Light's Criteria Calculator

Light's Criteria Calculator for Pulmonology. Any positive criterion classifies the biochemistry as exudative; zero positive criteria meets the transudative pattern. More positive criteria do not quantify severity or prove malignancy.

For shared guidance on choosing a tool and documenting results, read the clinical calculator workflow.

How this calculator works

This widget counts three externally assessed conditions; it does not accept and divide five raw laboratory measurements. Exudative biochemistry is present if the pleural/serum protein ratio exceeds 0.5, the pleural/serum LDH ratio exceeds 0.6, or pleural LDH exceeds two-thirds of the laboratory serum LDH upper limit. One positive condition is sufficient.

When to use this calculator

Use with appropriately paired pleural and serum results as part of evaluating a pleural effusion. The classification organizes the differential diagnosis; it does not establish its cause.

Inputs used

  • Pleural fluid protein
  • Serum protein
  • Pleural fluid LDH
  • Serum LDH
  • Serum LDH upper limit of normal when used

Clinical interpretation

Any positive criterion classifies the biochemistry as exudative; zero positive criteria meets the transudative pattern. More positive criteria do not quantify severity or prove malignancy.

Worked example

Pleural protein 3.8 and serum protein 6.6 g/dL give 0.576. Pleural LDH 180 and serum LDH 200 U/L give 0.90; with a serum LDH upper limit of 240, the third threshold is 160 U/L. All three are positive. Ratios exactly 0.5 and 0.6 and LDH exactly two-thirds of the upper limit do not exceed the thresholds.

Limitations and safety notes

Diuresis can concentrate a heart-failure effusion enough to meet exudative criteria. Reconcile discordant results with the clinical picture and selectively appropriate investigations; do not automatically relabel the fluid or order every possible pleural test.

Frequently asked questions

Do all three criteria have to be positive?

No. The rule uses an OR relationship: any one positive criterion is enough for exudative biochemistry.

References

  • Light et al. Original pleural-effusion study. 1972: https://pubmed.ncbi.nlm.nih.gov/4642731/
  • British Thoracic Society pleural disease guideline summary. 2023: https://doi.org/10.1136/thorax-2023-220304

Editorial review and citation methodology

Maintained by the Quick Medical Calculator Editorial Team. Content record date: September 7, 2026. A content date is not evidence of independent clinical review. Individual clinical sign-off is not recorded on this page.

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