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Pleural and peritoneal fluids

16 min

  • Name the pressure, permeability, or lymphatic change that produces an effusion
  • Classify a pleural effusion as transudate or exudate with Light's criteria
  • Calculate and interpret the serum-ascites albumin gradient

Read the full reference

Try first

Try first

Paired pleural fluid and serum give these results. Pleural protein 3.4 g/dL and serum protein 7.0 g/dL. Pleural LDH 160 U/L and serum LDH 300 U/L. The serum LDH upper reference limit is 222 U/L. How do Light's criteria classify the effusion?

The next section explains it.

The next section explains it.

Right. The next section explains why.

The next section explains it.

Get the idea

Why fluid collects

A thin layer of fluid normally lines the pleural, pericardial and peritoneal cavities. Fluid builds up when more enters a cavity than leaves it.1,2

ChangeEffectCommon examples
Higher hydrostatic pressurePushes more fluid out of capillariesHeart failure, venous obstruction
Lower plasma oncotic pressureLess protein holds fluid in the vesselsCirrhosis, nephrotic syndrome
Leakier membranesProtein, cells and fluid enter the cavityInfection, inflammation, malignancy
Poor lymphatic drainageFluid and protein are removed slowlyTumor, fibrosis, thoracic duct injury

The first two usually produce transudates, which are low in protein. The last two usually produce exudates. One effusion can have more than one cause, so an exudate in a patient with low albumin points to a second process.1,2,3 Serous fluids have few established reference intervals, so each result is read for its own cavity.2

Light's criteria for pleural fluid

A pleural effusion meets Light's criteria for an exudate if any one is present: pleural/serum protein ratio >0.5; pleural/serum LDH ratio >0.6; or pleural LDH >2/3 of the serum LDH upper reference limit. Match units within each ratio. Collect the blood and pleural fluid close together and measure them by comparable methods.3

Light's criteria catch nearly every exudate, and some transudates meet them too. A diuretic concentrates pleural fluid, so an effusion from treated heart failure can look exudative. In that setting, serum albumin minus pleural albumin above 1.2 g/dL supports a cardiac mechanism.3 Light's criteria apply to pleural fluid alone. Pericardial fluid and ascites are classified by other rules.2,3

The serum-ascites albumin gradient

SAAG (g/dL) = serum albumin (g/dL) − ascitic-fluid albumin (g/dL), from specimens collected at about the same time. A gradient of 1.1 g/dL or more supports portal hypertension from a hepatic, cardiac, or other portal-pressure cause. The gradient is a subtraction in g/dL. A high gradient reflects portal pressure and names no single organ disease. A gradient below 1.1 g/dL points away from portal hypertension, toward causes such as peritoneal malignancy, tuberculosis or pancreatic disease. A high gradient with ascitic protein above 2.5 g/dL supports cardiac ascites more than cirrhosis.4

References
  1. Keohane EM, Preston MM, Mirza KM, Walenga JM, eds. Rodak's Hematology: Clinical Principles and Applications. 7th ed. Elsevier; 2025. Accessed September 27, 2026. https://www.us.elsevierhealth.com/rodaks-hematology-9780323936507.html
  2. Milevoj Kopcinovic L, Culej J, Jokic A, Bozovic M, Kocijan I. Laboratory testing of extravascular body fluids: national recommendations on behalf of the Croatian Society of Medical Biochemistry and Laboratory Medicine. Part I: serous fluids. Biochem Med (Zagreb). 2020;30(1):010502. doi:10.11613/BM.2020.010502
  3. Roberts ME, Rahman NM, Maskell NA, et al. British Thoracic Society guideline for pleural disease. Thorax. 2023;78(suppl 3):s1-s42. doi:10.1136/thorax-2022-219784
  4. Biggins SW, Angeli P, Garcia-Tsao G, et al. Diagnosis, evaluation, and management of ascites, spontaneous bacterial peritonitis and hepatorenal syndrome: 2021 practice guidance by the American Association for the Study of Liver Diseases. Hepatology. 2021;74(2):1014-1048. doi:10.1002/hep.31884

Watch one

Ascitic fluid and serum are collected 30 minutes apart.

What is the serum-ascites albumin gradient, and what does it support?

TestResultPreviousReference intervalFlag
Serum albumin3.1 g/dL3.5–5.0 g/dLLow
Ascitic albumin1.6 g/dL
Ascitic total protein1.2 g/dL

Specimen: Not measured on ascitic fluid. Ascitic fluid, paracentesis, and serum collected 30 minutes apart

  1. Check the pairing: serum and ascitic fluid were collected 30 minutes apart.

    The gradient compares two fluids at one moment, so they must be drawn about the same time.

  2. Check the units: both albumins are in g/dL.

    The subtraction is only valid when both albumins share a unit.

  3. Subtract: 3.1 g/dL − 1.6 g/dL = 1.5 g/dL.

    The gradient is serum minus ascitic fluid, in that order.

  4. Compare: 1.5 g/dL is above 1.1 g/dL, so the gradient supports portal hypertension.

    The cutoff for portal hypertension is 1.1 g/dL.

  5. Read the protein: 1.2 g/dL is below 2.5 g/dL, the pattern of cirrhotic ascites more than cardiac ascites.

    Ascitic protein separates cardiac ascites from cirrhotic ascites among high gradients.

SAAG = 3.1 g/dL − 1.6 g/dL = 1.5 g/dL, which supports portal hypertension.

