Light's criteria calculator (pleural effusion)
Classify a pleural effusion as an exudate or a transudate with Light's criteria from 1972: enter pleural fluid and serum protein, pleural fluid and serum LDH and your laboratory's upper limit of normal for serum LDH, and see each ratio and which of the three criteria are met.
Guides
- Light's criteria false exudates in heart failureWhy Light's criteria call about 1 in 4 transudates exudates, mostly after diuretics, and how the albumin and protein gradients correct heart failure cases.
How this Light's criteria calculator works
This Light's criteria calculator compares pleural fluid with serum on three measurements and calls the effusion an exudate if any one of them is positive. Richard Light and colleagues at Johns Hopkins published the rule in the Annals of Internal Medicine in 1972 after studying 150 pleural effusions. Light later explained that no single cut-off of protein or LDH separated every transudate from every exudate, so he placed each of three cut-offs where no transudate lay above it. Used together, the three identified almost all transudates and exudates correctly. The three criteria are a pleural fluid to serum protein ratio above 0.5, a pleural fluid to serum lactate dehydrogenase (LDH) ratio above 0.6, and a pleural fluid LDH above two-thirds of the upper limit of normal for serum LDH. Serum values should come from blood taken at about the same time as the pleural fluid. Protein can be entered in g/dL or g/L, as long as both values use the same unit.
Exudate if at least one is true: pleural protein ÷ serum protein > 0.5, or pleural LDH ÷ serum LDH > 0.6, or pleural LDH > ⅔ × upper limit of normal for serum LDH
Transudate if none of the three is true. The protein ratio has no unit, so g/dL and g/L give the same answer. The 1972 paper wrote the third criterion as a pleural LDH above 200 IU; using two-thirds of your own laboratory's upper limit lets the cut-off follow your assay.
Each criterion is strict: a protein ratio of exactly 0.50 or an LDH ratio of exactly 0.60 is not met. This calculator rounds both ratios to two decimals, as it shows them, before comparing, and gives the serum minus pleural fluid protein gradient for any exudate so a borderline result in heart failure can be checked against the 3.1 g/dL cut-off described below.
Light's criteria results and what they mean
A Light's criteria result has only two outcomes: exudate, when one, two or all three criteria are met, and transudate, when none is met. A transudate forms when the pressures that move fluid across a normal pleura are out of balance, and Bielsa and colleagues note that pleural transudates are most often due to heart failure or hepatic hydrothorax. An exudate points to disease of the pleura or of its lymphatic drainage, such as infection or cancer, and usually needs further tests on the fluid to find its cause. The number of criteria met is not a severity grade: the 1972 rule treats one positive criterion the same as three. What the ratios do show is the margin. Light's 2013 review advises that when a disease that causes transudates is likely but the criteria suggest an exudate by only a small margin, the serum minus pleural fluid protein gradient should be checked.
| Criteria met | Result | What it means |
|---|---|---|
| 0 of 3 | Transudate | Fluid from a pressure imbalance across a normal pleura, most often heart failure or hepatic hydrothorax. |
| 1, 2 or 3 of 3 | Exudate | Fluid from pleural or lymphatic disease; the cause is sought with further pleural fluid tests. If heart failure is likely and the margin is small, check the serum minus pleural fluid gradients. |
Two gradients help sort out a borderline exudate in heart failure. Bielsa 2012 found that a serum minus pleural fluid albumin gradient above 1.2 g/dL (12 g/L) correctly identified 83% of heart failure effusions that Light's criteria had labelled exudates, against 55% for a protein gradient above 3.1 g/dL (31 g/L).
How accurate are Light's criteria?
