Tuberculous pleural fluid is characterized as straw yellow, partly straw green or light red, or even bloody. Pleural fluid sometimes coagulates outside the body. Tuberculous pleural fluid has a high protein content, usually greater than 30 g/L, specific gravity greater than 1.018, pH less than 7.3, and cell count usually greater than 500 × 10^6/L. Early leukocytes are predominantly neutrophils, but then they become predominantly lymphocytes. If the lymphocytes in the pleural fluid are greater than 75%, the pleural fluid is usually considered to be tuberculous. Mesothelial cells in the pleural fluid are usually less than 5%, and the protein content is high, suggesting that the protein ratio in pleural fluid and serum is greater than 0.5. The lactate dehydrogenase in pleural fluid is also mildly increased, and usually the lactate dehydrogenase in pleural fluid is greater than 2/3 of the upper limit of lactate dehydrogenase in serum, or the lactate dehydrogenase ratio in pleural fluid and blood is greater than 0.6. The positive rate of Mycobacterium tuberculosis in pleural fluid and TB culture is low. Even for pleural fluid culture and tuberculosis culture, the positive rate was only 30%. Meanwhile, tuberculous pleural fluid adenosine deaminase in pleural fluid is usually greater than 45 U/L, and it will be slightly lower than 27 U/L in elderly people, then it is tuberculous pleural fluid.