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Asthma presents with reversible narrowing of the airways, so pre-bronchodilator spirometry often shows an obstructive pattern. A reduced FEV1 (≤ 80% predicted) together with a low FEV1/FVC ratio (≤ 75%) reflects this obstruction and supports an asthma diagnosis in the right clinical context. This pattern means the airways are narrowed enough to limit the first-second flow and to decrease the proportion of air exhaled in the first second, which is characteristic of obstructive physiology like asthma.

If spirometry is normal before bronchodilator, or shows a high FEV1 with a normal or high FEV1/FVC ratio, it does not indicate obstruction and is less supportive of asthma on its own. The key point is the presence of airflow obstruction on initial testing, which, when combined with symptoms and reversibility after bronchodilator, reinforces the diagnosis of asthma.