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PulmonologyLight's Criteria

Light's Criteria Calculator for Pleural Effusion

Calculate all three classic Light criteria from paired serum and pleural-fluid protein and LDH values, with exact strict boundaries and optional pseudoexudate gradient context.

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Formula and medical content are based on the references listed on this page. See sources, About, and Sources and Review Process.

Paired-sample task: use serum and pleural-fluid protein and LDH measurements obtained at the same clinical time whenever possible. The serum LDH ULN must come from the reporting laboratory and is not the measured serum LDH.

Required paired protein values

Use the value from the pleural-fluid sample.

Use the paired serum result; it must be greater than zero.

Choose the unit reported for this paired protein record. Changing an already selected unit clears both values to prevent silent reinterpretation.

Required paired LDH values

Enter U/L (IU/L is numerically equivalent for this laboratory entry).

Enter the paired measured serum LDH in U/L—not the reference limit.

Enter the exact ULN printed by the reporting laboratory. No default is assumed.

Optional albumin gradient context

Enter both albumin values or leave both blank. This adjunct never changes the Light's Criteria classification.

Optional; requires the paired serum albumin.

Optional; requires the paired pleural-fluid albumin.

Select only when both optional albumin values are entered.

Enter the paired protein and LDH record to calculate all three Light's Criteria. Optional paired albumin values add separate pseudoexudate context.

About

This Light's Criteria calculator uses paired pleural-fluid and serum total protein and LDH measurements to classify a pleural effusion as an exudative or transudative pattern. Any one of the three classic criteria is sufficient for an exudative pattern. [1, 3, 4]

The result classifies the biochemical pattern; it does not identify the cause, diagnose infection or malignancy, determine whether thoracentesis is safe, or select drainage or treatment. Light's Criteria apply to pleural fluid, not ascitic fluid. [4, 5]

Formula

Pleural-fluid protein / serum protein > 0.5. [1, 4]
Pleural-fluid LDH / measured serum LDH > 0.6. [1, 4]
Pleural-fluid LDH > 2/3 × the reporting laboratory's serum LDH upper limit of normal. [1, 4]
Exudative pattern = any criterion met. Transudative pattern = none met. All three operators are strict >; exact equality does not meet the original criterion.

Interpretation

Light's Criteria interpretation boundaries
PatternArithmetic definitionInterpretation limit
Exudative patternAt least 1 of 3 strict criteria is metDoes not identify malignancy, infection, tuberculosis, pulmonary embolism, autoimmune disease, chylothorax, or hemothorax.
Transudative pattern0 of 3 strict criteria are metDoes not establish heart failure, cirrhosis, kidney disease, or another cause, and does not exclude a coexisting process.

The number of criteria met is an audit count, not a severity scale. A count of 3 is not a validated measure of greater disease severity than a count of 1. [1, 3]

How to use Light's Criteria safely

Use paired serum and pleural-fluid measurements

Use pleural-fluid and serum values obtained at the same clinical time whenever possible. Enter the measured serum LDH separately from the laboratory-specific serum LDH ULN: they are different inputs, and this page never supplies or guesses the ULN. [1, 4, 5]

Strict boundaries and unit normalization

The original operators are greater-than: exactly 0.5, exactly 0.6, and pleural LDH exactly equal to two-thirds of the lab ULN do not meet their respective criteria. Some secondary calculators reproduce these boundaries as greater-than-or-equal; this implementation retains the source-specific strict operators. Protein and albumin values in g/L are divided by 10 exactly before use.

Possible pseudoexudate context

Light's Criteria are intentionally sensitive and can classify some clinically transudative effusions—especially heart-failure effusions after diuresis—as exudative. If the clinical picture strongly favors a transudative cause despite an exudative Light pattern, a serum–pleural albumin gradient >1.2 g/dL is the preferred adjunct when available; a serum–pleural protein gradient >3.1 g/dL can be used when albumin values are unavailable. These adjuncts do not erase or overwrite the Light result and do not exclude a second process. [2, 7, 8, 9]

Are there modified Light's criteria?

