Evaluation of Supranormal Spirometry Values with an Obstructive Ratio for Airway Hyperreactivity

Background: Published guidelines on spirometry interpretation suggest an elevated FVC and FEV₁ > 100% of predicted with an obstructive ratio may represent a physiological variant. Further evidence is needed on whether this finding indicates symptomatic airways obstruction and what additional evaluation should be done. Methods: Participants were prospectively enrolled to undergo additional testing for a technically adequate spirometry study with an FEV₁ > 90% of predicted, and FEV₁/FVC below the lower limit of normal, based on 95th percentile confidence intervals. Further testing consisted of full pulmonary function testing, impulse oscillometry (IOS), post-bronchodilator testing, fractional exhaled nitric oxide (FeNO), and methacholine challenge testing (MCT). Results: A total of 49 patients meeting entry criteria enrolled and completed testing. Thirty-three were considered symptomatic based on clinical indications for initial testing and 16 were considered asymptomatic. Baseline pulmonary function test values were not different between groups while IOS R₅ values (% predicted) were higher in the symptomatic group (126.5 ±0.37 vs 107.1 ±0.31). Bronchodilator responsiveness on PFT or IOS was infrequent in both groups. There was a 29% positivity rate for MCT in the symptomatic group compared to one borderline study in asymptomatic participants. FeNO was similar for symptomatic, 26.17 ±31.3 ppb, compared to asymptomatic, 22.8 ±13.5 ppb (p= 0.93). The dysanapsis ratio was higher in the symptomatic (0.15 ±0.03) compared to the asymptomatic (0.13 ±0.02) (p< 0.05). Conclusion: Normal FEV₁ > 90% of predicted and obstructive indices may not represent a normal physiological variant in all patients. In symptomatic patients, a positive MCT and elevated baseline IOS values were more common than in asymptomatic patients with similar PFT characteristics. These findings suggest that clinicians should still evaluate for airway hyperresponsiveness in patients with exertional dyspnea with airway obstruction and FEV₁ > 90% of predicted and consider alternative diagnoses to include a normal physiologic variant if non-reactive.