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 false positive result making the fluid appear as an exudate. It is believed to be due to the action of the diuretic that causes fluid to shift out of the pleural space.
So in cases of diuretic-treated patients having exudative fluid by Light’s criteria, it is recommended to measure the serum : pleural effusion albumin gradient. If the serum albumin minus pleural fluid albumin is > 1.2 g/dL, the patient is likely to have a transudative effusion. Also in cases where one or more of Light’s criteria are met but clinically the patient is thought to have a condition producing a transudative effusion, then we can also measure the protein levels in the serum and pleural fluid. If the difference between these two levels is > 3.1 g/dL, then the fluid is transudative.