The Light criteria: the beginning and why they are useful 40 years later.

The Light criteria serve as a good starting point in the separation of transudates from exudates. The Light criteria misclassify about 25% of transudates as exudates, and most of these patients are on diuretics. If a patient is thought likely to have a disease that produces a transudative pleural effusion but the Light criteria suggest an exudate by only a small margin, the serum-pleural fluid protein gradient should be examined.