Under normal conditions, only a small amount of fluid acts as a lubricant in the pleural cavity and only a small number of cells are seen, so there is no normal percentage of single nucleated cells. Single nucleated cells within the pleural effusion include lymphocytes and monocytes, and there is no normal percentage for a normal healthy population because the number of cells is very small. Pleural effusion occurs only in pathological conditions, which are categorized as leaky fluid and exudate, and the percentage of single nucleated cells is different in the two conditions. In leaky fluid the number of cells is low and usually cannot be categorized, mainly seen in hypoalbuminemia, nephrotic syndrome, cirrhosis, malnutrition and other people. On the other hand, the number of cells in exudate is higher and can be categorized into single nucleated cells and multinucleated cells. An increased proportion of single nucleated cells >50% or more, combined with past medical history and other auxiliary tests, is considered to be a chronic infection of the lungs, Mycobacterium tuberculosis infection, and so on.