloodstream. Unlike basic spirometry, which only measures airflow, a complete PFT typically includes three core components: spirometry, lung volume measurement, and diffusion capacity testing. The full battery takes between 15 and 45 minutes, depending on which tests your doctor orders and whether any need to be repeated.
What Makes It “Complete”
Simple spirometry is the most common lung test and can be done in almost any doctor’s office. It tells you how fast and how much air you can blow out. A complete PFT goes further by adding tests that measure the total size of your lungs and how well oxygen crosses from your lungs into your blood. Together, these three types of measurement give a full picture of lung function that spirometry alone can’t provide.
Your doctor might order a complete PFT to diagnose a new breathing problem, track how a known lung condition is progressing, evaluate your lungs before surgery, or figure out whether shortness of breath is coming from your lungs or somewhere else, like your heart.
Spirometry: Airflow and Speed
Spirometry is the foundation of every PFT. You take the deepest breath you can, then blow out as hard and as long as possible into a mouthpiece connected to a machine called a spirometer. A nose clip keeps air from escaping through your nostrils. This produces two key numbers.
The first is your forced vital capacity (FVC), the total volume of air you can push out in one full, forceful exhale. The second is your FEV1, the volume of air you push out in just the first second. It’s the ratio between these two numbers that matters most. A low ratio, meaning you’re moving a smaller-than-expected share of your air in that first second, points toward an obstructive pattern. This is what doctors see in conditions like asthma and COPD, where the airways are narrowed. When both the total volume and the first-second volume are reduced but the ratio between them stays normal or goes up, the pattern suggests restriction, meaning the lungs can’t fully expand. This happens in conditions like pulmonary fibrosis or chest wall problems.
Bronchodilator Reversibility
If spirometry shows obstruction, the technician will often have you inhale a short-acting bronchodilator (a medication that relaxes airway muscles) and then repeat the test about 15 minutes later. This is the reversibility portion of the PFT. If your FEV1 improves by at least 12% and at least 200 milliliters after the medication, the obstruction is considered reversible. Reversible obstruction is a hallmark of asthma, while obstruction that doesn’t budge much after a bronchodilator is more typical of COPD.
Lung Volumes: How Much Air Your Lungs Hold
Spirometry can only measure air that moves in and out. It can’t measure air that stays trapped in your lungs after you exhale, called residual volume. To get that number, and to calculate your total lung capacity, you need a separate lung volume test.
The most common method uses a body plethysmograph, a clear, phone-booth-sized box you sit inside with the door closed for about five minutes. While inside, you breathe against a closed shutter on the mouthpiece. The box senses tiny pressure changes as your chest expands and contracts, and those changes are used to calculate how much air is in your lungs at rest. It can feel a bit claustrophobic, but the box is transparent and the technician is right outside.
Alternative methods exist for people who can’t tolerate the box. In gas dilution, you breathe a harmless tracer gas (typically helium) from a closed system until it spreads evenly between the machine and your lungs. The degree of dilution reveals your lung volume. A nitrogen washout technique works similarly: you breathe pure oxygen while the machine measures how much nitrogen your lungs release. These gas-based methods tend to underestimate lung volume in people with severe airflow obstruction or emphysema, because gas can’t reach trapped pockets of air. The plethysmograph picks up all air in the chest, ventilated or not, so it gives a higher and generally more accurate reading in those cases.
The key numbers from this test are total lung capacity (TLC), residual volume (RV), and functional residual capacity (FRC, the amount of air left in your lungs after a normal, relaxed exhale). An abnormally high TLC and RV suggest air trapping and hyperinflation, common in emphysema. An abnormally low TLC confirms a restrictive pattern, something spirometry can suggest but not prove on its own.
Diffusion Capacity: Oxygen Transfer
The third major component, called a DLCO test, measures how efficiently gas moves from the air sacs in your lungs into your bloodstream. You breathe in a gas mixture that includes a tiny, harmless amount of carbon monoxide along with a tracer gas (often methane) that your blood doesn’t absorb. You hold your breath for about 10 seconds, then exhale into the machine. By comparing how much carbon monoxide went in with how much came back out, the machine calculates how much crossed into your blood.
Carbon monoxide is used because it behaves almost identically to oxygen when crossing the lung membrane, making it a reliable stand-in. A low diffusion capacity means something is interfering with gas transfer. This could be damage to the air sacs (as in emphysema), thickening of the lung tissue (as in pulmonary fibrosis), or reduced blood flow through the lungs. Diffusion capacity can also be elevated in certain conditions, such as polycythemia vera, where an excess of red blood cells pulls more gas from the lungs than normal.
How Results Are Interpreted
Your results are compared to predicted values based on your age, height, sex, and (in older reference equations) race. More recent standards from 2022 use race-neutral equations. Rather than a simple “you should be above 80%” cutoff, current guidelines define normal as falling above the lower limit of normal (LLN), which is the 5th percentile for your demographic group. If your result is below that threshold, it’s considered abnormal.
In practice, the three components work together like puzzle pieces. Spirometry might show an obstructive pattern, lung volumes might reveal air trapping, and diffusion capacity might point to emphysema rather than chronic bronchitis. No single test tells the whole story, which is exactly why a complete PFT includes all three.
How to Prepare
Preparation matters because many everyday habits can skew the results. Stop smoking at least four hours before the test. Avoid heavy exercise for at least 30 minutes beforehand. Don’t eat a large meal within two hours, and skip alcohol for at least eight hours. A full stomach or recent exercise can physically restrict how deeply you can breathe, and smoking temporarily changes airway function.
The trickiest preparation detail involves your inhalers. You’ll generally be asked to hold bronchodilator medications for at least four hours before the test, though your doctor may give different instructions depending on the type of inhaler and the purpose of the test. If the goal is to see how your lungs function without medication support, you may need to stop long-acting inhalers for a longer window. Always confirm the specifics with the office that scheduled the test.
Who Should Not Take the Test
Because PFTs require forceful breathing that raises pressure in your chest, certain conditions make the test risky. A heart attack within the past month, a known large aortic aneurysm (greater than 6 centimeters), an active pneumothorax (collapsed lung), or recent surgery on the chest, abdomen, brain, or eyes are all reasons to delay or skip testing. Modern eye surgery techniques heal quickly, and most patients can safely test about a week after a procedure like cataract surgery, but your doctor should make that call.
For bronchial challenge testing, a more provocative add-on that deliberately triggers airway narrowing to diagnose asthma, the thresholds are stricter: it’s generally not performed if your FEV1 is already below 50% of predicted, if you’ve had a heart attack or stroke within three months, or if your blood pressure is severely uncontrolled.
What the Test Feels Like
Most people find PFTs tiring rather than painful. The spirometry portion demands maximum effort: you blow as hard and as long as you possibly can, often repeating the maneuver three or more times until the technician gets consistent readings. It’s common to feel lightheaded or to cough. The body box portion is passive by comparison, just gentle breathing in a sealed booth, though the closed door bothers some people. The diffusion capacity test involves a single deep breath and a brief breath-hold, which most people find easy.
Between the different tests, the technician will give you breaks. The entire session, including instructions and rest periods, typically wraps up within 45 minutes. Results are usually available within a few days, interpreted by a pulmonologist who reads them alongside your symptoms, imaging, and medical history.