Understanding the FEV1/FVC Ratio
FEV1 (forced expiratory volume in one second) and FVC (forced vital capacity) are both measured during the same spirometry maneuver: you take a full breath in, then exhale as hard and fast as possible into the spirometer. FEV1 is the volume of air pushed out in the first second; FVC is the total volume exhaled. The FEV1/FVC ratio compares the two and is one of the most widely used screening numbers for airflow obstruction.
How the ratio is calculated
The formula is simple: FEV1/FVC ratio (%) = (FEV1 ÷ FVC) × 100. For example, an FEV1 of 3.2 L and an FVC of 4.0 L give a ratio of 3.2 ÷ 4.0 × 100 = 80%. Clinicians typically use the post-bronchodilator value (measured after an inhaled bronchodilator) for diagnosis, since it reflects the airway's fixed, non-reversible narrowing rather than a temporary spasm.
Obstructive vs. restrictive patterns
A ratio below 70% (0.70) is the GOLD-criteria threshold for an obstructive pattern — airways are narrowed, so air empties more slowly and FEV1 drops faster than FVC (seen in COPD and asthma). A restrictive pattern looks different: lung tissue or chest-wall stiffness reduces both FEV1 and FVC roughly together, so the ratio can stay normal or even climb above 85% even though the lungs hold less air overall. Ratio alone cannot tell obstructive from restrictive apart from a normal lung — that requires the absolute FVC and FEV1 % predicted values from a full pulmonary function test.
Why the fixed 70% cutoff has limits
Lung elastic recoil naturally declines with age, so a healthy person's FEV1/FVC ratio trends downward over a lifetime even without disease. Using one fixed 70% cutoff for everyone means older adults (roughly 65+) can be flagged as obstructive when their result is actually normal for their age, while younger adults with early obstruction can test "normal" at a ratio like 72-74% that is already below their personal baseline. Many pulmonologists prefer age-, sex-, height-, and ethnicity-adjusted lower-limit-of-normal (LLN) reference equations for this reason, though the fixed 70% threshold remains the standard used in GOLD guidelines for COPD.