Light's Criteria Calculator

Light's Criteria Calculator for Pulmonology. The output is binary: exudate (at least one criterion positive) or transudate (all criteria negative). An exudate result means the pleural surface is diseased and the effusion warrants cytology, microbiology, adenosine deaminase, and often imaging or pleural biopsy. A transudate result points to a systemic imbalance of hydrostatic or oncotic pressure and directs treatment to the underlying organ failure rather than pleural sampling. Because the rule is tuned for sensitivity (roughly 98 percent for exudates), it misclassifies about 20 to 25 percent of true transudates as exudates, most often in diuresed heart-failure patients; when the clinical picture strongly suggests a transudate despite a positive result, a serum-to-pleural albumin gradient greater than 1.2 g/dL or a serum-to-pleural protein gradient greater than 3.1 g/dL supports reclassifying the fluid as transudative.

How this calculator works

Light's Criteria classify a pleural effusion as exudative if at least one of three conditions is met: pleural fluid protein divided by serum protein exceeds 0.5, pleural fluid lactate dehydrogenase (LDH) divided by serum LDH exceeds 0.6, or pleural fluid LDH exceeds two-thirds of the upper limit of the laboratory's normal serum LDH reference range. If none of the three is satisfied, the fluid is labeled a transudate. The tool therefore requires four inputs (paired serum and pleural protein, paired serum and pleural LDH) plus the local serum LDH upper reference limit, and it is deliberately a "high-sensitivity" rule that errs toward calling fluid exudative.

When to use this calculator

Use Light's Criteria on any newly sampled pleural effusion of unclear cause to separate transudates (heart failure, cirrhosis, nephrotic syndrome) from exudates (parapneumonic effusion, malignancy, tuberculosis, empyema, pulmonary embolism), which drives whether further pleural workup is needed. It applies to the initial diagnostic thoracentesis in adults and is the reference standard endorsed by the British Thoracic Society and ATS-informed practice. It should not be relied upon in isolation when the patient has been on diuretics, which can spuriously push a cardiac transudate into the exudate range, and it does not identify the specific cause of an exudate.

Inputs used

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

Clinical interpretation

The output is binary: exudate (at least one criterion positive) or transudate (all criteria negative). An exudate result means the pleural surface is diseased and the effusion warrants cytology, microbiology, adenosine deaminase, and often imaging or pleural biopsy. A transudate result points to a systemic imbalance of hydrostatic or oncotic pressure and directs treatment to the underlying organ failure rather than pleural sampling. Because the rule is tuned for sensitivity (roughly 98 percent for exudates), it misclassifies about 20 to 25 percent of true transudates as exudates, most often in diuresed heart-failure patients; when the clinical picture strongly suggests a transudate despite a positive result, a serum-to-pleural albumin gradient greater than 1.2 g/dL or a serum-to-pleural protein gradient greater than 3.1 g/dL supports reclassifying the fluid as transudative.

Worked example

A patient with dyspnea has thoracentesis: pleural protein 3.8 g/dL, serum protein 6.6 g/dL (ratio 0.58, which is greater than 0.5); pleural LDH 180 U/L, serum LDH 200 U/L (ratio 0.90, greater than 0.6); serum LDH upper reference limit 240 U/L so two-thirds is 160 U/L, and pleural LDH 180 exceeds it. All three criteria are met, so the fluid is an exudate, prompting evaluation for infection, malignancy, or embolism rather than simple diuresis.

Limitations and safety notes

The criteria misclassify a substantial minority of transudates as exudates, especially after diuretic therapy in heart failure, so a "positive" result near the thresholds should be reconciled with the clinical context and an albumin gradient. They tell you the effusion is exudative but not why, and they perform poorly at the boundary when only one criterion is marginally positive. Accuracy also depends on drawing paired serum and pleural samples close in time and on using the reporting laboratory's own LDH reference range, since the two-thirds cutoff is lab-specific.

Frequently asked questions

How many of the three criteria must be positive to call an effusion exudative?

Only one. Light's Criteria are a logical OR: if any single criterion (protein ratio greater than 0.5, LDH ratio greater than 0.6, or pleural LDH greater than two-thirds of the serum upper limit) is met, the fluid is classified as an exudate. All three must be negative for a transudate.

Why does a heart-failure patient on diuretics sometimes get a false exudate?

Diuresis concentrates protein and LDH in the pleural fluid, raising the ratios above threshold even though the effusion is hydrostatic in origin. In this setting, check the serum-to-pleural albumin gradient; a value above 1.2 g/dL reclassifies the fluid as a transudate despite the positive Light's result.

Do I need both serum and pleural samples?

Yes. Two of the three criteria are ratios of pleural to serum values, so paired samples drawn around the same time are required. The third criterion also needs your laboratory's serum LDH upper reference limit to compute the two-thirds cutoff.

What alternative tests help when Light's Criteria are borderline?

When the result is only weakly positive and heart failure is suspected, use the serum-minus-pleural protein gradient (transudate if greater than 3.1 g/dL) or the albumin gradient (transudate if greater than 1.2 g/dL). Pleural NT-proBNP can also support a cardiac transudate.

References

  • Light RW, Macgregor MI, Luchsinger PC, Ball WC. Pleural effusions: the diagnostic separation of transudates and exudates. Ann Intern Med. 1972. PMID: 4642731.
  • Hooper C, Lee YCG, Maskell N; BTS Pleural Disease Guideline Group. Investigation of a unilateral pleural effusion in adults: British Thoracic Society Pleural Disease Guideline 2010. Thorax. 2010. PMID: 20696692.

Editorial review and citation methodology

Reviewed by the Quick Medical Calculator Editorial Team. Last reviewed: April 11, 2026. The review checks calculator inputs, intended population, interpretation, limitations, and source alignment.

  • Prefer original validation studies for scoring systems and prediction tools.
  • Use current specialty society guidance, transplant allocation policy, public health guidance, or regulator resources when they govern clinical use.
  • Include limitations and safety notes when a calculator is population-specific, context-dependent, or unsuitable as a standalone decision tool.

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