This ratio compares how much air a person can force out in one second to their total forced exhaled volume, and is the primary number used to detect obstructive lung disease.
No obstructive pattern (compare to the age-based lower limit of normal).
Obstructive pattern present.
Moderate obstruction.
Severe obstruction.
Given: FEV₁ 3.0 L, FVC 4.0 L.
In obstruction, less of the total volume escapes in the first second, lowering the ratio.
No — it rules out an obstructive pattern specifically. A restrictive pattern (low FVC with a preserved or even high ratio) can still indicate disease such as pulmonary fibrosis.
Roughly 0.75–0.80 in young adults, declining somewhat with normal aging. A ratio below 0.70 — or below the lower limit of normal for age — suggests obstruction.
Obstructive diseases: COPD and asthma are the most common, along with bronchiectasis. Airflow out is slowed, so less of the total volume comes out in the first second.
Restrictive disease shrinks both FEV₁ and FVC proportionally (or FVC more), so the ratio is preserved even though lung volumes are reduced.