What pulmonary function test (PFT) is considered hallmark for interstitial lung disease?

The hallmark pulmonary function test finding associated with interstitial lung disease (ILD) is a decreased diffusing capacity of the lungs for carbon monoxide (DLCO). ILD often affects the alveolar-capillary membrane, impairing gas exchange and leading to lower DLCO values. This decrease is primarily due to the lung's reduced ability to transfer gas from the alveoli into the blood, which occurs in many forms of ILD, including pulmonary fibrosis and sarcoidosis.

In contrast, the other options represent different patterns that are not characteristic of ILD. For example, an increased FEV1/FVC ratio typically suggests a restrictive lung disease, but in conditions like asthma or COPD, it does not apply to ILD. Increased total lung capacity (TLC) may indicate hyperinflation which is not typical in ILD, as these conditions lead to a reduction in lung volumes. Lastly, a normal airflow obstruction would be expected in healthy individuals or those with obstructive lung diseases but does not reflect the changes present in patients with ILD where airflow is usually preserved but lung volumes and diffusing capacity are affected.