One of the benefits of having a spirometry test is that it can detect abnormalities in lung function even if signs and symptoms are not yet present. Not only will a spirometry test help your doctor diagnose COPD and other lung diseases in their earliest stages, it can also help your doctor determine how much damage has been done within your lungs.
Four important measurements that are obtained during a spirometry test are: 1
The following chart may help you better understand these concepts:
|OBSTRUCTIVE AND RESTRICTIVE LUNG PATTERNS CHART|
|Measurement||Obstructive Pattern||Restrictive Pattern|
|Forced vital capacity (FVC)||Decreased or normal||Decreased|
|Forced expiratory volume in one second (FEV1)||Decreased||Decreased|
|FEV1/FVC ratio||Decreased||Normal or increased|
|Total lung capacity (TLC)||Normal or increased||Decreased|
Once your doctor determines whether your lung disorder is obstructive or restrictive, she should grade the severity of the abnormality based on the FEV1 percentage of predicted. This will help your doctor choose appropriate treatment options. Although there are others equally as accurate, the following chart represents the American Thoracic Society’s system for determining the severity of lung disease:
|American Thoracic Society Grading System for Pulmonary Function Test Abnormality|
|Severity of Lung Disease||FEV1 Percentage of Predicted|
|Very severe||< 35|
If your spirometry results indicate an obstructive defect, your doctor may administer a bronchodilator challenge to determine if the obstruction is reversible. It’s considered reversible if the FEV1 or the forced vital capacity (FVC) increase by more than 12% and 200 milliliters in adults 18 years of age and older. Please note: obstruction is usually fully reversible in asthma whereas it may be only partially reversible in people with COPD.
For more information about forced vital capacity and other pulmonary function test measurements, talk to your pulmonologist or primary care provider.
 Johnson, Jeremy D., MD, MPH & Theurer, Wesley M., DO, MPH. A Stepwise Approach to the Interpretation of Pulmonary Function Test. Am Fam Physician. 2014 Mar 1;89(5):359-366.
 American Thoracic Society. Gading System for Pulmonary Function Test Abnormality. Accessed March 17, 2017.