Light's criteria: how the pleural fluid ratios are calculated
Light's criteria compare pleural fluid with blood using three values: the pleural-to-serum protein ratio, the pleural-to-serum LDH ratio, and the pleural LDH measured against the lab's upper limit of normal for serum LDH. For pleural protein 3.0 g/dL and serum protein 6.0 g/dL, the protein ratio is 3.0 ÷ 6.0 = 0.5.
Not a medical device. Not a diagnosis or dosing recommendation.
What are Light's criteria?
Light's criteria are a published rule for sorting a pleural effusion, fluid that collects between the lung and the chest wall, into one of two groups: a transudate or an exudate. The rule was set out by R. W. Light and colleagues in 1972 and uses protein and lactate dehydrogenase (LDH) measured in both the pleural fluid and the blood.
The rule is also known as Light criteria, the transudate vs exudate rule, or the Light 1972 effusion criteria. Under the rule, meeting any one of the three criteria is enough for the exudate group.
This page explains how the three values are calculated. The limits appear in the exact expression below; this page doesn't say what a result means for a patient or what caused an effusion.
The Light's criteria formula
The calculator works out three numbers and compares each with a fixed published limit from the 1972 paper:
protein_ratio = pleural_protein_g_dl / serum_protein_g_dl
ldh_ratio = pleural_ldh_u_l / serum_ldh_u_l
pleural_ldh_u_l compared with a fixed fraction of ldh_uln_u_lExact expression the calculator uses:
exudate if pleural_protein_g_dl / serum_protein_g_dl > 0.5 or pleural_ldh_u_l / serum_ldh_u_l > 0.6 or pleural_ldh_u_l > (2/3) * ldh_uln_u_lMagent's calculator reports the two ratios to 3 decimal places, and the comparison value derived from the LDH upper limit to 1 decimal place.
What each part means
| Term | Plain-English meaning | Unit | Accepted range |
|---|---|---|---|
pleural_protein_g_dl | Total protein in the pleural fluid | g/dL | 0.1–20 |
serum_protein_g_dl | Total protein in the blood | g/dL | 0.1–20 |
pleural_ldh_u_l | LDH in the pleural fluid | U/L | 1–10000 |
serum_ldh_u_l | LDH in the blood | U/L | 1–10000 |
ldh_uln_u_l | The lab's upper limit of normal for serum LDH | U/L | 50–2000, required |
protein_ratio | Pleural ÷ serum protein | ratio | 3 decimal places |
ldh_ratio | Pleural ÷ serum LDH | ratio | 3 decimal places |
How the formula works
- Divide pleural fluid protein by serum protein. That's the protein ratio.
- Divide pleural fluid LDH by serum LDH. That's the LDH ratio.
- Compare the pleural fluid LDH with a fixed fraction of the lab's upper limit of normal for serum LDH.
- Each of the three comparisons is checked separately. If any one exceeds its published limit, the rule places the fluid in the exudate group.
- Each limit is "strictly greater than": a value exactly equal to the limit doesn't meet that criterion.
The two ratios have no units, because the same unit sits on the top and the bottom. The third criterion isn't a ratio; it compares an LDH value in U/L with another value in U/L.
Units
Protein goes in as g/dL and LDH as U/L, for both the pleural fluid and the blood. The upper limit of normal for LDH is your lab's own number, printed with the serum result. Magent's calculator requires it and doesn't assume a default such as 200 U/L, because labs differ.
Worked examples
These use Magent's own expected values for the calculator. They show the arithmetic of the ratios.
Example 1: protein 3.0 / 6.0, LDH 100 / 200
- Protein ratio: 3.0 ÷ 6.0 = 0.500
- LDH ratio: 100 ÷ 200 = 0.500
Example 2: protein 4.0 / 6.0, LDH 100 / 200
- Protein ratio: 4.0 ÷ 6.0 = 0.6666… → 0.667
- LDH ratio: 100 ÷ 200 = 0.500
Example 3: protein 3.0 / 6.0, LDH 130 / 200
- Protein ratio: 0.500
- LDH ratio: 130 ÷ 200 = 0.650
Example 4: protein 5.0 / 10.0, LDH 80 / 200
- Protein ratio: 5.0 ÷ 10.0 = 0.500
- LDH ratio: 80 ÷ 200 = 0.400
Examples 1 and 4 have different protein values but the same protein ratio. The ratio depends on the relationship between the two samples, not on either number alone.
