Understanding Your Pulmonary Function Test Results

What Is a Pulmonary Function Test and Why Do Doctors Order It?

A pulmonary function test, often called a PFT or spirometry test, measures how well your lungs work. These tests check how much air your lungs can hold and how quickly you can move air in and out of your lungs. Doctors order these tests for many reasons. They might want to check your lung health before surgery, monitor a lung condition like asthma or chronic obstructive pulmonary disease (COPD), figure out why you feel short of breath, or track how well a lung treatment is working.

The tests are non-invasive, meaning nothing goes inside your body. You breathe into a machine called a spirometer, which records your breathing patterns. The whole process usually takes 15 to 45 minutes, depending on which specific tests your doctor wants performed. About 6 million Americans take pulmonary function tests each year, making it one of the most common lung tests in medical practice.

Understanding your results matters because your lungs are vital to your survival. They take in oxygen and remove carbon dioxide from your blood. When your lungs don't work properly, your body doesn't get enough oxygen, which can affect your heart, brain, muscles, and other organs. By understanding what your test results mean, you can better understand your own health and work with your doctor on any needed treatment plans.

Your doctor might order a PFT if you have symptoms like persistent cough, shortness of breath during normal activities, wheezing, chest pain when breathing, or a history of smoking. People with known lung diseases like asthma, emphysema, or pulmonary fibrosis often have these tests done regularly. Workers exposed to dust, chemicals, or other lung irritants may also need periodic testing.

Practical Takeaway: Pulmonary function tests provide objective measurements of how your lungs perform. Before your test, wear loose, comfortable clothing and avoid heavy meals right before the appointment. Let your doctor know about any medications you take, as some can affect results.

Understanding FEV1 and FVC: The Two Most Important Measurements

The two most important numbers on your pulmonary function test results are FEV1 and FVC. FEV1 stands for Forced Expiratory Volume in 1 second—this is the amount of air you can forcefully exhale in the first second after taking the deepest breath possible. FVC stands for Forced Vital Capacity—this is the total amount of air you can forcefully exhale after the deepest breath. Both measurements are given in liters and as a percentage of what is predicted for someone your age, height, sex, and race.

Think of your lungs like a balloon. FVC tells you how big the balloon can get when fully inflated. FEV1 tells you how quickly you can let the air out of that balloon. Your lungs need to do both things well. A normal FEV1 is usually 80 percent or higher of the predicted value. A normal FVC is also usually 80 percent or higher of predicted. When doctors look at these two numbers together, they calculate something called the FEV1/FVC ratio. This ratio tells them what percentage of your total lung capacity you can empty in just one second.

In healthy adults, the FEV1/FVC ratio is usually 70 percent or higher. If your ratio is lower than 70 percent, it might suggest an obstructive lung disease like asthma or COPD. This means the airways in your lungs are narrowed, making it harder to push air out quickly. If your FEV1 and FVC are both low but the ratio stays above 70 percent, it might suggest a restrictive lung disease, meaning your lungs can't expand fully to hold as much air as they should.

The predicted values your results are compared to come from large studies of healthy people. Doctors use equations that account for your specific characteristics because lung capacity varies by body size and other factors. A man who is 6 feet tall will naturally have higher lung volumes than a woman who is 5 feet tall, so doctors don't compare them to the same standard. This is why your results show both your actual numbers and your percentage of predicted—the percentage tells you whether your lungs are working as expected for someone like you.

Practical Takeaway: Focus on your FEV1/FVC ratio first. If it's below 70 percent, your airways may be narrowed. If both FEV1 and FVC are low but the ratio is normal, your lungs may not be expanding fully. Ask your doctor which pattern your results show and what it means for your health.

Interpreting Obstruction, Restriction, and Mixed Patterns

Pulmonary function test results fit into patterns that help doctors understand what's happening in your lungs. The three main patterns are obstructive, restrictive, and mixed. An obstructive pattern means your airways are narrowed, making it hard to exhale. The hallmark is a low FEV1/FVC ratio (below 70 percent) with a reduced FEV1. Common causes include asthma, COPD, bronchiectasis, and cystic fibrosis. People with obstruction often describe feeling like they can't get the air out of their lungs—like breathing through a straw.

A restrictive pattern means your lungs can't expand fully, so they hold less air than normal. In restriction, both FEV1 and FVC are reduced, but the FEV1/FVC ratio stays normal (70 percent or higher). It's like your lungs are being squeezed from the outside or the lung tissue itself is stiff. Causes include pulmonary fibrosis, sarcoidosis, neuromuscular weakness, chest wall problems, or conditions where fluid builds up around the lungs. People with restriction often feel short of breath but don't have trouble getting air out.

A mixed pattern shows features of both obstruction and restriction. The FEV1/FVC ratio is low (obstructive), but both FEV1 and FVC are also reduced. This can happen in people with advanced COPD combined with other lung problems, or in other complex situations. Mixed patterns require careful interpretation because treating obstruction differs from treating restriction, so your doctor needs to understand which component is most prominent.

Normal results show an FEV1 of 80 percent or higher of predicted, an FVC of 80 percent or higher of predicted, and an FEV1/FVC ratio of 70 percent or higher. This means your lungs are working as expected. Mild abnormalities show similar patterns but with values in the 70-79 percent range. Moderate abnormalities fall in the 50-69 percent range, and severe abnormalities are below 50 percent. Your doctor uses these severity levels to decide on treatment and how often to repeat testing.

Practical Takeaway: Identify which pattern your results show by looking at the FEV1/FVC ratio first, then the actual values. Write down whether you have obstruction, restriction, mixed, or normal results. This helps you understand conversations with your doctor and track changes if you have repeat testing.

What Other Test Results Mean: DLCO, Flows, and Volumes

Beyond FEV1 and FVC, your pulmonary function test report may include other measurements. DLCO, which stands for Diffusing Capacity of the Lungs for Carbon Monoxide, measures how well oxygen moves from your lungs into your bloodstream. Your doctor may order this if they suspect pulmonary fibrosis, emphysema, or other conditions that damage the lung tissue. A low DLCO suggests the walls between air sacs and blood vessels are thickened or damaged, making oxygen transfer difficult. Normal DLCO is usually 80 percent or higher of predicted.

Flow measurements show how fast air moves through your airways at different lung volumes. Peak Expiratory Flow (PEF) is the fastest speed you can exhale. Forced Expiratory Flow at 25-75 percent (FEF 25-75) shows how fast air flows when you're in the middle of exhaling, after the first quick push. These flow measurements are sensitive to small airway problems that might not show up in FEV1 and FVC. Athletes and people with early asthma sometimes have normal FEV1/FVC but reduced flows.