Alternative biochemical strategies have been studied, but there is no single universal modified Light score on this page. Classic Light's Criteria remain the named primary model. Albumin and protein gradients are separate adjuncts for a suspected pseudoexudate; cholesterol-only or blood-free approaches are different strategies and are not silently added as a fourth Light criterion. [3, 8, 9]

Common exudative and transudative causes

Common causes associated with exudative and transudative pleural-effusion patterns
PatternCommon clinical contexts
TransudativeHeart failure, cirrhosis or hepatic hydrothorax, and nephrotic or other hypoalbuminemic states.
ExudativeInfection or parapneumonic effusion, malignancy, tuberculosis, pulmonary embolism, and inflammatory or autoimmune pleuritis.

These are context examples, not diagnoses generated from the submitted values. [4, 5, 6]

Pleural infection and management require separate evidence

An exudative Light result does not by itself mean a chest drain is required. When pleural infection is suspected, current BTS guidance uses the clinical state together with fluid appearance, pH, glucose, LDH, microbiology, ultrasound, and imaging findings to guide management. This calculator does not implement that pathway. [4, 5]

Light's Criteria and SAAG are different tasks

Light's Criteria compare pleural fluid with serum. The serum–ascites albumin gradient (SAAG) uses ascitic/peritoneal fluid for a different clinical question. These calculations are not interchangeable, and this page does not apply Light's Criteria to ascites.

Evidence boundary

The 1972 criteria remain the default biochemical reference framework in later reviews, while modern guidance emphasizes their false-exudate limitation and the need for etiology-specific investigation. Population performance does not become an individual disease probability. [1, 2, 3, 6]

References

  1. Light RW, Macgregor MI, Luchsinger PC, Ball WC Jr. Pleural effusions: the diagnostic separation of transudates and exudates. Ann Intern Med. 1972;77(4):507–513. PMID 4642731. DOI 10.7326/0003-4819-77-4-507.
  2. Light RW. The Light criteria: the beginning and why they are useful 40 years later. Clin Chest Med. 2013;34(1):21–26. PMID 23411053. DOI 10.1016/j.ccm.2012.11.006.
  3. Porcel JM, Light RW. Pleural Fluid Analysis: Are Light's Criteria Still Relevant After Half a Century? Clin Chest Med. 2021;42(4):599–609. PMID 34774168. DOI 10.1016/j.ccm.2021.07.003.
  4. Roberts ME, et al.; BTS Pleural Guideline Development Group. British Thoracic Society Guideline for pleural disease. Thorax. 2023;78(Suppl 3):s1–s42. PMID 37433578. DOI 10.1136/thorax-2022-219784.
  5. Shen-Wagner J, Gamble C, MacGilvray P. Pleural Effusion: Diagnostic Approach in Adults. Am Fam Physician. 2023;108(5):464–475. PMID 37983698.
  6. Sundaralingam A, et al. ERS statement on benign pleural effusions in adults. Eur Respir J. 2024;64(6):2302307. PMID 39060018. DOI 10.1183/13993003.02307-2023.
  7. Roth BJ, O'Meara TF, Cragun WH. The serum-effusion albumin gradient in the evaluation of pleural effusions. Chest. 1990;98(3):546–549. PMID 2152757. DOI 10.1378/chest.98.3.546.
  8. Mohan G, Bhide P, Agrawal A, Kaul V, Chaddha U. A practical approach to pseudoexudative pleural effusions. Respir Med. 2023;214:107279. PMID 37172787. DOI 10.1016/j.rmed.2023.107279.
  9. Christopher DJ, Gupta R, Thangakunam B, et al. Pleural effusion guidelines from ICS and NCCP Section 1: Basic principles, laboratory tests and pleural procedures. Lung India. 2024;41(3):230–248. PMID 38704658. PMCID PMC11093145. DOI 10.4103/lungindia.lungindia_33_24.

FAQ

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Disclaimer

Educational and informational reference only. Not intended to replace professional medical advice, diagnosis, treatment, or independent verification.