2025 ERS/ATS Update of the Classification of Interstitial Pneumonias: Key Changes

Interstitial pneumonia is a heterogeneous group of diffuse parenchymal lung diseases characterized by variable combinations of inflammation and fibrosis involving the interstitium, airways, and alveolar spaces. The classification of interstitial pneumonia has undergone a major update for the first time in 12 years since 2013. The 2025 European Respiratory Society/American Thoracic Society (ERS/ ATS) update introduces an integrated framework that extends beyond idiopathic entities to include secondary causes such as connective tissue disease and hypersensitivity pneumonitis. This update classifies interstitial pneumonias into ‘interstitial patterns’ and ‘alveolar filling patterns’ based on the predominant site of involvement, with interstitial patterns further subclassified into ‘fibrotic’ and ‘nonfibrotic’. Furthermore, it newly defines and reintroduces the ‘bronchiolocentric interstitial pneumonia’ pattern to replace the traditional ‘hypersensitivity pneumonitis pattern’ and renames the terms ‘acute interstitial pneumonia’ and ‘desquamative interstitial pneumonia’ to ‘diffuse alveolar damage’ and ‘alveolar macrophage pneumonia’, respectively, to more accurately reflect the underlying pathophysiology. Importantly, the update distinguishes morphologic patterns from multidisciplinary discussion diagnoses and formalizes the concept of diagnostic confidence, presenting a system of ‘confident’, ‘provisional’, or ‘unclassifiable interstitial lung disease’. Although this update is based on expert consensus, the new classification framework can provide a more comprehensive and practical approach for clinicians managing patients with complex interstitial pneumonias. This review summarizes these key changes and their clinical implications.