Best test for lung fibrosis-

Chest X ray may be normal in individuals with early or limited disease.HRCT typically demonstrates a patchy,predominantly peripheral,subpleural and basal reticular pattern, and,in more advanced disease,the presence of honeycombing cysts and traction bronchiectasis.When these features are present,HRCT has a high positive predictive value for the diagnosis of pulmonary fibrosis and recourse to biopsy is seldom needed.HRCT appearances may also be sufficiently characteristic to suggest an alternative diagnosis such as hypersensitivity pneumonitis or sarcoidosis or asbestosis. Reference:Davidson's Medicine-22nd edition,page no:708