The most important test of lung function is spirometry. Spirometry is used for many lung conditions. For people with asthma, the test provides information such as:1

  • Whether your lung disease is obstructive or restrictive
  • How blocked your airways
  • Whether medications can open the airways
  • How sensitive your airways are
  • Whether your medications are working
  • Whether your lung function has changed over time

Spirometry can be done in most adults and children older than five.1

Where is spirometry done?

If your family doctor or pediatrician has the equipment, spirometry can be done in the office.1 Spirometry is also done in a hospital pulmonary function laboratory. Some pharmacies provide spirometry.

How is spirometry for asthma done?

The test may be performed standing or sitting. You may be asked to wear nose clips or pinch your nose shut.2 You will be asked to breathe into a mouthpiece that is attached to a device called the spirometer.

There are three phases of the test: (1) taking the deepest breath possible, followed by (2) blasting air out of the lungs as rapidly as possible, and then (3) continuing to breathe out for as long as possible.2,3 You will be asked to do this at least three times to get a valid test. The attempt does not count if you cough, if there is a delay in exhaling, or if you stop breathing too early. It could take up to eight attempts to get a valid test.2

A valid test requires a lot of effort from you! The technician will encourage you and provide instructions throughout the test. The test can be tiring and uncomfortable. You may feel light headed or dizzy.1 You can take a break between attempts. Giving it your best effort is important so that the results lead to the right diagnosis.

The test may be repeated after you take a medication that opens your airways, called a bronchodilator. Albuterol is the medication typically used. Fifteen to 30 minutes after taking the medication, you will be asked to do spirometry again.4 This step is done to see if the airway blockage (obstruction) is “reversible.”

What are the results from spirometry?

There are three main results from spirometry:5

  • FEV1: The amount of air you can forcefully exhale in one second. FEV1 stands for forced expiratory volume in one second.
  • FVC: The maximum amount of air you can forcefully exhale. FVC stands for forced vital capacity.
  • FEV1/FVC: The percentage your total air capacity that you can forcefully exhale in one second.

The results are given in liters per minute and percentages. The percentage is based on “predicted normal values,” which are the expected results for someone your age, height, weight, gender, and race.3 Table 1 shows normal results in percentages.3

Table 1. Normal spirometry results

Normal Value
80% to 120%
80% to 120%
±5% predicted

Barreiro TJ, Perillo I. An approach to interpreting spirometry. Am Fam Physician. 2004;69:1107-1114. http://www.aafp.org/afp/2004/0301/p1107.html

How are spirometry results used?

To diagnose asthma: Having a low FEV1 and low FEV1/FVC means that the air has a hard time leaving the lungs, which indicates airway obstruction. Obstructive lung diseases include asthma and COPD (chronic obstructive pulmonary disease).

The results after taking albuterol (“post-bronchodilator results”) help separate asthma from COPD. If FEV1 increases by 12% or 200 mL after taking albuterol, it means that the airway blockage is reversible.1 Reversibility is a sign of asthma.

To figure out how severe asthma is: The results of spirometry indicate how severe your asthma is, as shown in Table 2.1

Table 2. Classifying asthma severity

Mild Asthma
Moderate Asthma
Severe Asthma
Adults: Normal Children: >80%
Adults: Reduced 5% Children: 75%-80%
Adults: Reduced >5% Children: <75%

National Heart, Lung, and Blood Institute. Expert panel report 3 (EPR-3): Guidelines for the diagnosis and management of asthma - Full Report 2007. Accessed 11/12/14 at: http://www.nhlbi.nih.gov/files/docs/guidelines/asthgdln.pdf

To see how well medications are working: Spirometry should be repeated after you have started taking medications and your symptoms have stabilized.1 The results provide information about how well controlled your asthma is.1

Table 3. Evaluating asthma control

Well Controlled
Not Well Controlled
Very Poorly Controlled

National Heart, Lung, and Blood Institute. Expert panel report 3 (EPR-3): Guidelines for the diagnosis and management of asthma - Full Report 2007. Accessed 11/12/14 at: http://www.nhlbi.nih.gov/files/docs/guidelines/asthgdln.pdf

What should I do to prepare for spirometry for asthma?

Wear comfortable, loose clothing.2 Do not exercise vigorously or eat a large meal before the test. You may need to stop taking some of your asthma medications before the test. Your health care provider will advise you about which medications to stop and which are fine to continue.2

How often do I need to do spirometry for asthma?

Spirometry can be used to see how lung function changes over time. A decline in lung function increases the risk of an asthma attack. Spirometry should be done after treatment has started and symptoms have stabilized. It should be repeated anytime symptoms start to worsen, and at least once every one to two years.1 For pregnant women, spirometry is the preferred way to monitor lung function at each prenatal visit.1

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Written by: Sarah O'Brien | Last Reviewed: May 2016.