Light's criteria are very sensitive for exudates but less specific, because a single positive test is enough to call an exudate. Porcel's 2013 review in Current Opinion in Pulmonary Medicine states that Light's criteria identify 98% of pleural exudates but misclassify about 25% of transudates as exudates, and Light's own 2013 review adds that most of these misclassified patients are on diuretics. During diuresis, the protein and LDH concentrations in pleural fluid rise. Chakko and colleagues sampled the effusions of eight patients with heart failure before and after treatment: the mean protein ratio rose from 0.34 to 0.47 and the mean LDH ratio from 0.39 to 0.64, and three effusions that began as transudates met the exudate criteria after diuresis. Blood in the fluid also raises pleural LDH. In a 2002 study of 175 patients with heart failure who had a thoracentesis, Eid and colleagues found a noncardiac cause for most exudates, and red cell contamination explained 4 of the 12 that heart failure alone produced.
Other cut-offs have been proposed. Heffner's 1997 meta-analysis of 1,448 patients from eight studies found by ROC analysis that a pleural LDH above 0.45 of the upper limit of normal worked as a threshold, but no test combination was clearly better than the rest. Romero-Candeira's 2002 study of 249 patients found Light's criteria 93% accurate, higher than clinical judgement alone (84%), the albumin gradient (87%) and the protein gradient (86%); in patients on diuretics their accuracy fell to 83%. Porcel and Light concluded in 2021 that the criteria remain the default reference test after 50 years. For a full walk-through of the heart failure problem, see the guide to false exudates in heart failure.
Questions
How many of Light's criteria must be met for an exudate?
One. An effusion that meets any one of the three criteria is an exudate; a transudate meets none of them.
Does it matter if protein is in g/dL or g/L?
No, as long as pleural fluid and serum protein are in the same unit. The ratio cancels the unit, so 3.5/6.5 g/dL and 35/65 g/L both give 0.54.
Why does the LDH cut-off use my lab's upper limit?
LDH assays and reference ranges differ between laboratories. The 1972 paper used 200 IU; Light and later reviews state the criterion as two-thirds of the upper limit of normal for serum LDH, so it follows each lab's range.
What if a heart failure patient's effusion is an exudate?
If the criteria are met only narrowly, check the serum minus pleural fluid albumin gradient (above 1.2 g/dL suggests a transudate) or protein gradient (above 3.1 g/dL). The guide to false exudates works through an example.
Is my data sent anywhere?
No. The calculation runs in your browser, and the values you enter are not sent or stored.
Sources
- 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
- Light RW. The Light criteria: the beginning and why they are useful 40 years later. Clin Chest Med. 2013;34(1):21–26. doi:10.1016/j.ccm.2012.11.006
- Porcel JM. Identifying transudates misclassified by Light's criteria. Curr Opin Pulm Med. 2013;19(4):362–367. doi:10.1097/MCP.0b013e32836022dc
- Bielsa S, Porcel JM, Castellote J, Mas E, Esquerda A, Light RW. Solving the Light's criteria misclassification rate of cardiac and hepatic transudates. Respirology. 2012;17(4):721–726. doi:10.1111/j.1440-1843.2012.02155.x
- Chakko SC, Caldwell SH, Sforza PP. Treatment of congestive heart failure. Its effect on pleural fluid chemistry. Chest. 1989;95(4):798–802. doi:10.1378/chest.95.4.798
- Eid AA, Keddissi JI, Samaha M, et al. Exudative effusions in congestive heart failure. Chest. 2002;122(5):1518–1523. doi:10.1378/chest.122.5.1518
- Heffner JE, Brown LK, Barbieri CA. Diagnostic value of tests that discriminate between exudative and transudative pleural effusions. Chest. 1997;111(4):970–980. doi:10.1378/chest.111.4.970
- Romero-Candeira S, Hernández L, Romero-Brufao S, et al. Is it meaningful to use biochemical parameters to discriminate between transudative and exudative pleural effusions? Chest. 2002;122(5):1524–1529. doi:10.1378/chest.122.5.1524
- 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. Pleural fluid analysis: are Light's criteria still relevant after half a century? Clin Chest Med. 2021;42(4):599–609. doi:10.1016/j.ccm.2021.07.003