Interstitial changes on Chest XRay on its own has limitations to a clinical diagnosis unless you have experienced difficulty in breathing (on exertion or on rest [which is highly significant]) and a HRCT
scan is the best modality of investigation in this context.
A functional test is the pulmonary function test with DLCO that measures CO transfer (ie., gas exchange) which is critical in understanding the current pathology.
Previously lung biopsies were done that is not the current practice unless there is no other option.
There are several reasons for interstitial changes on lung fields, but it is best you see a Chest Physician who will request the above investigations and take a detailed clinical history (such as if you keep birds etc or have asthma
) before a final diagnosis is made.