Pleural Fluid Protein Unit Converter
Pleural Fluid Protein Unit Converter
Convert pleural fluid total protein between g/dL, g/L, mg/dL and mg/L — the unit Light’s criteria are written in, and the unit most laboratories outside the United States report.
Pleural Fluid Protein converter
Mass units onlyPleural fluid protein 3.0 g/dL, with a serum protein of 6.8 g/dL
The conversion, and the criteria it feeds
mg/dL = g/dL × 1000
Light’s protein criterion: pleural fluid protein ÷ serum protein > 0.5 = exudate
Absolute alternative: fluid protein at or above 3 g/dL (30 g/L)
- no molar column
- pleural fluid protein is albumin plus hundreds of globulins in proportions that change with the cause of the effusion, so there is no grams per mole to divide by. Inventing an average mass would make a conversion look possible and the answer would mean nothing
- the ratio, not the value
- Light’s primary criterion is a ratio to serum protein, which is dimensionless — so g/dL and g/L give the identical answer provided both samples use the same unit
- 3 g/dL
- the absolute alternative, 90% sensitive and 90% specific in the MSD tabulation. It is weaker than the ratio and is what you fall back on when no paired serum was drawn
Worked example
Pleural fluid protein 3.0 g/dL, with a serum protein of 6.8 g/dL
3.0 × 10 = 30.0 g/L
3.0 g/dL = 3,000 mg/dL = 30,000 mg/L
Ratio = 3.0 ÷ 6.8 = 0.44 — below 0.5, so this criterion is not met
But the absolute figure sits exactly on the 3 g/dL alternative, and the serum-minus-fluid gradient is 3.8 g/dL, above the 3.1 g/dL transudate cut — three criteria, three different readings of one sample
The protein criteria, with the sensitivity of each
| Criterion | Threshold | Sensitivity | Specificity |
|---|---|---|---|
| Pleural fluid : serum protein ratio | > 0.5 | 91% | 89% |
| Pleural fluid protein, absolute | ≥ 3 g/dL (30 g/L) | 90% | 90% |
| Serum minus pleural fluid protein | ≤ 3.1 g/dL (31 g/L) favours exudate | 87% | 92% |
| Light’s criteria as a whole | any one of three met | 98% | 77% |
| Pleural fluid LDH | see the LDH converter | 66–93% | 82–100% |
The same thresholds in every unit this page offers
| Threshold | g/dL | g/L | mg/dL |
|---|---|---|---|
| Exudate, absolute protein | 3.0 | 30 | 3,000 |
| Transudate by protein gradient | 3.1 | 31 | 3,100 |
| A typical normal serum protein | 6.8 | 68 | 6,800 |
| Peritoneal fluid, secondary vs spontaneous bacterial peritonitis | 1.0 | 10 | 1,000 |
Why pleural protein is read as a ratio and not as a value
The question a pleural tap answers first is whether the fluid is a transudate or an exudate, because that single division redirects the entire workup. A transudate comes from altered hydrostatic or oncotic pressure — heart failure, cirrhosis, nephrotic syndrome — and is treated by treating that cause. An exudate comes from disease of the pleura or its capillaries — infection, malignancy, pulmonary embolism, connective tissue disease — and needs direct investigation of the pleural space.
Light’s insight in 1972 was that a ratio to serum separates the two far better than an absolute fluid concentration does, because the fluid protein depends on the serum protein it came from. A patient with a serum protein of 4.5 g/dL from cirrhosis can produce a transudate with 2.4 g/dL of protein — above half of serum, and an exudate by the ratio, correctly — while an absolute 3 g/dL cut would have called it a transudate. The ratio is 91% sensitive and 89% specific in the standard tabulation against 90% and 90% for the absolute figure, and the real gain is that it does not fail systematically in the hypoproteinaemic patients who are most likely to have an effusion in the first place.
The cost of Light’s design is specificity. Because meeting any one of the three criteria is sufficient, the set reaches 98% sensitivity and drops to about 77% specificity, and roughly a quarter of genuine transudates are labelled exudates. The commonest reason is diuresis: treating heart failure concentrates the protein and the LDH in the remaining fluid and pushes both ratios up. The standard rescue is a gradient rather than a ratio — a serum-minus-fluid protein gradient above 3.1 g/dL, or an albumin gradient above 1.2 g/dL, identifies a true transudate that the ratios have misclassified.
The unit point is that the criteria and the thresholds were published in g/dL and most laboratories outside the United States report g/L, a factor of ten apart, with two thresholds — 3.0 and 3.1 — that differ by only a thirtieth and point in opposite directions. There is no molar column on this page because there cannot be one: pleural fluid protein is albumin plus hundreds of globulins in proportions that change with the disease, the same reason CSF total protein and urine protein take mass units only.
Frequently asked questions
How do I convert pleural fluid protein from g/dL to g/L?
Multiply by 10. A pleural fluid protein of 3.0 g/dL is 30 g/L. Light’s criteria and the thresholds quoted with them were published in g/dL, so this is the conversion to make before comparing a g/L report against a remembered figure.
Does the unit affect Light’s protein ratio?
No, provided the fluid and the serum are in the same unit. The ratio is dimensionless, so 3.0/6.8 g/dL and 30/68 g/L give the identical 0.44. It is the absolute 3 g/dL threshold and the 3.1 g/dL gradient that are unit-sensitive, and those two differ from each other by only a thirtieth.
Can pleural fluid protein be converted to molar units?
No. It is a mixture of albumin and hundreds of globulins whose composition changes with the cause of the effusion, so there is no single grams per mole to divide by. Any factor quoted for it would be an assumed average mass rather than a conversion, and the site’s test suite rejects a molar unit declared against total protein for exactly that reason.
What pleural fluid protein means exudate?
A fluid-to-serum protein ratio above 0.5 is Light’s criterion, and meeting it alone is sufficient. Where no paired serum was drawn, an absolute fluid protein at or above 3 g/dL (30 g/L) is the fallback, 90% sensitive and 90% specific. The pleural fluid LDH criteria are independent of both and any one of the three suffices — see the Light’s criteria calculator.
The patient is on diuretics and the fluid looks like an exudate. What now?
Check a gradient rather than a ratio. Diuresis concentrates protein and LDH in the residual fluid and pushes Light’s ratios up, which is the commonest cause of the roughly 25% of transudates that Light’s criteria misclassify. A serum minus pleural fluid protein gradient above 3.1 g/dL, or an albumin gradient above 1.2 g/dL, identifies the true transudate.
Related calculators
References
- Criteria for identifying exudative pleural effusions. MSD Manual, Professional Edition; accessed October 2026. Tabulates Light’s three criteria with their individual sensitivities and specificities.
- Light RW, Macgregor MI, Luchsinger PC, Ball WC Jr. Pleural effusions: the diagnostic separation of transudates and exudates. Ann Intern Med. 1972;77(4):507-13.
- Mayo Clinic Laboratories. Protein, total, body fluid (TPBF) — interpretive thresholds for pleural and peritoneal fluid. Test catalogue; 2026.
Not medical advice. For healthcare professionals and education. Reference intervals vary by laboratory and assay — always use your own laboratory's. Never base a dose or a treatment decision on this page alone. Full disclaimer at calcengines.com/disclaimer/