Light's Criteria (Exudative vs Transudative Pleural Effusion)

An effusion meets Light's criteria for an exudate if any one of three laboratory comparisons is positive. Use paired pleural fluid and serum samples with consistent units (protein in the same mass/volume scale; LDH in the same enzymatic units). The third criterion uses your lab's reference upper limit of normal for serum LDH, not the patient's measured serum LDH alone.

Pleural fluid total protein

Serum total protein (same units)

Pleural fluid LDH (IU/L or local consistent units)

Serum LDH (same units as pleural LDH)

Upper limit of normal — serum LDH (lab reference)

Criterion 3: pleural LDH > (⅔) × this reference ULN.

Disclaimer: Light's criteria are a widely used laboratory screen and can be misclassified in a minority of cases (e.g. early heart failure on diuretics, some protein-poor exudates). They do not replace clinical judgment, imaging, or additional pleural fluid studies when indicated.

Light's criteria — reference summary

Overview

Light's criteria distinguish exudative from transudative pleural effusions using simultaneous pleural fluid and serum chemistries. An effusion is classified as an exudate if any one of the three rules below is satisfied. Reference: Light RW et al., Ann Intern Med 1972;77(4):507–513. DOI 10.7326/0003-4819-77-4-507.

The three criteria (any one → exudate)

#	Comparison	Exudative if
1	Pleural fluid protein ÷ serum total protein	> 0.5
2	Pleural fluid LDH ÷ serum LDH	> 0.6
3	Pleural fluid LDH vs laboratory reference for serum LDH (upper limit of normal)	PF LDH > (⅔) × (serum LDH ULN)

Use the same units for pleural and serum protein, and the same LDH assay units for pleural fluid, serum LDH, and the quoted ULN. If none of the three criteria are met, the fluid behaves like a transudate by this definition—still interpret in clinical context.

Clinical notes

Heart failure patients on diuretics may show "exudative" protein or LDH ratios despite a transudative process (diuretic effect).
Some exudates (e.g. early parapneumonic, select malignancies) may fall below thresholds; correlative tests (cell count, Gram stain/culture, cytology, pH, glucose, etc.) may still be indicated.
Alternative or adjunctive rules (e.g. cholesterol, serum–effusion albumin gradient) exist for selected scenarios—this tool implements only the classic Light triad.

Background

Pleural effusions are broadly categorized as transudates or exudates. That distinction is not merely academic: it shapes the differential diagnosis, the urgency of additional testing, and the need for pleural procedures. Transudates typically arise when systemic factors alter Starling forces or plasma oncotic pressure (for example, heart failure, cirrhosis with portal hypertension, or nephrotic-range proteinuria). Exudates more often reflect local pleural pathology—inflammation, infection, malignancy, pulmonary embolism with infarction, abdominal disease tracking through diaphragmatic defects, and several other processes—in which capillary permeability is increased or lymphatic drainage is impaired.

Light's criteria are a classic, widely taught laboratory framework that uses paired pleural fluid and serum chemistries obtained near the same time. The rules ask whether the pleural space enriches protein or lactate dehydrogenase (LDH) relative to blood in patterns that mirror exudative physiology. When any single criterion is met, the effusion is classified as an exudate by this definition; when none are met, the fluid is classified as a transudate by this definition. Clinicians should still integrate imaging, symptoms, comorbidities, and the rest of the pleural fluid analysis because no single rule is perfect in every scenario.

Why protein and LDH?

Protein leaks more readily across inflamed or disrupted pleural membranes; many exudative processes therefore produce fluid with a higher protein concentration relative to plasma. LDH is a cellular enzyme; higher pleural fluid LDH often accompanies increased cell turnover, tissue injury, or inflammation within the pleural compartment. The original Light formulation therefore combined a protein ratio, an LDH ratio, and a standalone pleural LDH threshold tied to the laboratory reference range for serum LDH. Together, these three checks capture complementary signals: relative enrichment versus serum (ratios) and an absolute pleural LDH elevation against a population-based upper limit of normal.

