Light's Criteria Calculator

What are Light's criteria?

Light's criteria are three biochemical rules used to classify a pleural effusion as an exudate or a transudate: (1) pleural fluid protein divided by serum protein is greater than 0.5; (2) pleural fluid LDH divided by serum LDH is greater than 0.6; (3) pleural fluid LDH is greater than two-thirds of the laboratory's upper limit of normal (ULN) for serum LDH. If one or more criteria are met, the effusion is classified as an exudate; otherwise it is a transudate. They were published by Light RW et al. in Annals of Internal Medicine in 1972.

What is the difference between an exudate and a transudate?

A transudate is caused by systemic pressure imbalances (such as heart failure, cirrhosis, or nephrotic syndrome) pushing fluid across a normal pleural membrane, so the fluid is low in protein and LDH. An exudate is caused by local pleural disease (such as infection, malignancy, or pulmonary embolism) that makes capillaries leak, so the fluid is rich in protein and LDH. The distinction guides the next diagnostic steps.

Why does the calculator use strictly greater than, not greater than or equal to?

Light's original criteria use strict inequality. A protein ratio of exactly 0.5, an LDH ratio of exactly 0.6, or a pleural LDH exactly equal to two-thirds of the ULN does NOT meet the criterion. This calculator implements the strict version, so borderline values are reported honestly as not met.

How accurate are Light's criteria?

Light's criteria are designed to catch nearly all exudates: sensitivity is about 98%, so a transudate result is rarely a missed exudate. Specificity is lower, about 70 to 83% in later studies, meaning up to roughly 25% of transudates are misclassified as exudates. Most misclassifications happen when LDH or protein values sit near the cutoff, or in heart-failure patients treated with diuretics, which concentrate pleural protein and LDH.

What helps when Light's criteria call a heart-failure effusion an exudate?

When the clinical picture says heart failure but Light's criteria say exudate, clinicians commonly check two tiebreakers: the serum-to-pleural-fluid albumin gradient (above 1.2 g/dL, i.e. 12 g/L, favors transudate) and pleural fluid NT-proBNP (above about 1500 pg/mL favors cardiac origin). Diuretic use is the classic cause of this mismatch and should be noted in the chart.

What commonly causes exudative pleural effusions?

Common causes of exudates include pneumonia (parapneumonic effusion), malignancy, pulmonary embolism, tuberculosis, rheumatoid arthritis, systemic lupus erythematosus, pancreatitis, esophageal rupture, and effusions after cardiac surgery. This list is educational background, not a diagnosis: the actual cause must be worked up by a clinician with imaging, cultures, and cytology.

What commonly causes transudative pleural effusions?

The most common cause of a transudate is heart failure. Other causes include cirrhosis, nephrotic syndrome, myxedema (severe hypothyroidism), and hypoalbuminemia. Transudates usually improve by treating the underlying systemic condition rather than by pleural procedures.

What units can I enter in the calculator?

Enter protein in g/dL or g/L; the calculator converts internally using 1 g/dL = 10 g/L exactly. Enter LDH values (pleural, serum, and the ULN) in U/L. The serum LDH upper limit of normal is lab-specific, so use the reference range from the same laboratory that reported the values.

Can this calculator diagnose my condition?

No. This is an educational tool that applies the published arithmetic of Light's criteria. It cannot diagnose, rule out disease, or replace clinical judgment. Pleural effusions always need evaluation by a qualified clinician who can integrate symptoms, examination, imaging, and the full fluid analysis.

Where do the cutoff numbers come from?

The cutoffs (0.5, 0.6, and two-thirds of the LDH ULN) come from the original study: Light RW, MacGregor MI, Luchsinger PC, Ball WC Jr. Pleural effusions: the diagnostic separation of transudates and exudates. Annals of Internal Medicine. 1972. The thresholds were chosen so that essentially no transudate in the original cohort exceeded them, maximizing sensitivity for exudates.