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 reveal whether you have an obstructive problem (like asthma or COPD), a restrictive problem (like pulmonary fibrosis), or normal lung function. Your results are compared against predicted values based on your age, height, sex, and ethnicity, and anything above 80% of predicted is generally considered normal.
How Airflow Is Measured: Spirometry
The core of most pulmonary function testing is spirometry, where you breathe into a mouthpiece as hard and fast as you can. This produces three key numbers:
- FVC (forced vital capacity): the total amount of air you can blow out after taking the deepest breath possible.
- FEV1 (forced expiratory volume in one second): how much of that air comes out in the first second of your exhale.
- FEV1/FVC ratio: the percentage of your total air that you can push out in that first second. A normal ratio is above 0.70, meaning at least 70% of the air comes out quickly.
These three values together tell your doctor whether air is flowing freely through your airways or whether something is causing a blockage or limitation. Both FEV1 and FVC should land above 80% of the predicted value for someone your size and age. When they don’t, the pattern of how they drop points toward a specific type of lung problem.
Obstructive Pattern: Narrowed Airways
An obstructive pattern shows up when air has trouble getting out of your lungs. The hallmark is an FEV1/FVC ratio below 0.70. Your lungs may actually hold a normal or even increased total volume of air, but you can’t push it out quickly because your airways are narrowed, inflamed, or partially collapsed.
This pattern is characteristic of asthma, COPD, chronic bronchitis, and emphysema. Because air gets trapped behind the narrowed airways, people with obstructive disease often have an elevated residual volume, the amount of air left in the lungs after you exhale as hard as you can. That trapped air is part of why breathing feels so effortful.
When the test includes a flow-volume loop, a graph that plots airflow speed against volume, obstructive disease produces a distinctive scooped-out shape on the exhale portion. This visual pattern helps clinicians pinpoint where and how severely the airways are affected.
Restrictive Pattern: Limited Lung Expansion
A restrictive pattern looks different. The FEV1/FVC ratio stays normal or even goes up (above 0.70), but total lung capacity drops below 80% of predicted. Your airways aren’t blocked; your lungs simply can’t expand fully. The flow-volume curve often takes on a tall, narrow, convex shape rather than the scooped-out look of obstruction.
Conditions that cause this include pulmonary fibrosis (scarring of lung tissue), chest wall deformities, neuromuscular diseases that weaken the breathing muscles, and severe obesity. In fact, current guidelines specify that obesity alone is only considered the cause of a restrictive pattern when BMI exceeds 40. Below that threshold, other causes need to be investigated even if the patient carries extra weight.
Bronchodilator Response: Testing for Reversibility
If your initial spirometry shows obstruction, the test often includes a second round after you inhale a quick-acting bronchodilator (typically albuterol). You wait a short time, then repeat the breathing maneuvers. A significant improvement, defined as an increase in FEV1 of at least 12% and 200 mL from baseline, suggests your airway narrowing is at least partly reversible. This is one of the diagnostic criteria for asthma, distinguishing it from conditions like COPD where the obstruction tends to be more fixed.
Even in COPD, some people show partial reversibility. The degree of improvement helps guide treatment decisions and gives your doctor a sense of how responsive your airways are to medication.
Lung Volumes Beyond Spirometry
Spirometry captures how air flows, but it can’t measure everything sitting in your lungs. Three important volumes, total lung capacity (TLC), functional residual capacity (FRC), and residual volume (RV), require a different technique: either breathing a tracer gas that gets diluted by the air in your lungs, or sitting inside a sealed booth called a body plethysmograph that detects pressure changes as you breathe.
These volumes fill in the picture that spirometry alone can’t complete. In obstructive disease, TLC and residual volume are often elevated because of trapped air. In restrictive disease, all three tend to be reduced because the lungs can’t fully inflate. Measuring total lung capacity is actually the definitive way to confirm a restrictive pattern; spirometry can suggest it, but TLC below 80% of predicted seals the diagnosis.
Gas Exchange: The Diffusion Test
Spirometry and lung volumes tell you about air movement, but they say nothing about whether oxygen is actually crossing from your lungs into your blood. That’s what a diffusion capacity test measures. You inhale a small, harmless amount of carbon monoxide, hold your breath briefly, then exhale. Because carbon monoxide binds strongly to hemoglobin and is normally absent from your blood, the rate at which it disappears from the air you breathe in reflects how efficiently gas moves across your lung membranes.
A low diffusion capacity can indicate damage to the tiny air sacs or the blood vessels surrounding them. It’s commonly reduced in emphysema (where the air sacs are destroyed), pulmonary fibrosis (where scarring thickens the membranes), and pulmonary hypertension. In chronic bronchitis, by contrast, diffusion capacity tends to stay normal because the air sacs themselves are intact. In asthma, diffusion is usually normal or even slightly elevated.
What Your Results Look Like on Paper
Your report will list each measurement alongside a “percent predicted” value or a z-score. Percent predicted compares your result to the expected value for a healthy person matching your demographics. Under current American Thoracic Society and European Respiratory Society guidelines updated in 2022, the preferred method uses z-scores, which express how far your result falls from the average of a healthy reference population. A z-score below negative 1.645 is considered below normal, placing you in the bottom 5% of healthy individuals.
Severity is graded on a three-level scale. A z-score between negative 1.65 and negative 2.5 indicates mild impairment. Between negative 2.5 and negative 4.0 is moderate. Below negative 4.0 is severe. This same scale applies to obstruction, restriction, and diffusion capacity, making it straightforward to compare severity across different types of lung problems.
If you’ve had previous tests on file, your doctor will also compare your current numbers to past results. A slow decline over time can reveal progressive disease even when individual results still fall within the normal range.
How to Prepare for the Test
Getting accurate results depends partly on how you prepare. Avoid eating a large meal within two hours of the test, and don’t exercise heavily for at least 30 minutes beforehand. No alcohol for four hours before, and no smoking at all on the day of the test. Wear loose, comfortable clothing that won’t restrict your ability to take a deep breath.
If you use inhalers, you’ll likely be asked to stop them before the test so your baseline lung function can be measured without medication. The timing depends on the type:
- Short-acting rescue inhalers (albuterol): stop 6 hours before
- Short-acting anticholinergics (ipratropium): stop 12 hours before
- Long-acting inhalers (formoterol, salmeterol): stop 24 hours before
- Ultra-long-acting inhalers (tiotropium, indacaterol): stop 36 hours before
The test itself is noninvasive and usually takes 30 to 60 minutes depending on which components are ordered. You’ll be coached through each maneuver, and the technician may ask you to repeat efforts several times to ensure consistent, reliable readings. It requires real effort on your part, especially during the forced exhale, but nothing about the process is painful.