[Interstitial lung disease revisited]

Interstitial lung disease (ILD), consisting of more than 200 subtypes of diseases, is a large group of heterogeneous diseases characterized by varying degrees of alveolitis and pulmonary parenchymal fibrosis. There are many challenges in its diagnosis and treatment. In recent years, new understanding of the clinical classification, disease behavior, early diagnosis, and treatment of ILD has been obtained. The anti-fibrotic drugs have been expanded successfully from treatment of idiopathic pulmonary fibrosis (IPF) to progressive-fibrosing interstitial lung diseases (PF-ILD). A new international clinical practice guideline categorized hypersensitivity pneumonitis (HP) into two clinical phenotypes, namely nonfibrotic and fibrotic HP. The clinical classification of ILD according to disease behavior and a progressing fibrotic phenotype is of important value for the establishment of treatment strategies for patients with ILD. Interstitial pulmonary abnormalities (ILA) may represent the subclinical stages of different types of ILD, and long-term follow-up of ILA is key to improving the early diagnosis of ILD. Interstitial pneumonia with autoimmune features (IPAF) may represent an early phase or prodromal state of a connective tissue disease (CTD), and patients with IPAF need to be under longitudinal surveillance for evolution to CTD.