Classifies a pleural effusion as an exudate or a transudate from pleural fluid and serum protein and LDH.
Result
Enter pleural and serum protein and LDH to classify the effusion.
Sources
3 studies
3
studies
1,810
citations
247
PubMed papers
Evidence
Known limitations
Meta-analysis of 1,448 patients from eight primary studies. All tests except the bilirubin ratio had similar accuracy; best thresholds were pleural LDH above 0.45 of the upper limit of normal and pleural cholesterol above 45 mg/dL. Combinations of two or three tests did slightly better than single tests.
In 59 consecutive patients, using a gradient of 1.2 g/dL or less for exudates and above 1.2 g/dL for transudates, 57 of 59 were classified correctly; the two errors were malignant effusions called transudates. Light’s criteria identified every exudate but misclassified five patients with heart failure as exudates, four of whom had received diuretics.
In 249 consecutive patients, Light’s criteria were 93% accurate, better than clinical judgement (84%) and the albumin (87%) and protein (86%) gradients. In patients on diuretics their accuracy fell to 83%, similar to the albumin gradient (88%).
The original paper has been cited 1,810 times, and 247 PubMed articles mention Light’s criteria in their title or abstract. Citation counts from OpenAlex and article counts from PubMed, retrieved November 15, 2026. Journal Metrics Scores and quartiles are from this site’s current release; follow the journal name for details.
After the result
Exudate · Transudate · Borderline exudate in heart failure
Find the cause in the pleura. Treat the underlying condition. Check the albumin gradient.
Next steps
Avoid common errors
5 common errors to avoid
and 3 more
When the criteria mislead
1Diuretics
Diuretics concentrate protein and LDH in pleural fluid, so a heart failure effusion can meet a criterion. In Roth 1990, 5 of 18 transudates were misclassified, all from heart failure.
2Mismatched samples
Take serum protein and LDH at the same time as the thoracentesis. Old serum values distort the ratios.
3Unknown upper limit
Without the laboratory’s upper limit of normal for LDH, only the two ratios can be checked. Most exudates meet a ratio criterion, but not all.
4Hemolysis
A traumatic tap or a hemolyzed pleural sample raises pleural LDH and can push a transudate over the LDH thresholds; a hemolyzed serum sample does the reverse.
5Classification is not a diagnosis
The criteria sort fluid into two groups. They do not name the cause, and a transudate can coexist with a second disease.
Formula
Exudate if protein ratio above 0.5, LDH ratio above 0.6, or pleural LDH above two-thirds of the upper limit of normal.
Exudate if protein ratio above 0.5, LDH ratio above 0.6, or pleural LDH above two-thirds of the upper limit of normal.
Criteria and results
| Criterion | Exudate if |
|---|---|
| Protein ratio | Pleural fluid protein ÷ serum protein above 0.5 |
| LDH ratio | Pleural fluid LDH ÷ serum LDH above 0.6 |
| Pleural LDH | Above two-thirds of the laboratory’s upper limit of normal for serum LDH (originally above 200 IU) |
| If borderline in heart failure | Serum minus pleural fluid albumin above 1.2 g/dL (12 g/L) suggests a transudate (Roth 1990) |
| Result | Criteria met | Common causes |
|---|---|---|
| Transudate | None | Heart failure, cirrhosis, nephrotic syndrome |
| Exudate | At least one | Pneumonia and empyema, cancer, tuberculosis, pulmonary embolism |
Light 1972
Heffner 1997
Units
Test cases
3 published cases, each also a test case
3 published cases; the calculator reproduces every result.
Worked examples
A previously healthy 24-year-old man with fever, breathlessness and cough had bilateral, lymphocyte-predominant pleural effusions, with paired serum and pleural fluid taken at thoracentesis. Pleural protein was 47 g/L against 67.6 g/L in serum, and pleural LDH 180 U/L against 205 U/L in serum, with a laboratory upper limit of normal of 260 U/L. Cytology and microbiology were negative, and lymph node biopsy showed non-caseating granulomas of sarcoidosis.
“All three criteria are satisfied, confirming that the effusion is exudative”
Protein ratio 47/67.6 = 0.70 (the authors reported 0.69), LDH ratio 180/205 = 0.88, and 180 U/L is above two-thirds of 260 U/L (173.3 U/L).
Entered as: Pleural fluid protein: 47 g/L; Serum protein: 67.6 g/L; Pleural fluid LDH: 180 U/L (IU/L); Serum LDH: 205 U/L (IU/L); Upper limit of normal for serum LDH (optional): 260 U/L (IU/L).
Source: Thaçi S et al., Respirol Case Rep 2026 (PubMed · Free full text · DOI)
A 75-year-old woman with heart failure with reduced ejection fraction had a large left-sided pleural effusion that did not respond to furosemide. Thoracentesis drained 1,500 mL of hazy yellow fluid: protein 2.9 g/dL (serum 5.7 g/dL) and LDH 152 U/L (serum 378 U/L). After the drainage a tension pneumothorax developed, and imaging then showed bone metastases that the effusion had hidden.
“Pleural fluid analysis demonstrated a discordant exudative effusion by Light's criteria.”
Protein ratio 2.9/5.7 = 0.51, just above 0.5; LDH ratio 152/378 = 0.40. The report gave no upper limit of normal for LDH, so the third criterion is left empty; with a laboratory upper limit of 228 U/L or less, pleural LDH 152 U/L would also meet it. A large one-sided effusion in heart failure that meets one criterion only just is a reason to look for another cause, as here.
