What Are Pulmonary Function Tests?
Pulmonary function tests (PFTs) are a group of measurements that show how well your lungs work. These tests measure 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 use PFTs to diagnose lung diseases, monitor existing conditions, and check how well treatments are working.
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Your lungs are two spongy organs in your chest that take in oxygen when you breathe in and release carbon dioxide when you breathe out. When something goes wrong with this process—whether from disease, injury, or weakness—your body doesn't get enough oxygen. PFTs help doctors figure out exactly what's not working and why. According to the American Lung Association, millions of Americans have chronic lung diseases like asthma, chronic obstructive pulmonary disease (COPD), and pulmonary fibrosis. PFTs play a key role in identifying and managing these conditions.
There are several different types of pulmonary function tests, and your doctor will order the ones most relevant to your situation. Some tests measure how much air your lungs can hold. Others measure how fast you can breathe air out. Still others measure how well oxygen moves from your lungs into your blood. The tests are painless and non-invasive, meaning nothing enters your body. You'll be asked to breathe into a machine called a spirometer, which records your breathing patterns.
Practical Takeaway: Before your PFT appointment, ask your doctor which specific tests you'll have and why they're recommending them. Understanding the purpose of your testing helps you prepare mentally and follow instructions more accurately during the procedure.
Common Types of Pulmonary Function Tests
Spirometry is the most common pulmonary function test. During this test, you'll breathe into a mouthpiece connected to a spirometer. The machine measures how much air you can breathe in (called inspiratory capacity) and how much you can breathe out (called vital capacity). It also measures how fast you can empty your lungs. You'll typically be asked to take several deep breaths and then blow out as hard and as fast as you can. The test usually takes about 15 minutes total.
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Lung volume tests measure how much air stays in your lungs even after you've exhaled completely. These tests use a technique called helium dilution or nitrogen washout. Some air always remains in your lungs—this is normal and necessary. However, if too much air gets trapped, it means your lungs aren't working efficiently. Lung volume tests help doctors see the total picture of your lung capacity, not just what you can actively breathe out. This test takes about 20 to 30 minutes.
Diffusion capacity testing (also called DLCO testing) measures how well oxygen moves from your lungs into your blood. For this test, you'll breathe in a small amount of harmless carbon monoxide gas mixed with room air, hold your breath for about 10 seconds, and then exhale into the machine. The machine measures how much of the carbon monoxide your lungs absorbed. A normal result means oxygen transfer is working well. Low results may suggest diseases like pulmonary fibrosis or emphysema. This test typically takes 5 to 10 minutes.
Bronchial challenge tests measure how sensitive your airways are. During these tests, you'll inhale increasing amounts of a substance like methacholine or exercise on a treadmill while your airflow is measured. If your airways narrow significantly in response, it suggests you may have asthma or reactive airways. These tests help doctors distinguish asthma from other lung conditions and assess the severity of airway sensitivity.
Practical Takeaway: Write down the names of each test you'll undergo and what it measures. This helps you understand your results when your doctor reviews them with you and makes it easier to explain your testing to other healthcare providers.
How to Prepare for Your Pulmonary Function Tests
Preparation is important for getting accurate results. Several days before your test, avoid respiratory infections if possible. If you have a cold or other infection, contact your doctor to ask whether to reschedule. Infections can temporarily affect your breathing and may alter your results. You should also avoid heavy exercise for at least 30 minutes before testing, as this can make your breathing rate elevated and affect measurements.
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On the day of your test, wear loose, comfortable clothing that doesn't restrict your chest or abdomen. Tight clothing makes it harder to take deep breaths. Avoid eating a large meal right before testing—a full stomach can restrict your ability to breathe deeply. However, don't skip meals entirely, as low blood sugar can make you feel faint during the test. Eat a light meal 2 to 3 hours before your appointment.
Medication timing matters for PFTs. If you use inhalers or other breathing medications, your doctor will give you specific instructions about when to stop using them before testing. Some medications can affect your results, so honesty with your healthcare team is essential. Bring a list of all medications and supplements you take. Also, avoid caffeine before testing, as it can stimulate your breathing and affect measurements.
Arrive at least 15 minutes early so you're not rushed. You'll be asked to remove any jewelry around your neck and to empty your bladder before testing. The technician will explain each test step-by-step and may do a practice run before the actual measurement. Tell the technician about any fears or concerns—they're trained to work with anxious patients and can explain exactly what will happen. Being relaxed and well-informed leads to better cooperation and more reliable results.
Practical Takeaway: Create a checklist for the day before your test: confirm your appointment time, plan your clothing, review medication instructions, and arrange transportation. Having everything organized reduces stress and helps you focus on following the testing instructions correctly.
What to Expect During Your Pulmonary Function Tests
When you arrive, a respiratory technician or pulmonologist will take your medical history, including information about your symptoms, medications, smoking history, and any previous lung problems. They'll measure your height and weight, as these measurements are used to calculate what your "normal" results should be. Everyone's lungs are different sizes based on body size, so doctors compare your results to predicted values for someone your age, sex, and size.
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Before any testing begins, the technician will show you the equipment and explain exactly what you'll be asked to do. For spirometry, you'll sit upright in a chair, place a clip on your nose to prevent air from escaping through your nostrils, and put a mouthpiece in your mouth. You'll then breathe normally for a few breaths, then take the deepest breath possible and blow out as hard and fast as you can until your lungs feel empty. You'll repeat this several times to make sure the results are consistent. The technician will watch your effort and give you encouragement like "keep going" or "blow harder."
During the tests, you might feel slightly lightheaded or dizzy after blowing out hard repeatedly. This is normal and temporary. Some people experience a slight cough or throat irritation from the clip or mouthpiece. These sensations go away quickly. The entire testing session typically takes 45 minutes to an hour, depending on which tests are being performed and how many times you need to repeat them for consistent results.
Throughout testing, communication is important. If you feel uncomfortable, dizzy, or need to stop, let the technician know immediately. They can pause and let you rest. If you don't understand an instruction, ask for clarification. The technician needs your full effort and cooperation to get meaningful results, so they're invested in making sure you're comfortable and understand what's expected.
Practical Takeaway: During testing, focus on following instructions precisely. When asked to blow out hard, really push the air out. When asked to breathe normally, relax. The effort you put into following directions directly affects the quality of your results and your doctor's ability to make an accurate diagnosis.
Understanding Your Pulmonary Function Test Results
Your results will be compared to "predicted values"—what researchers have determined is normal for someone your age, height, weight, and sex. Results are typically reported as a percentage of predicted normal values. For example, if your FEV1 (forced expiratory volume in one second) is 85% of predicted, it means you're