What Does Spirometry Measure?

Spirometry Is Important For Your Asthma Control

Spirometry. Photo © CDC

Question: What Does Spirometry Measure?

Answer: There is an adage that says what is not measured cannot be changed. If you want to improve your asthma control, you will need to consider performing home spirometry or peak flows on a regular basis. Spirometry is one of the most commonly performed pulmonary function tests when you have asthma.

When you inhale and exhale into the mouthpiece of a spirometer, it measures airflow.

Spirometers can not only measure how much airflow, but also measure airflow per unit time. Some examples of parameters spirometry can measure include:

Both FVC and FEV1 are important in the diagnosis and treatment of lung problems such as asthma. The values will depend on your The FEV1 to FVC ratio is the most used component to determine if an obstructive process like asthma is present. The degree of obstruction is generally classified in patients as:

> 80% = minimal obstructive defect

65 - 80% = mild obstructive defect

50 - 65% = moderate obstructive defect

< 50% = severe obstructive defect

The spirometer can accurately determine a number of other values and compare them to predicted values in healthy populations.

You will need to talk with your doctor or asthma educator to determine which test is most appropriate for you and how often the test will need to be performed. The FEV1 to FVC ratio is often used to determine if obstruction is present.

Other parameters that can be measured with spirometry include:

  • Forced expiratory flow or volume 25% to 75%. This measures the flow or volume in the middle of your exhalation. This test is thought to decrease before the FEV1/ FEVC ratio decreases so that a level below 50% may indicate that an obstructive process like asthma is present.
  • Peak expiratory flow (PEF). Your PEF will vary and an estimated value depends on your gender, race, height, weight, and age. In terms of your asthma control it is important to know your personal best peak flow or the highest peak flow achieved during a 2 week well period. This number is often part of your asthma action plan and decreases of a certain amount will trigger an action by you.
  • Total lung capacity (TLC). The total amount of air in your lungs after you inhale as deeply as possible.
  • Maximum voluntary ventilation (MVV). The total amount of air that you can breathe in and out in one minute.
  • Functional residual capacity (FRC). The amount of air that remains in your lung after a normal breath.

If your spirometry indicates poor asthma control, you will need to make sure that you are using your metered dose inhaler or other asthma medications correctly. You also need to make sure that you are adherent, taking medications regularly, and understand your complete asthma action plan.

Why Would My Doctor Order Spirometry?

There are a number of different reason why your doctor might order spirometry including:

  • Determine how much air you can breathe in and out of your lungs.
  • Determine how fast you can move air in and out of your lungs.
  • Detrmine how well oxygen gets into your lungs while breathing.
  • Help determine the cause of your symptoms such as wheezingchest tightness, shortness of breath, or cough.
  • Monitor the severity of your symptoms (mild, moderate, severe) over time
  • Determine how effective your current asthma treatment plan is.
  • Evaluate your respiratory function before surgery.

What Do I Need To Do To Prepare For My Spirometry Test

Your doctor should tell you beforehand if you need to avoid the use of your inhaled medications before the test. You will also want to wear loose fitting clothes that will not interfere with your ability to take a deep breath.

Also do not eat a large meal before the test as this could also make it hard to breathe.

Are There Risks of Office Spirometry?

Generally office spirometry is a very safe procedure and without significant side effects or risks. Patients sometimes experience dizziness during the procedure that almost always resolves with rest and deep breathing. Recent abdominal, intracranial, or eye surgery or a pneumothorax are relative contraindications to performing spirometry due to the increased pressure caused by the procedure. Be sure to mention any of these procedures to your doctor before performing spirometry.


Clinical Pulmonary Function Testing, Exercise Testing, and Disability Evaluation. In Chest Medicine: Essentials Of Pulmonary And Critical Care Medicine. Editors: Ronald B. George, Richard W. Light, Richard A. Matthay, Michael A. Matthay. May 2005, 5th edition.

National Heart, Lung, and Blood Institute. Accessed: September 6, 2016. Expert Panel Report 3 (EPR3): Guidelines for the Diagnosis and Management of Asthma

Enright, PL, Studnicka, M, Zielinski, J. Spirometry to detect and manage COPD and asthma in the primary care setting. Eur Respir Mon 2005; 31:1.

Ferguson GT, Enright PL, Buist AS, Higgins MW. Office spirometry for lung health assessment in adults: A consensus statement from the National Lung Health Education Program. Chest 2000; 117:1146.

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