COPD is diagnosed primarily through a breathing test called spirometry, which measures how much air you can blow out and how fast. The key number doctors look for is whether the ratio of air you can force out in one second compared to your total exhaled breath falls below 0.7 after inhaling a bronchodilator medication. That single measurement confirms airflow obstruction, but most people go through several additional tests to determine severity, rule out other conditions, and guide treatment.
Spirometry: The Core Diagnostic Test
Spirometry is the test that actually confirms or rules out COPD. You breathe into a mouthpiece connected to a machine, inhale as deeply as you can, then blow out as hard and as long as possible. The machine records two critical measurements: the volume of air you force out in the first second (called FEV1) and the total volume you exhale (called FVC). If the ratio between these two numbers is less than 0.7 after you’ve inhaled a bronchodilator, you have the airflow obstruction that defines COPD.
The 2025 guidelines from the Global Initiative for Chronic Obstructive Lung Disease (GOLD) actually streamlined this process. Doctors now use a pre-bronchodilator reading first as a screening step. If your ratio comes back above 0.7 before any medication, COPD is essentially ruled out. Only if it falls below 0.7 do you then repeat the test after inhaling a bronchodilator to confirm the diagnosis.
Your FEV1 percentage also tells your doctor how severe the obstruction is. Higher percentages mean milder disease with more preserved lung function, while lower percentages indicate more advanced obstruction.
How Doctors Tell COPD Apart From Asthma
The bronchodilator used during spirometry does double duty. It confirms COPD, but it also helps distinguish COPD from asthma, which can look similar on initial testing. The key difference is reversibility. In asthma, inhaling a bronchodilator typically improves your FEV1 by more than 12% and more than 200 milliliters. That improvement means the airway narrowing is temporary and responsive to medication.
In COPD, the obstruction stays largely the same after the bronchodilator. The airways are permanently narrowed, not just temporarily constricted. If your numbers barely budge after the medication, that fixed obstruction points toward COPD rather than asthma. Some people have features of both conditions, which can complicate the picture, but this reversibility test remains the primary way to separate the two.
What to Expect Before Your Test
Pulmonary function tests require some preparation to get accurate results. The National Heart, Lung, and Blood Institute provides specific guidance: don’t eat a large meal within two hours, avoid heavy exercise for at least 30 minutes, skip alcohol for at least four hours, and don’t smoke on the day of your test. Wear loose, comfortable clothing that won’t restrict a deep breath.
Medication timing matters significantly. You’ll need to stop certain inhalers before the test so they don’t mask your baseline lung function. Short-acting rescue inhalers should be stopped 6 hours before, short-acting anticholinergic inhalers 12 hours before, long-acting inhalers 24 hours before, and ultra-long-acting inhalers 36 hours before. Bring a list of all your medications, including doses. If you arrive more than 30 minutes late, you may need to reschedule.
Imaging Tests
A chest X-ray or CT scan doesn’t diagnose COPD on its own, but imaging reveals structural damage that spirometry can’t show. High-resolution CT scans are particularly useful for identifying emphysema, one of the main forms of COPD. On these scans, destroyed air sacs appear as dark areas measured at specific density thresholds. The scans can pick up even relatively small patches of emphysema, showing doctors exactly where and how extensively the lung tissue has broken down.
CT scans also help rule out other conditions that cause similar symptoms, like lung cancer or pulmonary fibrosis. For people with COPD, imaging provides information about airway wall thickening (more common in the chronic bronchitis type) and hyperinflation, where damaged lungs trap air and expand beyond their normal size.
The Gas Transfer Test
A test called the diffusing capacity test measures how efficiently your lungs move oxygen from inhaled air into your bloodstream. You breathe in a small, harmless amount of carbon monoxide, hold your breath briefly, then exhale. The machine measures how much of that gas your lungs absorbed.
In emphysema, the tiny air sacs where gas exchange happens are destroyed, so less surface area is available to absorb oxygen. This shows up as a notably decreased diffusing capacity. The test is especially helpful for confirming emphysema specifically, since people with the chronic bronchitis form of COPD may have relatively normal gas transfer despite significant airway obstruction.
Measuring Trapped Air
COPD causes air to get trapped in the lungs because damaged airways collapse during exhalation before all the air can escape. A test called body plethysmography measures this directly. You sit inside a sealed, phone-booth-sized chamber and breathe against a closed shutter while sensors track pressure changes to calculate your lung volumes.
Doctors look at the residual volume, which is the air left in your lungs after you exhale as completely as possible. In COPD, this volume is elevated because trapped air can’t escape. The ratio of residual volume to total lung capacity serves as a direct measure of how much hyperinflation is present. People with COPD typically have elevated total lung capacity, elevated residual volume, and a higher ratio between the two compared to healthy lungs.
Blood Oxygen Levels
Arterial blood gas testing measures the actual oxygen and carbon dioxide levels in your blood by drawing a sample from an artery, usually in the wrist. This test determines whether COPD has progressed to the point of respiratory failure, defined as blood oxygen falling below 60 mmHg.
Two patterns emerge. In one type, oxygen drops but carbon dioxide stays normal or low, meaning the lungs still ventilate adequately but can’t match airflow to blood flow efficiently. In the more severe type, oxygen drops while carbon dioxide rises above 45 mmHg, indicating the lungs aren’t moving enough air in and out overall. These results help determine whether you need supplemental oxygen and how aggressively your COPD needs to be managed. A simpler version, pulse oximetry (the clip placed on your finger), gives a quick oxygen reading but doesn’t measure carbon dioxide or provide the same level of detail.
Genetic Screening
A blood test for alpha-1 antitrypsin deficiency checks for a genetic condition that causes COPD even in people who have never smoked. Alpha-1 antitrypsin is a protein that protects lung tissue from damage. People who produce too little of it can develop emphysema at younger ages, sometimes in their 30s or 40s.
Both the American Thoracic Society and the European Respiratory Society recommend this test for all patients diagnosed with COPD, regardless of age, ethnicity, or smoking history. Updated guidelines from the Alpha-1 Foundation reinforce this recommendation. Despite these guidelines, the test is frequently overlooked. Identifying the deficiency matters because specific treatments exist for it, and family members can be screened as well.
The 6-Minute Walk Test
This deceptively simple test measures how far you can walk on a flat surface in six minutes. It provides a real-world snapshot of your functional capacity that lab tests alone can’t capture. The distance you cover correlates strongly with prognosis.
Research on COPD patients in pulmonary rehabilitation found that those who walked 250 meters or more had an 81% three-year survival rate. Those who managed 150 to 249 meters had a 66% three-year survival rate. Patients who covered less than 150 meters had only a 34% three-year survival rate, a prognosis comparable to some advanced cancers. For every additional meter covered during the test, the likelihood of death decreased by about 1%. The test is repeated over time to track whether your functional capacity is stable, improving with treatment, or declining.