Light's Criteria Calculator for Pleural Effusion

Light's criteria establishes that a pleural effusion is classified as an exudate if it meets at least one of three specific parameters: 1) A pleural fluid-to-serum protein ratio greater than 0.5, 2) A pleural fluid-to-serum lactate dehydrogenase (LDH) ratio greater than 0.6, or 3) Pleural fluid LDH greater than two-thirds (0.67 times) of the upper limit of normal serum LDH. Mathematically, the ratios are computed as: Protein Ratio = Pleural Protein / Serum Protein, LDH Ratio = Pleural LDH / Serum LDH, and LDH Upper Ratio = Pleural LDH / Laboratory Serum LDH Upper Limit.

Consider a patient with a pleural effusion whose laboratory results show a pleural protein of 4.2 g/dL, serum protein of 6.0 g/dL, pleural LDH of 250 U/L, serum LDH of 300 U/L, and a laboratory upper limit of normal serum LDH of 200 U/L. First, calculate the protein ratio: 4.2 / 6.0 = 0.7 (exudate threshold exceeded). Second, compute the LDH ratio: 250 / 300 = 0.83 (exudate threshold exceeded). Third, calculate the upper LDH ratio: 250 / 200 = 1.25, which also exceeds two-thirds of the upper limit (133 U/L). Because all three criteria are met, this pleural effusion is definitively classified as an exudate.

Always draw serum and pleural fluid samples simultaneously to ensure accurate ratio comparisons. Keep in mind that diuretic use can falsely elevate pleural fluid protein and LDH concentrations, potentially turning a transudate into a pseudo-exudate. Always interpret these biochemical findings within the context of the patient's complete clinical presentation and imaging results.