Light's criteria

Pleural effusions are classified as transudative or exudative. Exudates have at least 1 of the following, transudates none: 1) pleural fluid/serum protein > 0.5 2) pleural fluid/serum lactate dehydrogenase > 0.6 3) pleural fluid lactate dehydrogenase > 2/3* the upper limit of normal for serum lactate dehydrogenase (LDH) * More recent data suggests that > 45% the upper limit of normal for serum LDH may be better.

References

Medical Knowledge Self Assessment Program (MKSAP) 11, American College of Physicians, Philadelphia 1998