Pleural effusion is part of a chest or systemic disease and must be evaluated in conjunction with patient history taking, imaging, laboratory and ancillary testing, etc. If there is an error in one of these areas, the identification of the cause may be compromised. 1. Incomplete history taking: Failure to identify the cause of pleural effusion may be due to incomplete history taking; some patients are unable to provide complete and valid information due to the overlap of multiple etiologic factors, which makes it impossible for them to fully understand their disease; or some patients are unconscious and unable to provide valid information. 2. Failure to perform relevant imaging tests: This may be due to the failure to perform relevant imaging tests; the main imaging tests for determining pleural effusion include chest X-ray, chest CT, and ultrasound. 3. Failure to perform relevant laboratory tests: probably because not all laboratory auxiliary tests on pleural effusion have been performed; laboratory auxiliary tests mainly include the following: (1) Thoracentesis and pleural fluid (fluid that stays in the chest cavity) examination: the appearance and odor of the pleural fluid, cell count and classification, biochemical examination, enzyme assay, tumor marker, immunological examination, cytological examination, and pathogenetic examination can be examined. (2) Pleural biopsy, its positive diagnostic rate is 40%~75%, pus thorax or bleeding tendency should not be pleural biopsy; thoracoscopy or open thoracic biopsy, thoracoscopy has the highest diagnostic rate of the etiology of malignant pleural effusion, which can reach 70%~100%. Clinically, there are still a few cases of pleural effusion whose etiology is still difficult to be determined by the above examinations, and the patient should consult a doctor as soon as possible for relevant treatment when physical discomfort occurs.