The FVC (the Forced Vital Capacity) is the total amount of air the patient can exhale from a full inspiration.

The FVC is necessary to use in comparison to the FEV1 (FEV1/FVC) for the evaluation of obstruction. In the apparent absence of obstruction (normal FEV1/FVC), the FVC can be looked at to assess the patient for possible restrictive lung disease (can only be suggested by a low FVC, it cannot be diagnosed by a low FVC).

Accuracy of the FVC can affect the ability to check for obstruction (since it is the denominator in the FEV1/FVC) or the ability to assess possible restriction. The FVC should be from an exhalation that is as long as the patient is able to perform. An effort that is ended while expiratory flow is still present is suspicious for underestimation of the FVC.