The criteria for separating transudates from exudates were published in 1972 by Light and coworkers. They were based on the measurements of serum and pleural fluid protein and LDH. The criteria are as follows: If at least one of the following 3 criteria is present, the fluid is virtually always an exudate and if none is present then the fluid is virtually always a transudate: 1) Pleural fluid : Serum protein ratio > 0.5 2) Pleural fluid LDH > 2/3 of the upper limit of the serum reference range 3) Pleural fluid : Serum LDH ratio > 0.6 An exception to using Light’s criteria is in the setting of CHF treated with diuretics. Normally, in CHF, the effusions are due to an increased capillary hydrostatic pressure and are therefore transudates. But the use of diuretics has been shown to increase the pleural fluid protein and LDH concentrations. Thus we will have a fal...