Light's Criteria for Pleural Fluid

Exudate or transudate, with the serum-pleural albumin gradient alongside · v1

  • Enter the paired serum and pleural protein and LDH, drawn within the same few hours as each other.
  • You get the exudate or transudate call with the three Light's ratios shown against their cut-offs.
  • Where the patient is on diuretics, enter the serum and pleural albumin as well. The serum-pleural albumin gradient rescues the diuresed heart failure effusion that Light's criteria overcall.
  • Enter the pleural ADA where tuberculosis is in the differential, which in India it usually is.
  • Light's criteria are 98% sensitive for exudates, and what that sensitivity costs is specificity, so an exudate call is the one worth a second look.

  • A diagnosis. It returns a category: exudate says the pleura itself is the problem, transudate sends you back to the heart, the liver and the kidney, and neither answer names the disease.
  • Whether the effusion should be drained.
  • Children. The criteria were derived in adults and have not been validated below that age.
  • The findings a fluid often gives away first: the appearance, the cell count and differential, the glucose and the pH. Frank pus in the syringe is an empyema whatever the arithmetic returns.
  • A tap that has not yet been done, or a serum sample taken days apart from the fluid. A serum LDH two days old will move the ratio on its own.

1. Patient Details

2. Clinical Context

Light's criteria are read against the clinical picture, not instead of it. What you expected before the tap is what decides how much weight a borderline ratio should carry.

3. Laboratory Values

Primary Light's Criteria *

All four values below are required, two from the serum and two from the fluid. Any one of the three criteria being met classifies the fluid as an exudate, so a transudate is the verdict only when all three are negative.

The 250 U/L default is a common figure and not a universal one. LDH reference ranges are analyser-dependent, so take the upper limit from your own laboratory's report: the third criterion is two-thirds of whatever number you enter here.

Additional Values (Optional)

The albumins matter when the patient is on diuretics. ADA matters when tuberculosis is in the differential, which in India it usually is.

Clinical Interpretation & Indian Context

1. The Foundation of Light's Criteria

What it is: A standard set of three biochemical rules to categorise pleural effusions as transudative or exudative. If any one of the three criteria is met, the fluid is classified as an exudate.

Why it matters: It directs the entire diagnostic pathway. Transudates suggest a systemic issue (organ failure, volume overload) requiring diuresis and medical optimisation. Exudates imply local pleural disease (infection, malignancy, inflammation) demanding further local investigations like cytology, cultures, or pleural biopsy.

2. The "Pseudoexudate" Caution

What it is: Approximately 20-30% of transudates in heart failure patients who are actively receiving diuretics will be falsely classified as exudates by Light's criteria.

At the bedside: Diuresis concentrates protein and LDH in the pleural space faster than they can equilibrate with the serum, so the fluid crosses a threshold that the underlying disease never did. If the clinical picture is heart failure and Light's criteria return an exudate, calculate the Serum-Pleural Albumin Gradient (SPAG). A SPAG > 1.2 g/dL identifies the fluid as a transudate that diuresis has concentrated.

3. The Indian Context: Tuberculosis

Local Epidemiology: In India, tuberculosis remains the commonest cause of an exudative pleural effusion, and in a young patient it should be the first thought rather than the last. Malignancy comes next, and the balance shifts towards it with age.

At the bedside: Routine pleural fluid AFB smears have very low yield (< 5%). Instead, an Adenosine Deaminase (ADA) level > 40 U/L in a lymphocyte-predominant exudate is highly sensitive and specific for TB pleuritis. Always send fluid for GeneXpert/CBNAAT and mycobacterial culture to confirm and check for drug resistance.

4. What Causes Each

Transudates (Systemic) Exudates (Local)
Heart failure (Most common) Tuberculosis (High prevalence in India)
Hepatic cirrhosis (Hepatic hydrothorax) Parapneumonic effusion / Empyema
Nephrotic syndrome Malignancy (Lung, breast, lymphoma)
Severe hypoalbuminaemia / Malnutrition Pulmonary Embolism (Can be either, usually exudate)
Peritoneal dialysis Connective tissue disease (RA, SLE)
Abbreviations ADA (Adenosine Deaminase) · AFB (Acid-Fast Bacilli) · BMI (Body Mass Index) · CBNAAT (Cartridge-Based Nucleic Acid Amplification Test) · CHF (Congestive Heart Failure) · JVP (Jugular Venous Pressure) · LDH (Lactate Dehydrogenase) · PF (Pleural Fluid) · RA (Rheumatoid Arthritis) · SLE (Systemic Lupus Erythematosus) · SPAG (Serum-Pleural Albumin Gradient) · TB (Tuberculosis) · ULN (Upper Limit of Normal)
References
  1. Light RW, Macgregor MI, Luchsinger PC, Ball WC. Pleural effusions: the diagnostic separation of transudates and exudates. Ann Intern Med. 1972;77(4):507-513.
  2. Porcel JM. Identifying transudates misclassified by Light's criteria. Curr Opin Pulm Med. 2013;19(4):362-367.
  3. Sharma SK, Suresh V, Mohan A, et al. A prospective study of sensitivity and specificity of adenosine deaminase estimation in the diagnosis of tuberculosis pleural effusion. Indian J Chest Dis Allied Sci. 2001;43(3):149-155.
  4. Udwadia ZF, Sen T. Pleural tuberculosis: an update. Curr Opin Pulm Med. 2010;16(4):399-406.
  5. Roth BJ, O'Meara TF, Cragun WH. The serum-effusion albumin gradient in the evaluation of pleural effusions. Chest. 1990;98(3):546-549.
  6. Heffner JE, Brown LK, Barbieri CA. Diagnostic value of tests that discriminate between exudative and transudative pleural effusions. Primary Study Investigators. Chest. 1997;111(4):970-980.
  7. Aggarwal AN, Agarwal R, Sehgal IS, Dhooria S. Adenosine deaminase for diagnosis of tuberculous pleural effusion: a systematic review and meta-analysis. PLoS One. 2019;14(3):e0213728.
  8. Liang QL, Shi HZ, Wang K, Qin SM, Qin XJ. Diagnostic accuracy of adenosine deaminase in tuberculous pleurisy: a meta-analysis. Respir Med. 2008;102(5):744-754.
  9. Porcel JM, Light RW. Pleural effusions. Dis Mon. 2013;59(2):29-57.
  10. Roberts ME, Rahman NM, Maskell NA, et al. British Thoracic Society Guideline for pleural disease. Thorax. 2023;78(Suppl 3):s1-s42.
How to Cite This Tool

DOIhttps://doi.org/10.5281/zenodo.22401614

AMA Style:Umakanth S. Light's Criteria for Pleural Fluid. Version 1. MEDiscuss Clinical Decision Support System. Published 2026. Accessed . https://mediscuss.org/cdss/lights-criteria. doi:10.5281/zenodo.22401614

Vancouver Style:Umakanth S. Light's Criteria for Pleural Fluid [Internet]. Version 1. MEDiscuss.org; 2026 [cited ]. Available from: https://mediscuss.org/cdss/lights-criteria. doi:10.5281/zenodo.22401614

Disclaimer and scope
Disclaimer: This decision-support system is written for healthcare professionals. It does not substitute clinical judgment, and the treating physician retains absolute responsibility for every diagnostic, dosing and therapeutic decision. The algorithms are built on the evidence as it stands, and the evidence moves. Verify the output against your current institutional protocol, and against the patient in front of you.
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Category Advanced DiagnosticsCalculator
Specialties Internal Medicine, Pulmonology

Written and maintained by

Dr Shashikiran Umakanth

Last revised 24 August 2026

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