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Pleural and peritoneal fluids

16 min

Skills you'll build3 skills
  • 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

At the bench: Coordinate cell examination and chemistry for a body fluid specimen

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?

Keep going to see the reasoning.

Keep going to see the reasoning.

Right. Keep going to follow the reasoning.

Keep going to see the reasoning.

Get the idea

Use the explanation to check your first decision.

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
Increased membrane permeabilityProtein, 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 indicates a second process.1,2,3 Serous fluids have few established reference intervals, so each result is interpreted 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 identify nearly every exudate, and some transudates also meet them. A diuretic concentrates pleural fluid, so an effusion from treated heart failure can appear exudative. In that setting, serum albumin minus pleural albumin above 1.2 g/dL (12 g/L) supports a cardiac mechanism.5 Light's criteria apply only to pleural fluid. 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 does not identify a single disease. A gradient below 1.1 g/dL argues against portal hypertension and suggests 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

Follow the bench demonstration

Sources for this step
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
  5. 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

Watch one

Read each decision and its explanation.

Ascitic fluid and serum are collected 30 minutes apart.

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

TestResultReference intervalFlag
Serum albumin3.1 g/dL Low3.5–5.0 g/dL3.5–5.0Low
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 time, so they are drawn close together.

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

    The subtraction is valid only when both albumins are in the same 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, which fits cirrhotic ascites more than cardiac ascites.

    Ascitic protein distinguishes cardiac ascites from cirrhotic ascites when the gradient is high.

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

Practice this skill

Your turn

Problem 1 of 3 · Hints available

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 are weighted equally, and meeting any one of them classifies the fluid as an exudate.

Hint
  1. Check each of the three criteria separately.
  2. Consider how many criteria are needed to classify an exudate.

Review Pleural-fluid classification

Problem 2 of 3 · Hints available

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 indicate?

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 are needed.

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

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

The low albumin can contribute fluid. The exudate indicates a change in membrane permeability or lymphatic drainage as well.

Hint
  1. Consider which type of fluid low oncotic pressure produces.
  2. Review which changes allow protein and cells into the cavity.

Review Serous-fluid formation

Problem 3 of 3 · On your own

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

Read the case evidence, then make each decision. The step is complete without it.

  • Walter Faber, 74, MRN 6620419, has heart failure and has taken a diuretic for 10 days.
TestResultReference intervalFlag
Pleural protein3.0 g/dL
Serum protein6.4 g/dL6.0–8.3 g/dL6.0–8.3
Pleural LDH150 U/L
Serum LDH240 U/L High140–222 U/L140–222High
Pleural albumin1.2 g/dL
Serum albumin2.7 g/dL Low3.5–5.0 g/dL3.5–5.0Low

Specimen: Not measured on pleural fluid. Pleural fluid by thoracentesis, and serum collected within the same 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 meets a criterion, so the effusion is an exudate. 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 criterion that is not met. It does not change the LDH criteria that are met.

A high serum LDH lowers the LDH ratio. It does not indicate 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 interpretation depends on the diuretic. Treated heart failure can move a transudate above Light's cutoffs, and the albumin gradient of 1.5 g/dL supports a cardiac mechanism. The ascites fits the same mechanism, with a SAAG of 1.6 g/dL and a high protein.

For reference

Sources checked