Light's Criteria Calculator for Pleural Effusion

Light's criteria help distinguish exudative pleural effusions from transudative effusions. The distinction narrows the differential diagnosis and guides further investigation. Exudates usually reflect local pleural inflammation, infection, malignancy, vascular injury, or impaired lymphatic drainage, while transudates more often result from systemic hydrostatic or oncotic pressure changes such as heart failure or cirrhosis. An effusion is classified as exudative when any one of three findings is present: the pleural-fluid to serum protein ratio is greater than 0.5; the pleural-fluid to serum lactate dehydrogenase ratio is greater than 0.6; or pleural-fluid LDH is greater than two-thirds of the laboratory upper limit of normal for serum LDH. If none are present, the fluid is likely transudative. The calculator shows both ratios and the number of criteria met so the classification can be checked directly. Pleural and serum samples should ideally be collected close together, and the serum LDH upper limit must come from the laboratory that performed the test. Units cancel in each ratio only when the paired values use matching units. The third criterion compares pleural LDH with a serum reference limit, not with the patient's measured serum LDH. Entering the wrong reference or mixing units can change the classification. The criteria are deliberately sensitive for exudates, but they can label some true transudates as exudative. This is particularly recognized in patients with heart failure after diuretic treatment. When the clinical picture strongly suggests a transudate despite a borderline result, clinicians may consider additional measures such as the serum-to-pleural albumin gradient, serum-to-pleural protein gradient, natriuretic peptides, imaging, and the overall response to treatment. Classification is the beginning of evaluation, not a diagnosis. Cell count, pH, glucose, Gram stain and culture, cytology, triglycerides, hematocrit, microbiology, and other studies may be appropriate depending on the presentation. Respiratory distress, fever, suspected empyema, trauma, or a large or rapidly accumulating effusion warrants prompt clinical assessment. This calculator supports professional interpretation and does not replace thoracentesis standards, laboratory review, imaging, or specialist judgment.