As you breathe into a mouthpiece that is attached to monitoring equipment, a series of measurements is made to evaluate how well your lungs are functioning and to detect any respiratory abnormalities.

Purpose of the Pulmonary Function Tests

  • To determine the cause of breathing difficulties, such as shortness of breath
  • To distinguish between obstructive lung disease (such as asthma or emphysema), which primarily inhibits expiration (breathing out), and restrictive lung disease (such as pulmonary fibrosis, respiratory muscle weakness, or tumors), which primarily inhibits inspiration (breathing in)
  • To assess the severity of known lung disease, monitor the course of lung disease, or evaluate the effectiveness of specific treatments (such as bronchodilator medications)
  • To evaluate patients before thoracic (chest) or abdominal surgery
  • To detect narrowing in the airways

Who Performs It

  • A doctor, a respiratory therapist, or a pulmonary lab technician

Special Concerns

  • Pulmonary function testing may not be safe for individuals with unstable asthma or respiratory distress; severe heart disease or a recent heart attack; pneumothorax (leakage of air outside the lungs and into the pleural cavity, resulting in a collapsed lung); or active tuberculosis.
  • Pregnancy or a distended stomach may affect test results.
  • Accurate results may not be possible in people who cannot follow the examiner’s instructions carefully, such as those with chest pain (for example, due to a fractured rib) or mental instability.

Before the Pulmonary Function Tests

  • Do not eat a heavy meal before the test.
  • Do not drink alcohol for at least 4 hours prior to the test.
  • Do not exercise heavily for a minimum of 30 minutes prior to the test.
  • Do not smoke and stay away from others who are smoking for at least 1 hour prior to the test.
  • Be sure to inform your doctor of all medications you are taking. Certain drugs, including bronchodilators, pain relievers, and sedatives, may affect the results and should be discontinued before the test.
  • If you have well-fitted dentures, wear them during the test.
  • Wear loose clothing that won’t inhibit your breathing.
  • Empty your bladder before the test.

What You Experience

  • A clip is placed on your nose to prevent air from passing through your nostrils during the procedure.
  • You are asked to seal your lips tightly around a mouthpiece that is connected by a tube to the monitoring device. At first, you should breathe normally while the examiner makes sure the test equipment is working properly.
  • Next, you are instructed to perform a variety of breathing exercises, such as breathing in as deeply as possible, briefly holding your breath, and exhaling as hard as you can. You will repeat these exercises several times. For the most accurate results, follow the examiner’s instructions as closely as possible.
  • In some cases, you will repeat the tests after inhaling a bronchodilator spray medication, or you may inhale a special mixture of air and carbon monoxide in order to assess how well your lungs deliver oxygen to your blood. Occasionally, breathing tests are performed while you exercise by walking on a treadmill or cycling on a stationary bike.
  • If at any point you feel tired or lightheaded, ask for a chance to rest between exercises.
  • The total process takes from 20 to 45 minutes, depending on how many tests are done.

Risks and Complications

  • Pulmonary function testing is generally safe, though it may temporarily irritate breathing symptoms in those with some types of lung disease.
  • Inhalation studies may trigger an asthmatic episode, so bronchodilators are kept available for immediate treatment.

After the Pulmonary Function Tests

  • You may leave the testing facility immediately after the test, and resume your normal diet and any medications that were withheld.
  • If you feel tired, dizzy, or lightheaded, rest until you are recovered.


  • A physician will assess your lung function by comparing your test values with the average values obtained from healthy individuals of your age, sex, height, and weight. Results are expressed as a percentage; in general, a score lower than 80 percent is considered abnormal.
  • Arterial blood gas levels, another valuable measure of lung function, are usually also considered when making a diagnosis.
  • Based on your test results, medical history, and physical exam, your doctor will recommend appropriate treatment, if necessary.
  • If your results are normal but your symptoms still suggest a diagnosis of asthma, the doctor may recommend a methacholine challenge test.


The Johns Hopkins Consumer Guide to Medical Tests

Simeon Margolis, M.D., Ph.D., Medical Editor

Updated by Remedy Health Media

Publication Review By: the Editorial Staff at

Published: 25 Jan 2012

Last Modified: 27 Jan 2015