[Isoniazid-induced pneumonitis]

An 82-year-old man was treated with isoniazid (INH) because of a low-grade fever. On the 9th day of treatment, dry coughing and general malaise developed. On the 30th day, he was admitted to our hospital. A chest-X ray film showed infiltrative shadows in the right middle and lower lung fields, but a chest CT scan showed an abnormal lung density in the right lower lobe. Abnormal laboratory findings included leucocytosis, liver dysfunction, hypoxemia, low vital capacity, low diffusing capacity and a high level of C-reactive protein. A differential cell count of the bronchoalveolar lavage fluid (BALF) showed many neutrophils and lymphocytes; examination of a specimen obtained by transbronchial lung biopsy (TBLB) revealed edema of alveolar walls, lymphocyte infiltration, and proliferation of type II alveolar epithelial cells. A drug lymphocyte stimulation test (DLST) against INH was positive. After discontinuation of INH, symptoms resolved, laboratory findings became normal, and the infiltrative shadows in the right middle and lower lung fields disappeared. The clinical course and the findings of BALF, TBLB, and DLST suggested the diagnosis of pneumonitis caused by INH.