Light's Criteria Calculator

Classify pleural effusion as exudate or transudate using Light's criteria with pleural fluid protein, serum protein, pleural fluid LDH, serum LDH, and the upper limit of normal serum LDH.

Last updated on: April 1, 2026

Calculate Light's Criteria

Enter pleural fluid total protein in g/dL or g/L.
Enter serum total protein in g/dL or g/L.
Enter pleural fluid LDH in U/L, IU/L, or µkat/L.
Enter serum LDH in U/L, IU/L, or µkat/L.
Enter the laboratory upper limit of normal for serum LDH in the same preferred unit system.

What Light's Criteria Measure

Light's criteria are used to classify pleural effusions as exudative or transudative using paired pleural fluid and serum biochemistry.

An effusion is classified as exudative if any one of the three criteria is positive. If none are positive, the effusion is classified as transudative.

Diagnostic Criteria

Criterion 1 uses the pleural fluid to serum protein ratio and is positive when the ratio is greater than 0.5.

Criterion 2 uses the pleural fluid to serum LDH ratio and is positive when the ratio is greater than 0.6.

Criterion 3 is positive when pleural fluid LDH exceeds two-thirds of the upper limit of normal for serum LDH.

Classification and Interpretation

Exudative Pleural Effusion: At least one Light's criterion is positive. This supports an exudative process and usually warrants evaluation for local pleural or pulmonary disease.
Transudative Pleural Effusion: None of the Light's criteria are positive. This supports a transudative process, often related to systemic hydrostatic or oncotic pressure imbalance.

Important Limitations

Light's criteria prioritize sensitivity for exudates and can overclassify some transudates as exudates in selected clinical settings.

  • Diuretic-treated heart failure can produce pseudoexudates.
  • Albumin or protein gradients may help in selected borderline cases.
  • Light's criteria should be interpreted together with the overall clinical picture and pleural fluid analysis.

This page has been medically reviewed by Dr. Khoulah Attia – PharmD, Immunology Specialist.

Related Calculators

Wells Criteria for PE Calculator
PESI Score Calculator
4PEPS Score Calculator
H. pylori Pre-Test Probability
Kt/V & Standardized Kt/V Calculator
PRECISE-DAPT Score