History of Light's criteria
The criteria come from R. W. Light, M. I. Macgregor, P. C. Luchsinger and W. C. Ball Jr, Pleural effusions: the diagnostic separation of transudates and exudates, published in the Annals of Internal Medicine in 1972 (volume 77, issue 4, pages 507–513).
More than 50 years later, a 2026 study of over 7,000 thoracenteses described Light's criteria as the standard for distinguishing exudative from transudative pleural effusions, and as the gold standard for that differentiation.[1]
When it's used
The criteria are applied to fluid removed from the pleural space. In the 2026 study, the patients were those who underwent diagnostic thoracentesis, a procedure that drains pleural fluid through a needle.[1]
Where it's used
The inputs come from two laboratory samples: pleural fluid, and blood. The 2026 study notes that Light's criteria require serum sampling alongside the pleural fluid, and it tested blood-free combinations, measured in pleural fluid alone, as an alternative.[1]
Who uses it
Light's criteria have been extensively evaluated in published studies, many of them small series with few transudates, which is why the 2026 authors set out to reassess them in a larger population.[1]
Limits of the calculation
- It sorts fluid into one of two groups. It doesn't name the cause of an effusion.
- It isn't Heffner's abbreviated two-test criteria, and it isn't an NT-proBNP rule.
- Magent doesn't interpret a chest x-ray.
- The LDH upper limit of normal is required input. The calculator doesn't assume 200 U/L.
- Equality with a published limit doesn't meet that criterion; the comparisons are strictly greater than.
- Inputs outside the accepted ranges are rejected, not estimated.
Clinical decisions belong to a qualified clinician who knows the patient.
How it differs from related calculators
- A–a gradient uses arterial blood-gas values to compare alveolar and arterial oxygen. It doesn't use pleural fluid.
- Blood-free pleural fluid combinations of protein, LDH and cholesterol have been studied as alternatives that avoid a blood sample.[1] Magent's calculator doesn't compute them.
Glossary
- Pleural effusion: fluid collected between the lung and the chest wall.
- Transudate / exudate: the two groups Light's criteria sort pleural fluid into.
- LDH: lactate dehydrogenase, an enzyme measured in U/L.
- Upper limit of normal (ULN): the top of a lab's reference interval for serum LDH.
- Thoracentesis: draining pleural fluid with a needle for testing.[1]
Common mistakes
- Leaving out the LDH upper limit of normal. The third criterion depends on the lab's own value.
- Swapping pleural and serum values. Pleural fluid goes on top of each ratio.
- Mixing protein units. Both protein values must be in g/dL.
- Treating equality as meeting a criterion. Each comparison is strictly greater than.
Source
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
Further reading
- Porcel JM, Porcel L, Palma R, Bielsa S. Revisiting Light's criteria: a validated blood-free triple combination matches diagnostic accuracy in over 7000 patients. ERJ Open Res. 2026;12(2):00821-2025.
Frequently asked questions
How do you calculate Light's criteria?
Divide pleural protein by serum protein, divide pleural LDH by serum LDH, and compare pleural LDH with a fixed fraction of the lab's LDH upper limit. For protein 3.0 and 6.0, the protein ratio is 0.500.
What are the cutoffs for Light's criteria?
The exact expression above shows the limits the calculator uses (strictly greater than 0.5, 0.6 and two-thirds of the LDH upper limit). What a result means needs a clinician's reading of the source.
Does a patient need a blood test for Light's criteria?
Yes. Two of the three values compare pleural fluid with blood, and the third uses the lab's serum LDH upper limit.
Why does the calculator need the LDH upper limit of normal?
Because labs report different upper limits, and the third criterion is measured against that value. Magent doesn't assume one.
What is the difference between a transudate and an exudate?
They're the two groups Light's criteria sort pleural fluid into. This page doesn't describe causes.
Who developed Light's criteria?
R. W. Light and colleagues, in a 1972 paper in the Annals of Internal Medicine.