The fluid is defined as an exudate if at least 1 of the following 3 criteria is met:
Ratio of pleural fluid protein to serum protein > 0.5 Ratio of pleural fluid lactate dehydrogenase (LDH) to serum LDH > 0.6 Pleural fluid LDH more than 2/3 the upper limits of the laboratory normal serum LDH
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