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For healthcare professionals

Light's Criteria Calculator

Light's criteria sort a pleural effusion into an exudate or a transudate from pleural fluid and serum protein and LDH. Meeting any one of the three criteria means an exudate; meeting none means a transudate. Light and colleagues published them in 1972. This tool is written for clinicians and is not a diagnosis.

Last reviewed: By the uncovermyrisk.com editorial team Educational only, not medical advice.

Severe shortness of breath, chest pain or confusion: call 911 now.General safety advice (not part of the published criteria). Do not wait for laboratory results.

Laboratory values

Use pleural fluid and serum samples taken at about the same time. Nothing you enter leaves this page.

Result appears here

What you will see

The two ratios, which of the 3 criteria are met, and whether the effusion is an exudate or a transudate.

Educational only, not medical advice. Not a diagnosis. Decisions belong to the treating team.

Source

Light RW and colleagues, 1972. The three criteria are taken from two open-access (CC BY) studies, Subedi AR and colleagues, Clinical Respiratory Journal 2025, and Eraslan BZ and colleagues, Medicine 2026. The tests check each criterion on its own, the exact edges (a ratio of exactly 0.5 or 0.6 does not count) and the two-thirds cut.

Validated in

Adults with a pleural effusion who had a diagnostic tap. Subedi 2025 studied 90 patients at a teaching hospital in Nepal; Eraslan 2026 studied 281 patients at one center in Turkey.

Limits

Light's criteria are sensitive but less specific: they label about 25% of transudates as exudates (Feller-Kopman 2026). Diuretic treatment can raise pleural fluid protein or LDH and cause this; a serum to pleural albumin gradient above 1.2 g/dL has been used to point to a transudate when the criteria are borderline (Eraslan 2026). The two-thirds cut depends on your laboratory's own upper limit of normal for serum LDH. The criteria do not name the cause of an effusion.

The three criteria

An effusion is an exudate if it meets at least one of these, and a transudate if it meets none (Subedi 2025) (Eraslan 2026):

The criteria were published by Light and colleagues (Light 1972).

Worked example: pleural fluid protein 4.0 g/dL and serum protein 7.0 g/dL give a ratio of 0.57, above 0.5, so the effusion is an exudate even when neither LDH criterion is met. In one laboratory with an upper limit of normal of 378 U/L, two thirds was about 250 U/L (Feller-Kopman 2026).

What the studies found

In 90 patients, Light's criteria had a sensitivity of 98.4% and a specificity of 85.7% against the clinical diagnosis (Subedi 2025). A review describes the criteria as consistently misclassifying about a quarter of transudates as exudates, and notes that they need pleural fluid and serum samples at the same time (Feller-Kopman 2026). Eraslan and colleagues describe a sensitivity of about 98% with a specificity of 70% to 80% (Eraslan 2026).

Related tools

For pneumonia severity, see the CURB-65 calculator. For suspected pulmonary embolism, see the Wells score for PE. Other bedside scores are listed under For healthcare professionals.

Sources

  1. Subedi AR, Shrestha TM, Shakya YL, Budha B. Comparative evaluation of pleural fluid cholesterol and total protein versus Light's criteria in differentiation of exudates and transudates: a cross-sectional analysis at Tribhuvan University Teaching Hospital. Clin Respir J. 2025;19(9):e70121. doi:10.1111/crj.70121. PMID 40867084. Open access (CC BY). Methods (Light's criteria) and Results (90 patients; sensitivity 98.4%, specificity 85.7%). pmc.ncbi.nlm.nih.gov/articles/PMC12390823/
  2. Eraslan BZ, Yilmaz E, Kodalak Cengiz S, Comert S. Do pleural fluid pH and lactate improve classification beyond Light's criteria?: a single-center retrospective study. Medicine (Baltimore). 2026;105(19):e48606. doi:10.1097/MD.0000000000048606. PMID 42116308. Open access (CC BY). Introduction (sensitivity, specificity, diuretics, serum-pleural albumin gradient above 1.2 g/dL), Methods (281 patients) and Tables (criteria). pmc.ncbi.nlm.nih.gov/articles/PMC13166768/
  3. Feller-Kopman DJ, Mounsey CA. Pleural effusions: seeing transudate-exudate differentiation in a different light. ERJ Open Res. 2026;12(2):01362-2025. doi:10.1183/23120541.01362-2025. PMID 41846695. Open access (CC BY). Text (about 25% of transudates misclassified; paired samples; upper limit of normal 378 IU/L giving about 250 IU/L). pmc.ncbi.nlm.nih.gov/articles/PMC12991025/

Written and maintained by the uncovermyrisk.com editorial team. We do not have a medical reviewer; every claim links to its primary source above. Last reviewed: 2026-11-02. Spotted an error? See About.