The presence of excessive fluid in the pleural cavity for any reason is called pleural effusion and is often referred to as pleural fluid. According to the classification of the characteristics of pleural effusion, pleural effusion can be divided into leakage, exudate, pus, hemothorax and celiac disease. Many diseases in the human body can lead to pleural effusion, and the characteristics of pleural effusion are different, so it is difficult for non-medical professionals to distinguish between exudate and leaky fluid, and to understand the biochemical indicators in the effusion laboratory. There are several diseases that commonly lead to pleural effusion and the characteristics of pleural effusion are summarized: 1. Infectious diseases Infection of the dirty or wall pleura itself, such as pleurisy caused by various reasons. Infectious diseases of the lungs can also lead to pleural effusion, such as lobar pneumonia and tuberculosis. In addition, lung infections are often combined with pleural infections or are a common cause of pleural effusions. Inflammatory effusion is mostly exudative, yellowish translucent fluid, often accompanied by chest pain and fever. 2.Malignant tumor Pleural effusion may appear in the late stage of malignant tumor, which often indicates tumor metastasis and poor prognosis. Common malignant tumors include lung cancer and pleural mesothelioma. If bleeding causes such as trauma are excluded, red or bloody pleural fluid often suggests malignant tumors. 3.Trauma surgery Surgery, trauma and some other factors that can cause rupture of pleural cavity, the body instinctively exudes fluid to fill the pleura. At this time, the fluid can be yellowish, reddish or red, and the different colors are mainly due to the different proportions of blood mixed in the pleural fluid. Because there is a clear history of surgery and trauma, it is easy to distinguish from malignant pleural effusion. 4.Circulatory system diseases The obstruction of venous blood return into the heart will lead to pleural effusion, which is clear and transparent as leakage fluid. It is commonly seen in superior vena cava obstruction, congestive heart failure and constrictive pericarditis. 5, hypoproteinemia, nephrotic syndrome, cirrhosis Nephrotic syndrome and cirrhosis can cause not only peritoneal effusion, but also pleural effusion. Hypoproteinemia caused by severe malnutrition can also lead to pleural effusion. At this time, pleural effusion is mostly transparent and clear leaking fluid. 6, special pleural effusion Celiac disease: due to obstruction or damage to the thoracic duct, lymphatic fluid accumulation in the pleural cavity leads to pleural effusion. Mostly milky yellow turbid fluid, positive pleural fluid celiac test can confirm the diagnosis. Hemothorax: a kind of pleural effusion caused by trauma and surgery, that is, the fluid in the pleural cavity is blood, mostly seen after chest puncture injury or surgery, is an emergency and needs timely treatment. Abscess pleural: caused by infection of the membrane cavity, the effusion is a cloudy pleural exudate with pus-like characteristics. Generally bacteria, Mycobacterium tuberculosis and fungi are the pathogens of pustothorax. Acute abscess thorax often has symptoms such as fever, chest pain and chest tightness. Chronic abscess chest has symptoms such as pleural thickening, thoracic collapse, and chronic wasting. Sometimes the etiology of pleural effusion can be assisted according to the nature of pleural fluid, but biochemical tests of pleural fluid, mycobacterium tuberculosis tests, and exfoliative cell tests often have more diagnostic value, and when pleural fluid examination still cannot make a clear diagnosis, pleural biopsy can be considered to make a clear diagnosis.