I. Etiology of pleural effusion There are many causes of pleural effusion, and sometimes the diagnosis is still unknown after examination. The meaning of undiagnosed pleural effusion and idiopathic pleural effusion is different, the latter still has unknown causes after systematic examination, while undiagnosed pleural effusion does not necessarily undergo extensive systematic examination. II. Analysis of pleural effusion with unknown diagnosis Pleural effusion with unknown diagnosis accounts for about 15-20% of the cases of pleural effusion. After careful analysis of clinical features and pleural fluid analysis, further investigations are selected according to the need for further diagnosis. Initial pleural fluid analysis includes the appearance of the pleural fluid, is it exudate or leaky? Identification of exudate or leaky fluid usually requires examination of pleural fluid proteins and LDH. In the case of exudate it is also necessary to examine cytological classification, cytopathology, Gram stain, bacterial culture, antacid stain and tuberculosis culture. Through history, physical examination, chest imaging, pleural fluid analysis, biochemistry, pathology, bacteriology and other examinations, the diagnosis can be obtained in about 70% of pleural effusions, and the remaining 30% can be basically diagnosed by needle pleural biopsy, angiography, fibrinoscopy, thoracoscopy, open pleural biopsy and other examinations. III. Possible causes of pleural effusion of unknown diagnosis There are also many causes of pleural effusion of unknown diagnosis, mainly considering the following 1. Tuberculosis is a common cause. It has been suggested that anti-tuberculosis treatment can be given if PPD is positive for pleural effusion of unknown diagnosis, but positive PPD cannot determine the presence of active tuberculosis, and some studies suggest that it is rare for idiopathic pleural effusion with positive PPD to be finally confirmed as tuberculosis, and the proportion of positive PPD in our general population is high, which cannot be used as an indication for anti-tuberculosis treatment, but strong positive PPD has greater significance for young children. Some foreigners have proposed that for pleural exudative effusion with unknown diagnosis, if lymphocytes predominate, ADA increase more than 45U/L and PPD positive, it is more likely to consider tuberculosis. 2, Malignant pleural effusion is also a common cause, and it is very important to distinguish it from benign pleural effusion. The most common one is pleural metastatic cancer (lung cancer, breast cancer, etc.). Most malignant pleural effusions are more voluminous and the cell classification is predominantly lymphatic. Lymphoma accounts for 10% of the cases, with non-Hodgkin’s pleural fluid appearing early, associated with direct lymphoma infiltration, and Hodgkin’s pleural fluid appearing late, associated with lymphovascular obstruction. Pleural mesothelioma is a tumor of the pleura itself. 3. Pulmonary embolism is a relatively easy diagnosis to overlook and should be considered as a mandatory differential diagnosis for pleural effusions of unknown diagnosis. Dyspnea is the most common symptom, pleural fluid is leakage are available, chest X-ray shows lung volume reduction and a small amount of pleural fluid. 4, A more difficult diagnosis is pleural effusion caused by viral infection, which accounts for a large proportion of idiopathic pleural effusion. However, it can be self-absorbed. 5.Other causes include connective tissue disease, Mycoplasma pneumoniae infection, etc. For pleural effusion whose possible causes are tuberculosis and cancerous (which is also the most common cause), more invasive tests may be needed to obtain a diagnosis. Because of the high number of tuberculosis patients in China, pleural effusion without too many diagnostic measures is more likely to be tuberculosis, but the more systematic and comprehensive the examination, the less likely tuberculosis is associated with and the more likely idiopathic pleural effusion is. Planned follow-up observation is also important in the management of pleural effusions with an unknown diagnosis. Fourth, the diagnostic value of pleural biopsy on pleural effusion of unknown diagnosis Pleural biopsy is an invasive test, which has the greatest diagnostic value for tuberculosis and malignant tumors. Because tuberculosis and malignant tumor are the common causes of pleural effusion of unknown diagnosis, pleural biopsy has great diagnostic value for pleural effusion of unknown diagnosis.