Your turn

Problem 1 of 3

Paired pleural and serum testing gives protein ratio 0.45, LDH ratio 0.70, and pleural LDH below two thirds of the serum upper reference limit. How do Light's criteria classify the fluid?

Correct. The LDH ratio of 0.70 exceeds the 0.6 cutoff, and meeting one Light criterion is enough to classify the fluid as an exudate.

Incorrect. Light's criteria classify a pleural effusion as exudative when at least one criterion is met, so the two unmet criteria do not outweigh the LDH ratio.

Incorrect. The three criteria carry equal weight, and meeting any one of them classifies the fluid as an exudate.

Hint
  1. Check each of the three criteria on its own.
  2. Ask how many criteria it takes to classify an exudate.

Review Pleural-fluid classification

Problem 2 of 3

A patient with nephrotic syndrome has a serum albumin of 1.9 g/dL. Her pleural effusion meets Light's criteria for an exudate on both LDH criteria. What do the results point to?

Low oncotic pressure lets fluid out of vessels and leaves most protein and cells behind, giving a transudate. An exudate needs another change as well.

Explained an effusion by one pressure change alone

Fluid accumulates when entry exceeds removal, through higher hydrostatic pressure, lower oncotic pressure, leakier membranes, or obstructed lymphatic drainage. Naming one mechanism, such as low albumin, can miss a coexisting permeability or lymphatic cause that changes which fluid tests matter.

Hydrostatic pressure also produces a transudate. It cannot explain the raised pleural LDH.

The results are internally consistent and fit a real exudate. A patient can have more than one reason for an effusion.

The low albumin can add fluid. The exudate points to a change in membrane permeability or lymphatic drainage as well.

Hint
  1. Ask which kind of fluid low oncotic pressure alone produces.
  2. Look back at which changes let protein and cells into the cavity.

Review Serous-fluid formation

Problem 3 of 3

Serum albumin is 3.0 g/dL, and ascitic albumin collected at the same time is 2.4 g/dL. What is the serum-ascites albumin gradient?

Show the answer

0.6 g/dL

SAAG = 3.0 g/dL − 2.4 g/dL = 0.6 g/dL. That is below 1.1 g/dL, so the gradient points away from portal hypertension. Dividing ascitic by serum albumin gives 0.8, a ratio with no SAAG cutoff.

Review Ascitic-fluid interpretation

Use it

  • Walter Pruitt, 74, MRN 6620419, has heart failure and has taken a diuretic for 10 days.
  • Pleural fluid and serum are collected within the same hour.
  • The serum LDH upper reference limit is 222 U/L.
TestResultPreviousReference intervalFlag
Pleural protein3.0 g/dL
Serum protein6.4 g/dL6.0–8.3 g/dL
Pleural LDH150 U/L
Serum LDH240 U/L140–222 U/LHigh
Pleural albumin1.2 g/dL
Serum albumin2.7 g/dL3.5–5.0 g/dLLow

Specimen: Not measured on pleural fluid. Pleural fluid by thoracentesis, and serum collected within the hour

Decision 1 of 3

How do Light's criteria classify the effusion?

The LDH ratio is 150 ÷ 240 = 0.63, above 0.6. The pleural LDH of 150 U/L is above two thirds of 222 U/L, which is 148 U/L. One criterion would have been enough.

The protein ratio is 3.0 ÷ 6.4 = 0.47. The LDH criteria carry equal weight, and both are met.

Any one criterion classifies an exudate, and two are met here.

Required all three criteria to label an exudate

Meeting any one criterion classifies the effusion as an exudate. With a protein ratio of 0.45, an LDH ratio of 0.70, and pleural LDH below two thirds of the serum upper limit, the LDH ratio makes it exudative. Requiring all three reports it as a transudate.

Review Pleural-fluid classification

Decision 2 of 3

Which result supports a cardiac mechanism for this effusion?

A protein ratio below 0.5 is one unmet criterion. It does not undo the met LDH criteria.

A high serum LDH lowers the LDH ratio. It says nothing about the pleural mechanism.

2.7 − 1.2 = 1.5 g/dL, above 1.2 g/dL. After diuretics, that gradient supports a cardiac mechanism for an effusion that meets Light's criteria.

Review Pleural-fluid classification

Decision 3 of 3

Ascitic fluid from the same hour has albumin 1.1 g/dL and total protein 3.0 g/dL. The order asks for Light's criteria on it. What do you report?

Light's criteria are defined for pleural fluid. Ascites is classified by the serum-ascites albumin gradient.

2.7 − 1.1 = 1.6 g/dL, which supports portal hypertension. With ascitic protein above 2.5 g/dL, the pattern fits cardiac ascites.

1.1 ÷ 2.7 = 0.41 is a ratio. The SAAG is a subtraction in g/dL.

Used a ratio or reversed the albumin subtraction

SAAG subtracts ascitic albumin from serum albumin. With serum albumin 3.6 g/dL and ascitic albumin 1.9 g/dL, the gradient is 1.7 g/dL, above the 1.1 g/dL cutoff that supports portal hypertension. Dividing gives 0.53, a ratio with no SAAG cutoff, and reversing the subtraction gives −1.7 g/dL, below the cutoff.

Review Ascitic-fluid interpretation

The clue that settled this case is the diuretic. Treated heart failure can push a transudate over Light's cutoffs, and the albumin gradient of 1.5 g/dL shows the cardiac mechanism behind it. The ascites tells the same story through its SAAG of 1.6 g/dL and its high protein.

Keep

Sources checked