Light's criteria

Exudate versus transudate: any one of pleural/serum protein > 0.5, pleural/serum LDH > 0.6, or pleural LDH above two-thirds of the serum LDH upper limit.

Pleural protein(g/L)Serum protein(g/L)Pleural LDH(U/L)Serum LDH(U/L)Lab's serum LDH upper limit(U/L)

Result

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Interpretation: One met criterion makes the effusion an exudate.
Limitations: Sensitive but imperfectly specific: diuresed heart failure misclassifies as exudate in about a quarter of cases; a serum-pleural albumin gradient above 12 g/L reclassifies.
Reference: Light RW et al. Ann Intern Med 1972;77:507-13.

Connected

FluidUse it: the febrile effusion