Definition and diagnostic approach to interstitial lung disease: a narrative review

Purpose: Interstitial lung disease (ILD) is an umbrella term encompassing more than 200 heterogeneous parenchymal lung disorders that share overlapping clinical and imaging features but differ substantially in etiology, disease course, and treatment response. This review summarizes contemporary terminology and classification systems for ILD and outlines a practical diagnostic approach applicable in clinical practice.Current concepts: ILD is best understood as a clinical–radiologic–pathologic spectrum rather than a single disease entity. Contemporary classification is therefore hybrid and multi-axial, integrating (1) etiology, (2) morphologic pattern, (3) disease behavior, and (4) the presence or absence of fibrosis. The 2025 European Respiratory Society/American Thoracic Society update focused on revising the classification framework for interstitial pneumonias, a subset within the broader ILD spectrum, and emphasized grouping entities according to shared morphologic patterns regardless of etiology. In routine clinical practice, however, clinicians continue to prioritize etiology-based categories while also recognizing pattern-predominant disease groups that may be suggested primarily by imaging findings. Diagnostic evaluation begins with a detailed history and physical examination and typically includes high-resolution computed tomography as the cornerstone of morphologic assessment, together with pulmonary function testing and serologic studies as part of the initial evaluation. Bronchoalveolar lavage and lung biopsy are selectively performed when the results are expected to influence management decisions.Discussion and conclusion: Because no single test is sufficient for diagnosing all ILDs, diagnosis relies on integrating clinical, radiologic, laboratory, and, when available, histopathologic information, ideally through multidisciplinary discussion. This integrated approach maximizes diagnostic confidence and supports individualized management strategies.