In pulmonary function testing, what does a low FEV1/FVC ratio indicate?

A low FEV1/FVC ratio is indicative of obstructive lung disease. This ratio measures the proportion of a person's vital capacity that they are able to expire in the first second of forced expiration (FEV1) compared to the total amount of air exhaled (FVC). In obstructive diseases, such as asthma and chronic obstructive pulmonary disease (COPD), the airflow is limited, making it difficult to exhale rapidly. As a result, the FEV1 decreases more significantly compared to the FVC, leading to a reduced FEV1/FVC ratio.

In contrast, restrictive diseases, characterized by reduced lung volumes and difficulty with lung expansion, typically present with a normal or high FEV1/FVC ratio, despite overall decreases in both FEV1 and FVC. Normal lung function would show a healthy FEV1/FVC ratio, usually around 70-80% in adults, while mixed disease patterns may have a transiently low or normal FEV1/FVC depending on the severity and type of diseases involved. Thus, the identification of a low FEV1/FVC ratio is crucial in diagnosing obstructive lung disorders.