FEV1/FVC ratio
The FEV1/FVC ratio asks a simple question: of all the air you can blow out, what fraction came out in the first second? It is the single most important spirometry number for spotting blocked airways, because the bottleneck is about speed, not size.
Healthy adults clear roughly 70 to 80 percent of their vital capacity in that first second. When airways are narrowed — by asthma, COPD or bronchiectasis — air comes out more slowly, FEV1 falls faster than FVC, and the ratio drops. A reduced ratio is the defining feature of an obstructive pattern.
In restrictive disease the lungs are small but the airways are clear, so both FEV1 and FVC shrink together and the ratio stays normal or even rises. Because the normal ratio declines with age, modern practice compares it to the lower limit of normal rather than to a fixed 0.70 cut-off, which over-diagnoses obstruction in the elderly and misses it in the young.
Two patients each have an FEV1 of 70 percent predicted; one has a ratio of 0.62 (obstruction) and the other 0.84 (likely restriction) — the ratio is what tells them apart.
The ratio is the key to telling obstruction from restriction.
The GOLD criteria for COPD still use a fixed post-bronchodilator ratio below 0.70, while many physiology guidelines prefer the lower limit of normal; the two can disagree at the margins.