Light's Criteria Calculator

What Are Light's Criteria?

Light's criteria help distinguish exudative from transudative pleural effusions using pleural fluid and serum laboratory values. An exudate suggests local pleural disease such as infection, malignancy, or inflammation, while a transudate points to systemic causes like heart failure or cirrhosis. For related pulmonary assessment, see the Dead Space Calculator and BODE Index Calculator.

How Light's Criteria Work

Three ratios are calculated from a pleural fluid sample paired with serum labs. If any one criterion is met, the effusion is classified as an exudate. Criterion 1: pleural protein divided by serum protein is at least 0.5. Criterion 2: pleural LDH divided by serum LDH is at least 0.6. Criterion 3: pleural LDH divided by the upper limit of normal serum LDH is at least 0.67.

Transudate vs Exudate

Transudates occur when hydrostatic or oncotic pressure shifts move fluid into the pleural space without significant pleural inflammation. Common causes include congestive heart failure, nephrotic syndrome, and hepatic hydrothorax. Exudates reflect active pleural injury or capillary leak and warrant further workup such as cytology, culture, or imaging based on clinical context.