What Lung Function Tests Measure and Why Doctors Order Them
Lung function tests, also called pulmonary function tests (PFTs), measure how well your lungs work. These tests evaluate how much air your lungs can hold, how quickly you can move air in and out, and how effectively your lungs transfer oxygen into your bloodstream. Doctors order these tests for many reasons: to diagnose breathing problems, monitor chronic conditions like asthma or COPD, check lung health before surgery, or track how well a treatment is working.
Learn About DMV Bill of Sale Forms →
The most common lung function test is called spirometry. During this test, you breathe into a machine called a spirometer that records the amount of air you inhale and exhale, as well as how fast you breathe it out. The test typically takes 15 to 30 minutes and requires you to follow specific breathing instructions. You may be asked to breathe normally, then take the deepest breath possible and exhale as hard and fast as you can.
According to the American Lung Association, about 1 in 4 American adults have some form of lung disease. Lung function tests help doctors identify problems early, when treatment options are often more effective. These tests can detect conditions like asthma, COPD (chronic obstructive pulmonary disease), pulmonary fibrosis, and restrictive lung diseases. They can also show how severe a condition is and whether it's getting better or worse over time.
The information from lung function tests combines with your medical history, physical exam, and other tests to give doctors a complete picture of your respiratory health. Understanding what these numbers mean helps you participate in conversations with your healthcare team about your condition and treatment options.
Practical Takeaway: Lung function tests measure three main things: lung volume (how much air your lungs hold), airflow (how fast air moves in and out), and gas exchange (how well oxygen gets into your blood). Knowing what the test measures helps you understand why your doctor ordered it and what information it will provide.
Understanding FEV1 and FVC Numbers
Two of the most important numbers on a lung function test report are FEV1 and FVC. FEV1 stands for "forced expiratory volume in one second"—it measures how much air you can breathe out forcefully in just one second. FVC stands for "forced vital capacity"—it measures the total amount of air you can breathe out after taking the deepest breath possible. These two measurements are reported in liters and also as a percentage of what is predicted for someone your age, height, sex, and race.
Free Guide to Making Bread in a Bread Maker →
The FEV1/FVC ratio is particularly useful. This ratio is calculated by dividing your FEV1 by your FVC. In healthy adults, this ratio is usually 70% or higher, meaning at least 70% of the total air you can breathe out comes out in that first second. If your FEV1/FVC ratio is less than 70%, it may suggest airflow obstruction—a sign that something is partially blocking air from moving out of your lungs quickly. This pattern is common in conditions like asthma and COPD.
When you see your results, you'll notice they compare your numbers to "predicted values." These are normal ranges based on large studies of healthy people who share your characteristics. For example, a predicted FEV1 might be listed as 3.5 liters for a person of your size and age. If your actual FEV1 is 2.8 liters, your result would be shown as 80% of predicted. Results between 80% and 100% of predicted are generally considered normal. Results between 70% and 79% of predicted may indicate mild changes. Below 70% suggests more significant airflow reduction.
Interpreting these numbers requires context. A single test gives a snapshot of your lung function on that particular day. Factors like how hard you try during the test, respiratory infections, or not following instructions perfectly can affect results. This is why doctors often order repeat tests and look at changes over time rather than focusing on one result alone.
Practical Takeaway: FEV1 measures how much air exits in one second; FVC measures total air exhaled. The ratio between them (FEV1/FVC) helps identify airflow obstruction. Compare your actual results to "predicted values" for your age and size—results between 80% and 100% of predicted are typically normal, while lower numbers suggest lung changes that warrant monitoring.
Decoding Other Important Measurements
Beyond FEV1 and FVC, lung function test reports include several other measurements that provide additional information about how your lungs work. Understanding these helps you grasp the complete picture of your respiratory health.
Your Free Guide to CDL License Requirements →
FEF25-75 (forced expiratory flow) measures how fast air flows out of your lungs during the middle portion of exhalation. This measurement is sensitive to changes in small airways—the tiny tubes deep in your lungs that carry air. A low FEF25-75 can be an early sign of small airway disease, sometimes before FEV1 changes. It's expressed as liters per second, and normal values vary based on your size and age.
TLC (total lung capacity) measures the maximum amount of air your lungs can hold after the deepest possible breath. This is often measured using a technique called helium dilution or body plethysmography. Normal TLC is typically between 80% and 120% of predicted. Higher TLC values might suggest air trapping (air that gets stuck in your lungs), which occurs in emphysema or asthma. Lower TLC values might suggest restrictive patterns, where lungs cannot expand fully due to stiffness or weakness.
RV (residual volume) is the amount of air that remains in your lungs even after you exhale as completely as possible. Normally, RV is about 20-35% of your total lung capacity. An elevated RV suggests air trapping. The RV/TLC ratio helps doctors determine whether the problem is obstruction or restriction.
DLCO (diffusing capacity) measures how well oxygen moves from your lungs into your bloodstream. During this test, you breathe in a mixture of gases containing carbon monoxide in tiny amounts. The machine measures how much carbon monoxide you exhale—less exhaled means more was transferred into your blood, indicating good gas exchange. Low DLCO can suggest pulmonary fibrosis, emphysema, or blood vessel problems in the lungs.
Bronchodilator response testing compares lung function before and after using a fast-acting bronchodilator medication (usually albuterol). If results improve significantly after the medication—typically a 12% and 200 mL increase in FEV1—it suggests reversible airflow obstruction, supporting an asthma diagnosis.
Practical Takeaway: Beyond FEV1 and FVC, reports include FEF25-75 (small airway flow), TLC (total lung capacity), RV (air that remains after exhaling), and DLCO (oxygen transfer ability). Each measurement reveals different aspects of lung function; together they help doctors identify the type and severity of lung problems.
What Normal, Abnormal, and Borderline Results Look Like
Lung function results fall into several general patterns. Understanding these patterns helps you know whether your results suggest healthy lungs or point toward specific types of problems.
Free Guide to Watching Fox News Online →
Normal results typically show FEV1 and FVC both at 80% or higher of predicted values, with an FEV1/FVC ratio of 70% or higher. All other measurements (TLC, RV, DLCO) fall within expected ranges. Normal results suggest your lungs are functioning as expected for your age, size, and sex. However, "normal" doesn't mean perfect—it means your results are within the range seen in healthy people.
Obstructive patterns show a reduced FEV1/FVC ratio (below 70%), while FVC may be normal or only slightly reduced. FEF25-75 is typically quite low. This pattern occurs in asthma, COPD, and bronchiectasis. In obstructive patterns, the problem is that air gets trapped—it flows out slowly because airways are narrowed by inflammation, mucus, or structural damage. TLC and RV are often elevated because air doesn't leave the lungs completely.
Restrictive patterns show reduced