Light's criteria is a traditional method used to determine whether pleural fluid is an exudate by measuring serum and pleural fluid protein and LDH. At least one of the following three criteria must be true for the fluid to be an exudate.
* Pleural fluid protein/serum protein ratio greater than 0.5
* Pleural fluid LDH/serum LDH ratio greater than 0.6
* Pleural fluid LDH greater than two thirds the upper limits of the laboratory's normal serum LDH

Light's criteria are often criticized for being redundant, and other diagnostic criteria are often used (with similar diagnostic accuracy), because they do not require concurrent measurement of serum protein or LDH.

Two-test rule
Pleural fluid cholesterol greater than 45 mg/dL
Pleural fluid LDH greater than two-thirds the upper limit of the laboratory's normal serum LDH

Three-test rule
Pleural fluid protein greater than 2.9 mg/dL
Pleural fluid cholesterol greater than 45 mg/dL
Pleural fluid LDH greater than two-thirds the upper limit of the laboratory's normal serum LDH