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Understanding Pulmonary Function Tests: What They Measure

Pulmonary function tests, often called PFTs or spirometry, are medical measurements that show how well your lungs work. These tests measure how much air your lungs can hold, how quickly you can breathe air in and out, and how effectively your lungs move oxygen into your bloodstream. Doctors use these tests to detect lung diseases, monitor existing conditions, and assess how well treatments are working.

The most common pulmonary function test is spirometry. During this test, you breathe into a machine called a spirometer. The device measures the amount of air you inhale and exhale, along with the speed of your breathing. Another type measures how much air stays in your lungs after you exhale completely. A third type, called diffusion testing, measures how well oxygen passes from your lungs into your blood.

According to the American Lung Association, about 16 million Americans have chronic obstructive pulmonary disease (COPD), yet many cases go undiagnosed. Pulmonary function tests play a crucial role in catching these conditions early. The tests can detect problems like asthma, emphysema, bronchitis, restrictive lung diseases, and other respiratory conditions before symptoms become severe.

Different measurements have specific names. FVC stands for forced vital capacity—the total amount of air you can forcefully exhale after breathing in deeply. FEV1 refers to forced expiratory volume in one second—how much air you can exhale in just one second. Your FEV1 compared to your FVC helps doctors understand your lung function patterns.

Practical takeaway: Pulmonary function tests provide objective measurements of lung health. Understanding these basic measurements helps you discuss results with your healthcare provider and recognize what information the tests provide about how your lungs are functioning.

Who Might Benefit From Pulmonary Function Testing

Pulmonary function tests may be recommended for various reasons. People experiencing persistent cough, shortness of breath, wheezing, or chest pain during breathing often receive these tests to determine the cause. The tests help distinguish between different types of lung problems and identify which specific areas of lung function are affected.

Individuals with a history of smoking—whether current or past—are candidates for pulmonary function testing. The CDC reports that smoking damages the lungs in measurable ways that often appear on these tests before symptoms develop. Even people who quit smoking years ago may benefit from baseline testing, as it can reveal long-term damage and help monitor whether lung function stabilizes over time.

People with occupational exposure to dust, chemicals, or fumes may have pulmonary function tests as part of workplace health monitoring. Construction workers, farmers, factory workers, and healthcare workers exposed to aerosol particles sometimes receive regular testing. These occupational exposures can gradually reduce lung function over months or years.

Patients about to undergo surgery may need pulmonary function tests to assess whether their lungs can handle anesthesia safely. People with existing lung conditions like asthma or COPD often have baseline tests, then repeat testing to see whether medications are helping. Additionally, people with family histories of lung disease may receive testing to detect early changes.

Individuals taking medications known to affect lung function—such as certain chemotherapy drugs or heart medications—sometimes undergo pulmonary function testing to monitor for side effects. Workers in environments with air quality concerns, or people living in areas with significant air pollution, may also benefit from these baseline measurements.

Practical takeaway: Multiple situations warrant pulmonary function testing. If you have symptoms like persistent cough or shortness of breath, a relevant medical history, or occupational exposures, discussing pulmonary function testing with your healthcare provider can help determine whether these measurements would provide useful information about your lung health.

How Pulmonary Function Tests Are Performed

Pulmonary function testing typically happens in a clinic, hospital, or specialized laboratory. A respiratory technician guides you through the process. The test is non-invasive, meaning nothing enters your body. Most people can complete testing within 30 to 90 minutes, depending on which tests are performed and how many times you need to repeat measurements for accuracy.

Before testing begins, the technician records your height, weight, age, and sex, as these factors affect expected lung measurements. You'll be asked about your medical history, current medications, and recent respiratory infections. It's important to mention if you've had a cold, flu, or other respiratory illness recently, as this can temporarily affect results. The technician explains the testing process and answers questions before you begin.

During spirometry, you sit in a comfortable chair wearing a nose clip to prevent air from escaping through your nose. You breathe normally through a mouthpiece connected to the spirometer. The technician instructs you to take a deep breath, then exhale as hard and fast as possible for at least six seconds. Most people repeat this maneuver three to eight times to ensure consistent, reliable measurements. You rest briefly between efforts.

If your test includes lung volume measurements, you'll breathe into the machine while wearing headphones. The machine measures air that remains in your lungs after you exhale completely. Diffusion testing, if performed, involves breathing in a harmless gas mixture for a few seconds, holding your breath briefly, then exhaling. None of these procedures causes pain or significant discomfort, though some people feel lightheaded from repeated deep breathing—a sensation that passes quickly.

After testing, results are recorded and sent to your doctor. Your doctor interprets the results in the context of your age, height, sex, and medical history. Results typically include comparisons to predicted normal values for someone your size and age. Your doctor discusses what the results mean for your specific situation.

Practical takeaway: Knowing what happens during pulmonary function testing reduces anxiety about the procedure. The test involves breathing into a machine multiple times—it's safe, quick, and provides precise measurements that help doctors understand your lung function.

Understanding Your Pulmonary Function Test Results

Pulmonary function test results appear as numbers and percentages compared to predicted values based on your age, height, sex, and race. Results are typically presented as a percentage of predicted normal values. A result of 80% to 120% of predicted often falls within normal range, though your doctor determines what's normal for your specific situation. Results below 80% may indicate reduced lung function, while results above 120% are less common and may reflect measurement variation or other factors.

The FEV1/FVC ratio—your one-second exhale volume divided by total exhale volume—provides important information. In healthy people, this ratio is typically 70% or higher. A lower ratio suggests obstruction in airways, meaning air gets trapped in your lungs. This pattern appears in conditions like asthma and COPD. A normal or high ratio combined with low overall volumes may suggest restrictive disease, where lungs have reduced capacity to fill with air.

Results may also include flow measurements showing how quickly air moves through your airways at different points during exhalation. These measurements help identify where blockages occur—large airways, small airways, or throughout. Diffusion test results indicate whether oxygen efficiently crosses from your lungs into your blood. Low diffusion values can suggest conditions like interstitial lung disease or emphysema.

Your doctor interprets results considering your symptoms, medical history, and physical examination. Someone with mild reductions in lung function but no symptoms may not need immediate treatment, while another person with similar test results but significant symptoms might need intervention. Results may show patterns suggesting specific conditions—for example, reversibility (improvement after using a bronchodilator medication) often indicates asthma, while irreversible obstruction suggests COPD.

It's common for results to raise questions. Ask your doctor what specific numbers mean, whether your lungs show obstruction, restriction, or other patterns, and what next steps might involve. Request written results to keep in your personal medical records. Repeat testing over months or years shows trends, helping your doctor determine whether your lung function is stable, improving, or declining.

Practical takeaway: Pulmonary function test results provide specific measurements compared to predicted normal values. Understanding the main numbers—FEV1, FVC, and their ratio—helps you grasp what your lungs are doing and makes conversations with your healthcare provider more productive.

Information Resources About Pulmonary Function Testing

The American Lung Association maintains extensive educational materials about pulmonary function