Entered as: Pleural fluid protein: 2.9 g/dL; Serum protein: 5.7 g/dL; Pleural fluid LDH: 152 U/L (IU/L); Serum LDH: 378 U/L (IU/L).
Source: Ordookhanian C et al., Clin Case Rep 2026 (PubMed · Free full text · DOI)
A 66-year-old woman with cirrhosis, on diuretics she was not taking regularly, came in for the fourth time in five months with a left-sided pleural effusion and no ascites on examination (ultrasound showed only mild ascites). Pleural protein was 16.5 g/L against 70 g/L in serum, and pleural LDH 74 U/L against 615 U/L in serum. Cytology and microbiology were negative, and the effusion was put down to hepatic hydrothorax.
“pleural fluid analysis showed transudative effusion according to Light's criteria”
Protein ratio 16.5/70 = 0.24 and LDH ratio 74/615 = 0.12. The reference range printed for serum LDH was unclear, so the third criterion is left empty; a pleural LDH of 74 U/L is well below two-thirds of any usual upper limit.
Entered as: Pleural fluid protein: 16.5 g/L; Serum protein: 70 g/L; Pleural fluid LDH: 74 U/L (IU/L); Serum LDH: 615 U/L (IU/L).
Source: Shalatouni D et al., Cureus 2025 (PubMed · Free full text · DOI)
FAQ
6 questions and answers
and 4 more answers
Questions
Three tests that classify a pleural effusion. It is an exudate if the pleural fluid to serum protein ratio is above 0.5, the pleural fluid to serum LDH ratio is above 0.6, or pleural fluid LDH is above two-thirds of the upper limit of normal for serum LDH. If none is met it is a transudate.
One is enough. The criteria are designed to be sensitive for exudates, so meeting any single one classifies the fluid as an exudate.
Light’s 1972 paper used a pleural LDH above 200 IU, about two-thirds of the upper limit of normal in their laboratory. LDH assays and reference ranges now differ between laboratories, so the criterion is expressed against your laboratory’s upper limit of normal.
Diuretics concentrate pleural fluid, so a heart failure effusion can meet a criterion and look like an exudate, usually by a small margin. A serum minus pleural fluid albumin gradient above 1.2 g/dL (12 g/L) suggests a transudate in this setting (Roth 1990). The gradient can also call a malignant effusion a transudate, so a large one-sided effusion, or one that does not respond to treatment, still needs cytology.
Transudates come from pressure changes with a normal pleura: heart failure, cirrhosis (hepatic hydrothorax), nephrotic syndrome and hypoalbuminemia. Exudates come from disease of the pleura or nearby tissue: pneumonia and empyema, cancer, tuberculosis, pulmonary embolism, autoimmune disease and pancreatitis.
Yes, as close as possible to the thoracentesis. The ratios compare the two samples, so a serum result from days earlier can shift the classification.
Further reading
8 references with PubMed and DOI links
8
references
3
free full text
From Light RW et al., Ann Intern Med 1972 to the newest case reports.
References
clinical grade of aneurysmal subarachnoid hemorrhage, I to V.
total calcium adjusted for albumin, in mg/dL or mmol/L.
stroke severity from 15 bedside items, 0 to 42.
MAP from systolic and diastolic pressure, against the 65 mm Hg target.
eye, verbal and motor scores for preverbal children, 3 to 15.
oxygenation ratio with the Berlin ARDS severity ranges.
serum sodium corrected for high glucose, Katz or Hillier factor.
sodium − (chloride + bicarbonate), with the albumin-corrected gap.
urea nitrogen to creatinine ratio, in US or SI units.
estimated creatinine clearance with actual, ideal or adjusted weight.
cardiac output and index from oxygen consumption and saturations.
ANC from the white cell count, neutrophils and bands.
insulin resistance from fasting glucose and insulin, in US or SI units.
FENa from plasma and urine sodium and creatinine: prerenal, indeterminate or tubular pattern.
three-item stroke severity screen for large vessel occlusion, 0 to 4.
water deficit in hypernatremia from sodium, weight and body water.
expected PaCO2 in metabolic acidosis, 1.5 × HCO3 + 8 ± 2.
corrected QT interval by Bazett, Fridericia, Framingham or Hodges.
head CT decision rule for children with minor head trauma.
blood loss before hemoglobin or hematocrit reaches a chosen lowest value.
eye, verbal and motor response, 3 to 15, with GCS-Pupils.
Devine ideal body weight and adjusted weight from height and sex.
cuffed and uncuffed tube size and oral depth by age.
calculated osmolality, osmolal gap and ethanol correction.
severity of alcohol-associated hepatitis from prothrombin time and bilirubin.
MELD, MELD-Na and MELD 3.0 for chronic liver disease.
early stroke risk after a transient ischemic attack.
whether a patient with blunt trauma needs cervical spine imaging.
prognosis from a Bruce protocol exercise test.
alveolar–arterial oxygen gradient from a blood gas.
whether an adult with minor head injury needs CT.
risk of advanced liver fibrosis from age, AST, ALT and platelets.
6-week risk of a major cardiac event in emergency chest pain.
cardiac risk before major noncardiac surgery, from six factors.
pre-test probability of pulmonary embolism, with the D-dimer step.
urine output by weight, KDIGO oliguria stage and fluid balance.
Which published the meta-analysis and the albumin gradient study, with its current metrics.
Contributed by Dr. Meng Zhao (MD · Former Neurosurgeon · Medical AI Researcher), from the original publications listed here. Last updated November 15, 2026. LinkedIn
The calculator code is tested against every worked example on this page before each release.
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