Classify pleural effusions as exudative or transudative.
An effusion is exudative if ANY of the following are met: pleural/serum protein ratio > 0.5, pleural/serum LDH ratio > 0.6, or pleural LDH > 2/3 the upper limit of normal serum LDH. If none are met, it is transudative. Light's criteria have ~98% sensitivity for exudates but may misclassify some transudates in patients on diuretics.