Light's criteria is used to determine whether the source of a pleural effusion is transudative or exudative.
If the ratio of pleural fluid protein to serum protein is greater than 0.5, the effusion fluid is exudative.
The pleural effusion is likely exudative if the ratio of pleural fluid LDH and serum LDH is greater than 0.6
If the pleural fluid LDH is greater than 0.6 or 2⁄3 times the normal upper limit for serum, it is likely to be exudative, based on Light's criteria. Different laboratories have different values for the upper limit of serum LDH, but examples include 200 and 300 IU/l.
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