What may be the earliest evidence on a standard pulmonary function test of the presence of ILD?

Question

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Abnormal DLCO (Diffusing Capacity of the Lungs for Carbon Monoxide) may be the earliest evidence of the presence of ILD (Interstitial Lung Disease). This is because the DLCO is affected by the gas exchange in the alveoli, which is impaired in ILD due to the thickening of the interstitial tissue. Other parameters on the pulmonary function test may also be abnormal in ILD, but abnormal DLCO is commonly seen even in early stages. Decreased FEV1/FVC (Forced Expiratory Volume in one second/Forced Vital Capacity) is more commonly seen in obstructive lung diseases, such as asthma or COPD, rather than ILD. Widened P(A-a)O2 (Alveolar-arterial Oxygen Gradient) or increased shunt may indicate ventilation-perfusion mismatch or intrapulmonary shunting, respectively, but these are not specific for ILD and can also occur in other lung diseases or conditions.

What May Be the Earliest Evidence on a Standard Pulmonary