When a pleural effusion is sampled by thoracentesis, the first and most important question is whether it is a transudate or an exudate — the answer determines whether the workup continues (exudate) or stops once the systemic cause is addressed (transudate). Light's Criteria, published in 1972, remains the standard bedside tool for making that call from routine lab values.
How Light's Criteria Was Developed
Richard Light and colleagues published the original criteria in the Annals of Internal Medicine in 1972, comparing pleural fluid protein and LDH against paired serum samples in patients with known transudative and exudative effusions. They found that 3 simple ratio-and-threshold rules — pleural/serum protein > 0.5, pleural/serum LDH > 0.6, and pleural LDH greater than two-thirds of the serum LDH upper limit of normal — reliably separated the two categories using labs that were already routinely drawn.
Critically, the rule is an OR, not a summed score: meeting any single one of the 3 criteria is sufficient to call the fluid an exudate. This is a different logical shape from additive risk scores like HAS-BLED or CURB-65, where points accumulate toward a threshold — here, one met criterion ends the question.
Using the Result in Practice
A transudate classification generally means the effusion is a downstream effect of a systemic condition — congestive heart failure, cirrhosis, nephrotic syndrome, or hypoalbuminemia — and the clinical focus shifts to treating that underlying condition rather than the fluid itself.
An exudate classification points to a local pleural or lung process — infection, malignancy, pulmonary embolism, tuberculosis, or pancreatitis — and typically triggers further pleural fluid workup: cell counts and differential, pH, glucose, Gram stain and culture, and cytology, guided by the clinical context.
Limits and Edge Cases
Light's Criteria is highly sensitive (around 98%) for exudates but only moderately specific (roughly 74-83%), meaning it occasionally misclassifies a true transudate as an exudate. The best-documented failure mode is heart failure patients on diuretic therapy: diuresis concentrates the pleural fluid, pushing the protein and LDH ratios above the exudate thresholds even though the underlying process is transudative. When this is suspected, calculating the serum-to-pleural-fluid albumin gradient (a gradient greater than 1.2 g/dL favors a transudate) can help correct a borderline or unexpected exudate call.
The serum LDH upper limit of normal used in the third criterion is lab- and assay-specific, not a universal constant — always substitute your own laboratory's reported reference range rather than relying on the 200 U/L default used here for illustration.