- After diagnostic thoracentesis, draw a paired serum sample on the same day.
- Enter the pleural-fluid protein and serum protein (g/dL), the pleural-fluid LDH and serum LDH (U/L), and your laboratory's serum LDH upper limit of normal (U/L).
- The three Light's ratios/values and the exudate vs transudate verdict update automatically — an exudate is flagged if any one criterion is met.
- Use the result to direct further workup; if the clinical picture suggests a transudate but the result is "exudate," check the serum–pleural albumin or protein gradient (see Pearls).
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When to Use
Apply Light's criteria to every pleural effusion sampled by thoracentesis to classify it as an exudate or a transudate. The classification is the first fork in the diagnostic pathway: a transudate points toward systemic causes of altered hydrostatic/oncotic pressure (heart failure, cirrhosis, nephrotic syndrome) and usually needs no further pleural workup, whereas an exudate signals local pleural pathology (infection/parapneumonic effusion or empyema, malignancy, pulmonary embolism, tuberculosis, and others) and warrants a focused diagnostic evaluation of the pleura.
Appropriate population
Adults with a newly sampled pleural effusion of unknown cause, where paired same-day pleural and serum protein and LDH are available. Light's criteria are intended to be applied at the time of the first thoracentesis to triage subsequent investigation.
Caveat — diuretics and "false exudates"
Light's criteria are very sensitive for exudates but trade off specificity: they misclassify roughly 25% of transudates as exudates, especially in patients on diuretics (which concentrate pleural protein and LDH). If the effusion is labeled an exudate but you clinically suspect a transudate (e.g. decompensated heart failure on furosemide), confirm with a gradient: a serum-minus-pleural albumin gradient > 1.2 g/dL or a serum-minus-pleural total protein gradient > 3.1 g/dL reclassifies the fluid as a transudate.
Pearls & Pitfalls
"Any one" rule — high sensitivity by design
Because an effusion is an exudate if it meets any single criterion, Light's criteria rarely miss a true exudate (sensitivity ~98%). This is intentional: missing an exudate (malignancy, empyema, TB) is more dangerous than over-calling one. The price is the ~25% transudate over-classification handled by the albumin/protein gradient.
Get a paired same-day serum
The protein and LDH ratios require a contemporaneous serum sample — without it, only the absolute pleural LDH criterion can be evaluated, and the classification is incomplete. Send pleural fluid for protein, LDH, pH, glucose, cell count/differential, Gram stain, culture, and cytology at the same draw. Confirm the LDH upper limit of normal used is your own lab's (it varies by assay).
Pitfalls
(1) A grossly bloody tap (trauma/needle) can spuriously raise LDH. (2) The criteria classify but do not diagnose — an exudate still needs a cause established. (3) Do not apply them to fluid other than pleural. (4) Do not ignore a strongly suggestive clinical picture: a transudative-appearing patient with an "exudate" result deserves the gradient check before an unnecessary invasive pleural workup. (5) Very low pleural pH/glucose or frank pus indicates a complicated parapneumonic effusion/empyema needing drainage regardless of the ratios.
Why Use It
Distinguishing exudate from transudate is the single most useful early step in evaluating a pleural effusion, because it splits the differential diagnosis — and therefore the entire workup — in two. Before Light's 1972 study, no reliable biochemical rule existed; protein concentration alone misclassified many effusions. The combination of the pleural/serum protein ratio, the pleural/serum LDH ratio, and the absolute pleural LDH proved far more accurate, and the "any one positive = exudate" rule became the reference standard taught and used worldwide. It is simple, uses routinely available labs, and reliably identifies the exudates that require further investigation while sparing transudative patients unnecessary procedures.
Light's Criteria — Exudate vs Transudate
Enter the paired pleural-fluid and serum values plus your laboratory's serum LDH upper limit of normal. The three Light's ratios/values and the exudate/transudate verdict update automatically.
⚕ Light RW, Macgregor MI, Luchsinger PC, Ball WC Jr. Ann Intern Med. 1972;77(4):507–513. An effusion is an exudate if it meets any one of the three criteria. Light's criteria are highly sensitive but misclassify ~25% of transudates as exudates (especially on diuretics) — confirm a discordant result with the serum–pleural albumin gradient (> 1.2 g/dL) or protein gradient (> 3.1 g/dL). For licensed clinicians; does not replace individualized assessment.
Next Steps
Use the classification to direct the next stage of the workup.
- Exudate: pursue local pleural pathology — workup for infection / parapneumonic effusion / empyema (pH, glucose, Gram stain, culture), malignancy (cytology, consider pleural biopsy/thoracoscopy), pulmonary embolism, and tuberculosis (adenosine deaminase, AFB/TB culture) as the clinical picture dictates.
- Transudate: evaluate and treat the systemic cause — heart failure, cirrhosis with hepatic hydrothorax, or nephrotic syndrome — and generally no further pleural-specific workup is required.
- Discordant result: if the fluid is labeled an exudate but a transudate is clinically expected (e.g. heart failure on diuretics), compute the serum-minus-pleural albumin gradient (> 1.2 g/dL) or total-protein gradient (> 3.1 g/dL); a positive gradient reclassifies it as a transudate.
- Always interpret alongside pleural pH, glucose, cell count/differential, cytology, and the overall clinical context; complicated parapneumonic effusions and empyema need drainage regardless of the ratios.
Evidence & References
Light's Criteria — an exudate meets ANY ONE
| # | Criterion | Exudate threshold |
|---|---|---|
| 1 | Pleural fluid protein ÷ serum protein | > 0.5 |
| 2 | Pleural fluid LDH ÷ serum LDH | > 0.6 |
| 3 | Pleural fluid LDH (absolute) | > ⅔ × serum LDH upper limit of normal |
If none of the three is met, the effusion is a transudate. Meeting any one classifies it as an exudate. In the original derivation the rule was ~98% sensitive and ~83% specific for exudates; specificity is limited mainly by transudates over-classified as exudates (commonly diuretic-treated heart failure), reclassifiable with the serum–pleural albumin or protein gradient.
References
- Light RW, Macgregor MI, Luchsinger PC, Ball WC Jr. Pleural effusions: the diagnostic separation of transudates and exudates. Ann Intern Med. 1972;77(4):507–513. doi:10.7326/0003-4819-77-4-507.
- Roth BJ, O'Meara TF, Cragun WH. The serum-effusion albumin gradient in the evaluation of pleural effusions. Chest. 1990;98(3):546–549. doi:10.1378/chest.98.3.546.
- Porcel JM, Light RW. Diagnostic approach to pleural effusion in adults. Am Fam Physician. 2006;73(7):1211–1220.
