The 2025 ATS / ERS update of the international multidisciplinary classification of the interstitial pneumonias: implications for the pathologist

A consensus classification for idiopathic interstitial pneumonias was first published in 2002, providing terminology for clinicians, radiologists and pathologists as well as highlighting that a multidisciplinary approach was best clinical practice. This comprised seven histological patterns that had corresponding idiopathic multidisciplinary diagnoses. This classification was updated in 2013, with the addition of an eighth histological pattern. Since 2013, there have been further advances in our understanding of interstitial pneumonias, in particular relating to idiopathic versus secondary disease, advances in molecular pathology and recognition of progressive pulmonary fibrosis (PPF), which have led to a further update in 2025. This review highlights the changes relevant to pathologists reporting interstitial pneumonias. Major changes include (1) expansion beyond idiopathic interstitial pneumonias to also include secondary causes, (2) subclassification as interstitial (fibrotic vs non-fibrotic) and alveolar filling disorders, (3) expansion to include additional patterns (e.g. bronchiolocentric interstitial pneumonia), (4) improved terminology that better reflects histogenesis and (5) consideration of diagnostic confidence in biopsy evaluation. Pathologists also need to be aware of the advantages and limitations of cryobiopsy interpretation and the importance of reporting features that point towards a secondary cause rather than idiopathic disease. The 2025 classification provides a framework for a methodological approach to reporting biopsies in patients with interstitial pneumonia, which should be used prospectively for both diagnosis and research.