Evaluating How Post-Bronchodilator Vital Capacities Affect the Diagnosis of Obstruction in Pulmonary Function Tests

Background: Although the ratio of FEV₁ to the vital capacity (VC) is universally accepted as the cornerstone of pulmonary function test (PFT) interpretation, FVC remains in common use. We sought to determine what the differences in PFT interpretation were when the largest measured vital capacity (VC_max) was used instead of the FVC.

Methods: We included 12,238 consecutive PFTs obtained for routine clinical care. We interpreted all PFTs first using FVC in the interpretation algorithm and then again using the VC_max, obtained either before or after administration of inhaled bronchodilator.

Results: Six percent of PFTs had an interpretive change when VC_max was used instead of FVC. The most common changes were: new diagnosis of obstruction and exclusion of restriction (previously suggested by low FVC without total lung capacity measured by body plethysmography). A nonspecific pattern occurred in 3% of all PFT interpretations with FVC. One fifth of these 3% produced a new diagnosis of obstruction with VC_max. The largest factors predicting a change in PFT interpretation with VC_max were a positive bronchodilator response and the administration of a bronchodilator. Larger FVCs decreased the odds of PFT interpretation change. Surprisingly, the increased numbers of PFT tests did not increase odds of PFT interpretation change.

Conclusions: Six percent of PFTs have a different interpretation when VC_max is used instead of FVC. Evaluating borderline or ambiguous PFTs using the VC_max may be informative in diagnosing obstruction and excluding restriction.