What a Lung Function Test Measures

A lung function test, also called spirometry or pulmonary function testing, measures how much air your lungs can hold and how fast you can breathe air in and out. The test does not diagnose a specific disease — instead, it shows whether your lungs are working as they should for someone your age and size.

During the test, you breathe into a machine called a spirometer. The machine records how much air you inhale, how much you exhale, and how quickly you exhale. A technician or respiratory therapist guides you through a series of breaths: normal breathing, then a deep breath in followed by a hard, fast breath out. The whole test usually takes 15 to 30 minutes.

Doctors order lung function tests when they suspect a breathing problem — chronic obstructive pulmonary disease (COPD), asthma, or restrictive lung disease — or to check how well a known lung condition is being managed. The test can also be part of a routine checkup before surgery, especially if you have a history of smoking or lung problems.

Key Takeaways

  • A lung function test measures how much air your lungs hold and how fast you can breathe it out, using a machine called a spirometer.
  • You will be asked to breathe normally, then take a deep breath and exhale as hard and fast as you can into a tube.
  • The test takes 15 to 30 minutes and produces numbers that your doctor compares to what is normal for your age, height, and sex.
  • You should avoid heavy exercise, large meals, and certain medications for a few hours before the test, depending on what your doctor tells you.
  • The test itself is painless and safe, though some people feel lightheaded or tired afterward because of the forced breathing.

How to Prepare for the Test

Before you arrive, ask your doctor whether you should stop taking any medications. Some inhalers and bronchodilators can affect the results, so your doctor may ask you to skip them for 4 to 24 hours before the test. Bring a list of all medications you take, including over-the-counter drugs and supplements.

Wear loose, comfortable clothing that does not restrict your chest or belly. Avoid a large meal right before the test — eat something light a few hours ahead instead. Do not do heavy exercise or strenuous activity for at least 30 minutes before you arrive. If you use an inhaler for rescue breathing, bring it with you in case you need it after the test.

Arrive a few minutes early so you can check in and let the staff know if you have any questions. Tell the technician if you have had recent chest or abdominal surgery, heart problems, or if you feel unwell that day — any of these may mean the test should be rescheduled.

What Happens During the Test

When the test begins, the technician will explain each step and show you how to use the spirometer. You will sit upright in a chair and put a soft clip on your nose to keep your nostrils closed. You then place your mouth around a tube connected to the spirometer.

First, you will breathe normally through the tube for a few breaths so the machine can record your regular breathing pattern. Then the technician will ask you to take the deepest breath you can, hold it for a moment, and then exhale as hard and fast as you can until your lungs feel empty. This forced exhale is the most important part of the test. You may be asked to repeat this maneuver three to eight times so the technician can get consistent, reliable results.

Some versions of the test include a second part where you breathe in a harmless gas for a few seconds, then exhale into the machine. This measures how well gas moves from your lungs into your bloodstream. The technician will tell you what to expect before each step.

Understanding Your Results

The spirometer produces several numbers. The main ones are FEV1 (the amount of air you can force out in one second) and FVC (the total amount of air you can exhale after a deep breath). Your doctor compares these numbers to predicted values based on your age, height, sex, and race.

If your results are 80 percent or higher than predicted, your lung function is considered normal. Results between 50 and 79 percent suggest mild to moderate obstruction. Below 50 percent suggests more severe obstruction. Your doctor will explain what your specific numbers mean and whether they point to a particular condition.

You will not receive results on the spot. A doctor must review the test and compare it to your medical history and symptoms. Your doctor's office will contact you with results and next steps, which may include starting a medication, repeating the test in a few months, or referring you to a lung specialist.

Risks and Side Effects

Lung function testing is safe for most people. The test itself is painless and does not use radiation or needles. However, the forced breathing can cause some temporary discomfort. You may feel lightheaded, dizzy, or short of breath during or right after the test. These feelings usually pass within a few minutes.

In rare cases, the forced exhale can trigger a coughing fit or cause chest discomfort, especially if you have asthma or COPD. If you have uncontrolled asthma or a very low FEV1, your doctor may decide the test is too risky or may use a gentler version. Tell the technician when ready if you feel chest pain, severe shortness of breath, or fainting during the test.

After the test, rest for a few minutes before standing up. If you brought a rescue inhaler, you can use it if you feel wheezy. Most people return to normal activity right away, though some feel tired and should avoid strenuous exercise for the rest of the day.

When Your Doctor May Order This Test

Your doctor may order a lung function test if you have a persistent cough, shortness of breath, wheezing, or chest tightness that lasts more than a few weeks. The test helps determine whether these symptoms come from a lung problem or something else.

If you have already been diagnosed with asthma, COPD, or another lung disease, your doctor may order the test every year or two to see whether your condition is stable or getting worse. The test also helps measure how well your medication is working. Before major surgery, especially if you are over 65 or have a smoking history, your doctor may order lung function testing to make sure your lungs can handle the stress of anesthesia and recovery.

Frequently Asked Questions

Will the test hurt?

No. The test is painless. You may feel pressure in your chest or throat from the forced breathing, and some people feel lightheaded or tired, but these sensations pass quickly. If you feel pain during the test, tell the technician right away.

Can I eat or drink before the test?

You can drink water. Avoid a large meal for at least two hours before the test because a full stomach can make it harder to take a deep breath. Avoid caffeine and sugary drinks, which can make your heart race and affect your breathing pattern.

What if I cannot do the forced exhale?

The technician will coach you through it. If you have trouble with the maneuver the first time, they will explain it again and let you practice. Most people can do it after one or two tries. If you truly cannot perform the test, your doctor will know and can order a different type of breathing test or imaging instead.

How long does it take to get results?

The test itself takes 15 to 30 minutes, but you will not receive results that day. A doctor must review the numbers and compare them to your history. Most offices contact you within a few days to a week with results and any next steps.

Can I take my regular medications before the test?

Ask your doctor before the test. Some medications, especially inhalers and bronchodilators, can change the results. Your doctor will tell you which ones to skip and for how long. Always bring a list of everything you take so the technician knows what to account for.