Correctly categorizing pleural fluid of uncertain etiology as an exudate increases clinical diligence to seek the cause of the effusion from a concerning list of inflammatory and malignant conditions. The ability to separate transudative from exudative pleural effusions by chemical analysis has gone through a gradual, historical evolution. Initially, specific gravity and absence or presence of clotting were used to try to distinguish these two types of pleural effusions. Later, a protein in the fluid greater than three g/dL or a high lactate dehydrogenase (LDH) level was utilized to diagnose an exudative process. In the classic article by Light et al,1the presence of one or more of a combination of test results (pleural fluid-to-serum protein ratio > 0.5, pleural fluid LDH > 200 IU, and a pleural fluid-to-serum LDH ratio > 0.6) diagnosed an exudate. Light’s criteria continue to be the most widely accepted method to distinguish pleural fluid transudate from exudate. A variety of other tests (cholesterol, albumin, and bilirubin) have been tested to try and improve accuracy on Light’s criteria and on clinical impressions.2–4 No single test or cluster of tests, even when combined with clinical circumstances, have been sufficient to absolutely distinguish exudate from transudate.