Bronchoalveolar lavage is a test in which possible secretions, pathogenic bacteria, cells, and associated factors in the bronchi and alveoli are prepared for subsequent analysis by flushing with sterile saline and recovering them. In idiopathic interstitial pneumonia, bronchoalveolar lavage is relatively non-specific and has a minor diagnostic role, but it is of great interest in differential diagnosis, e.g., to differentiate from infection, if the lavage fluid culture has bacterial growth that meets the requirements, it can be considered a bacterial infection or a co-infection, and in some diseases it can be used for prognostic analysis. In diffuse parenchymal lung disease, some diseases can be diagnosed by alveolar lavage fluid, such as alveolar protein deposition.