Spirometry is not the same as a PFT, but it is part of one. A pulmonary function test (PFT) is an umbrella term for a group of tests that measure how well your lungs work. Spirometry is the most commonly ordered test within that group. When doctors order a “complete PFT,” they typically mean spirometry plus lung volume measurement plus a diffusion capacity test, all done in the same session.
How Spirometry Fits Into a PFT
Think of PFT as the full menu and spirometry as the most popular item on it. Spirometry measures how much air you can blow out and how fast you can do it. It’s the first-line test when a doctor suspects asthma or COPD based on symptoms like chronic cough, wheezing, or chest tightness. Many people who get “a PFT” actually only get spirometry, which is part of the reason the two terms get used interchangeably in casual conversation.
A complete PFT adds two more layers of information beyond what spirometry can provide: lung volumes and diffusion capacity. Your doctor chooses the full battery when your symptoms don’t fit a straightforward pattern, when you’re unexpectedly low on oxygen, or when there’s concern about lung tissue damage from medications or other causes.
What Spirometry Measures
During spirometry, you take a deep breath and blow out as hard and fast as you can into a mouthpiece. The two key numbers it produces are FVC (the total amount of air you can force out) and FEV1 (the amount you push out in the first second). The ratio between these two values is the single most important number for detecting airflow obstruction.
In healthy lungs, you exhale about 75% to 85% of your total air in that first second. If your ratio falls below normal, it points toward an obstructive condition like asthma or COPD, where the airways are narrowed. If both numbers drop proportionally and the ratio stays normal or even goes up, that pattern suggests a restrictive problem, where the lungs can’t fully expand. The traditional cutoff for a normal ratio has been 0.70, though pulmonary specialists increasingly use age-adjusted thresholds because the fixed cutoff tends to overdiagnose older adults and miss younger patients with early disease.
Sometimes spirometry is repeated after you inhale a bronchodilator (a medication that opens the airways). If your numbers improve significantly, it suggests the obstruction is at least partially reversible, which is a hallmark of asthma.
What a Full PFT Adds
Spirometry has a blind spot: it can only measure air that moves in and out. It cannot measure the air that stays trapped in your lungs after you exhale completely. That leftover air, called residual volume, requires a separate lung volume test to detect.
Lung Volumes
The most accurate way to measure total lung capacity is body plethysmography. You sit inside a sealed, transparent booth (it looks like a phone booth) and breathe against a closed mouthpiece for a few seconds. Pressure changes inside the booth allow the equipment to calculate the total gas in your chest, including air trapped in poorly ventilated areas. This is especially important in COPD, where air trapping is a defining feature. Plethysmography consistently picks up higher, more accurate measurements of trapped air than other methods in these patients.
The main values reported include total lung capacity, functional residual capacity (how much air sits in your lungs at the end of a normal breath), vital capacity, and inspiratory capacity. Together, these numbers reveal whether your lungs are hyperinflated, restricted, or normal in size.
Diffusion Capacity
This test answers a different question entirely: once air reaches the deepest part of your lungs, how efficiently does oxygen cross into your blood? You inhale a gas mixture containing a tiny amount of carbon monoxide plus a tracer gas, hold your breath for about 10 seconds, then exhale. The equipment measures how much carbon monoxide your lungs absorbed during that brief hold.
Carbon monoxide is used instead of oxygen because it binds so strongly to red blood cells that its uptake depends almost entirely on the health of the membrane between your air sacs and blood vessels, not on blood flow. A low result can indicate damage to that membrane from conditions like pulmonary fibrosis, emphysema, or certain drug toxicities. A normal spirometry result combined with a low diffusion capacity is a pattern that often points toward early interstitial lung disease or pulmonary vascular problems, something spirometry alone would miss entirely.
When Doctors Order Each Test
Spirometry alone is appropriate for straightforward scenarios: evaluating a smoker’s chronic cough, confirming suspected asthma, staging known COPD, or clearing someone for surgery when airway disease is a concern. It’s quick, widely available (many primary care offices have a spirometer), and gives enough information to start or adjust treatment for common airway diseases.
A complete PFT gets ordered when the picture is more complex. Typical reasons include:
- Unexplained shortness of breath that doesn’t clearly fit asthma or COPD
- Low oxygen levels without an obvious cause
- Concern about lung tissue damage from medications, autoimmune disease, or occupational exposures
- Spirometry results that look restrictive, since lung volumes are needed to confirm true restriction versus a patient who simply didn’t blow hard enough
There’s also a methacholine challenge test, which uses spirometry equipment but in a different way. You inhale increasing doses of a substance that temporarily narrows the airways. If your spirometry numbers drop during the challenge, it confirms airway hyperreactivity consistent with asthma. This test is reserved for cases where asthma is suspected but standard spirometry comes back normal.
What the Testing Experience Is Like
Spirometry alone typically takes 15 to 20 minutes, including repeated blows to ensure consistent results. You’ll be coached by a technician to blast air out as forcefully as possible, which can feel tiring after several attempts. A complete PFT session runs closer to 45 minutes to an hour because of the additional lung volume and diffusion tests.
None of the tests are painful, but they do require effort and cooperation. The body plethysmography booth can feel confining for some people, though the door isn’t locked and you’re only inside for a few minutes. For the diffusion test, holding your breath for 10 seconds while sitting still is the main challenge. You may be asked to avoid using inhalers for a set number of hours beforehand, depending on whether your doctor wants to see your baseline lung function or your function on medication.
Results are typically compared against predicted values based on your age, height, sex, and ethnicity. Your report will show your actual numbers alongside the predicted range, making it easy to see where you fall. If spirometry is the only test ordered and results raise more questions, your doctor may follow up with a complete PFT to fill in the gaps.