A BAL fluid analysis includes not only microorganism culturing but also an analysis of the cellular constituents or CD4/CD8 ratio of lymphocytes for the differential diagnosis of interstitial lung diseases, including idiopathic pulmonary fibrosis (IPF), interstitial pneumonia with connective tissue disease (CTD-ILD), sarcoidosis, or eosinophilic, drug-induced, or hypersensitivity pneumonia. The gene discussed is CD4; the disease is idiopathic pulmonary fibrosis.