Light's Criteria Calculator

Light's criteria separate pleural effusions into transudates — fluid pushed out by pressure changes, as in heart failure or cirrhosis — and exudates, which come from inflamed, infected or malignant pleura and usually need further investigation. Described by Richard Light and colleagues in 1972, the criteria remain the standard first step after a diagnostic thoracentesis. The original paper used a fixed pleural LDH cut-off of 200 IU; it was later replaced by two-thirds of the laboratory’s upper limit of normal for serum LDH so that results are comparable between LDH assays, which is the version used here.

Enter total protein and lactate dehydrogenase (LDH) measured in the pleural fluid and in a blood sample taken at about the same time, plus the upper limit of normal for serum LDH from your laboratory. The effusion is an exudate if ANY one of three criteria is met: pleural/serum protein ratio above 0.5, pleural/serum LDH ratio above 0.6, or pleural LDH above two-thirds of the upper limit of normal for serum LDH.

The criteria are very sensitive for exudates (about 98%), but later studies found a specificity of only about 75–80%, so roughly one transudate in four or five is labelled an exudate, especially in patients with heart failure who have been treated with diuretics. For that situation the calculator also shows the serum-minus-pleural protein gradient: a gradient above 3.1 g/dL suggests the effusion is really a transudate. The result supports, but does not replace, clinical assessment.

Transudates form when hydrostatic or oncotic pressures change while the pleura itself is healthy, so the fluid has little protein and LDH. Exudates arise from increased capillary permeability or impaired lymphatic drainage caused by pleural disease, raising protein and LDH. Meeting any one of the three criteria classifies the fluid as an exudate, which makes the rule highly sensitive (about 98% in pooled data) but less specific (about 75–80%).

When an effusion in a patient with heart failure on diuretics meets the exudate criteria only narrowly, a serum-minus-pleural total protein gradient above 3.1 g/dL (or an albumin gradient above 1.2 g/dL) indicates the fluid is probably a transudate concentrated by diuresis.