From 小麻, in those of research they distribute pleural effusion into two topics; Transudates and Exudates. Roughly separate this two categories; transudates = congestive heart failure, exudates = infection. There is a great description about them in 小麻. When we talk about pleural effusion, we should know about the Light's criteria and exudative criteria. Last but not least, same as @JYC replyed 請愛用小麻.