The three criteria (any one positive defines an exudate)

All calculations assume internally consistent units: pleural fluid and serum total protein must be reported in the same units (commonly grams per deciliter), and pleural fluid LDH, serum LDH, and the serum LDH upper limit of normal must be expressed in the same enzymatic units (commonly international units per liter). Mixing unit systems invalidates the arithmetic even if the numbers look plausible.

Criterion	Computation	Exudative threshold
1 — Protein ratio	Pleural fluid total protein ÷ serum total protein	> 0.5
2 — LDH ratio	Pleural fluid LDH ÷ serum LDH	> 0.6
3 — Pleural LDH vs reference	Compare pleural fluid LDH to two-thirds of the laboratory upper limit of normal for serum LDH	Pleural LDH > (⅔ × serum LDH ULN)

Note carefully that the third criterion uses the reference ULN for serum LDH from your assay—not the patient's measured serum LDH by itself. The second criterion, by contrast, uses the patient's contemporaneous serum LDH value in the denominator. This pairing is easy to confuse at the bedside and in documentation, yet it is essential for correct application.

How to collect and pair samples

Ideally, pleural fluid and peripheral blood are drawn close together in time, before large-volume therapeutic drainage materially changes pleural fluid composition, and with clear labeling that ties the specimens to the same thoracentesis episode. If a patient has had substantial fluid removed or received interventions that may alter chemistries, interpret ratios with caution. When serial thoracenteses are performed, use the chemistries that correspond to the clinical question you are answering.

Interpreting a positive versus negative result

Any criterion positive: The effusion meets Light's laboratory definition of an exudate. That finding should prompt consideration of exudative etiologies and often supports expanded pleural fluid testing (for example, cell count with differential, additional chemistries, microbiology, and cytology when appropriate). It does not, by itself, establish a specific diagnosis.

All criteria negative: The fluid behaves like a transudate under Light's rules. That pattern is compatible with common transudative mechanisms, yet clinicians should remain alert for clinical scenarios where an exudative process is still present (see pitfalls below).

Common pitfalls and modifiers

Diuretic therapy in heart failure: Concentrating pleural fluid protein and LDH after diuresis can push ratios upward and create an "pseudoexudate" pattern despite a fundamentally transudative process. Interpretation should incorporate timing of diuretics, volume status, and corroborating features.
Assay and unit mismatch: Using different protein assays or LDH methods between pleural fluid and serum, or mixing unit systems, can produce misleading ratios. Always verify the laboratory report footnotes.
Early or evolving effusions: Some exudative processes may not immediately satisfy thresholds, particularly early in the illness course or with very small samples.
Protein-poor exudates: Selected malignancies or other conditions may yield unexpectedly low protein relative to expectations; reliance on protein ratio alone can miss some exudates.
Serum LDH confounders: Hemolysis, muscle injury, or other causes of markedly elevated serum LDH change the LDH ratio denominator; integrate the blood specimen quality and the clinical context.

Beyond Light's criteria

Light's rules remain a cornerstone teaching tool and a practical first screen, but they are not the only useful discriminator. In selected cases, clinicians add other measurements—such as cholesterol-based criteria or serum–effusion albumin or protein gradients—when the pretest probability and the Light result disagree, or when institutional protocols recommend adjunctive tests. The calculator on this page implements the classic three-criterion Light framework only; it does not replace comprehensive pleural fluid interpretation or procedural planning.

Using this calculator responsibly

Enter pleural fluid and serum total protein in matching units, pleural fluid and serum LDH in matching units, and the laboratory upper limit of normal for serum LDH from the same assay family. The tool reports each criterion as positive or negative and states whether the overall pattern meets exudative criteria. This output supports education and structured bedside checking; it is not a substitute for independent verification of laboratory values, clinical reasoning, or institutional pathways for thoracentesis and pleural disease management.

CalcMD.

Light's Criteria for Exudative Pleural Effusion

Learn how Light's criteria use pleural fluid and serum protein, LDH ratios, and pleural LDH versus two-thirds of the serum LDH upper limit of normal to classify pleural effusions. Covers sampling, interpretation, and common pitfalls such as diuretics.