Closed- versus Open-Circuit Spirometry Tests. Spirometry is a pulmonary function test (PFT) used to diagnose airway obstructions with forced vital capacity (FVC) and forced expiratory volume (FEV-1) measurements. It’s also used to determine risk factors for certain pulmonary diseases, the severity and prognosis of lung function, detect early lung disease, and monitor lung growth or functional decline. Two types of spirometry procedures exist: closed-circuit and open-circuit testing, both can be conducted either sitting or standing. Clinicians should take note of how the baseline test was conducted as all future spirometry tests need to be performed in the same position.