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Transudate or Exudate

Blood Sciences Test



Samples should ideally be collected into heparin tubes to prevent clot formation. Samples for glucose should be adequately preserved with fluoride. Samples for pH should be collected anaerobically into a heparinised blood gas syringe.

Blood stained samples are not suitable for analysis of total protein and LDH.

All samples should be centrifuged prior to analysis.


The serous body cavities are mesothelial lined potential spaces surrounding the lungs, heart and abdomen. Normally, they contain a small amount of fluid that is an ultrafiltrate of plasma. When production and resorption of this ultrafiltrate are not balanced, fluid may accumulate, resulting in an effusion. Effusions may be classified as transudates or exudates. Transudates are usually bilateral and arise from either increased capillary hydrostatic pressure or decreased oncotic pressure secondary to congestive heart failure, fluid overload, cirrhosis or hypoalbuminemia. Exudates are usually unilateral and result from increased capillary permeability or decreased lymphatic resorption associated with infection, connective tissue disease, pancreatitis or cancer.

Several laboratory tests are helpful in distinguishing transudates from exudates including pH, total protein, lactate dehydrogenase (LD), amylase, glucose, white cell count and differential. Only one of these values has to fall into the exudate range for the effusion to be classified as an exudate. Large chemistry panels should not be ordered on body fluids.

To distinguish exudates from transudates if the patient’s serum total protein is normal and the pleural fluid protein is less than 25g/L the fluid is a transudate. If the pleural fluid protein is greater than 35g/L the fluid is an exudate.

If the pleural fluid protein is between 25 and 35 g/L Lights criteria can be used.

Light’s Criteria

Lab Test




clear, pale yellow

turbid, bloody

Fluid total protein

30 g/L or less

>30 g/L

Fluid/serum protein



Fluid/serum LD



Fluid LD

<0.67 x ULN serum

>0.67 x ULN serum


<1.2 mmol/l

1.2 mmol/l or greater

Specific gravity



Exudates typically have higher protein concentration and LD activity and lower pH and glucose values than transudates. The protein concentration of an exudate usually exceeds 3 g/dL. Exudate LD activity is greater than 0.67 times the upper limit of normal for serum.

Pleural Effusions

Other tests may also be helpful in evaluating pleural effusions. Pleural fluid pH is useful to evaluate the prognosis of effusions associated with pneumonia. Normal pleural fluid pH is 7.6. A pleural fluid pH >7.3 suggests that resolution is possible with medical therapy alone.

Note: If no simultaneous serum sample is received then pleural fluid total protein and LDH may be sufficient to distinguish between an exudates and a transudate. If the pleural fluid protein is between 25 and 35 g/L: the pleural fluid is an exudate if pleural fluid LDH greater than 66% of the upper limit of normal of the serum LDH method.

Measurement of glucose is only recommended in effusions suspected of being due to rheumatoid arthritis.

Additional pleural fluid tests are only required if a specific question is being asked:

Amylase in acute pancreatitis or oesophageal rupture

Pleural fluid triglyceride in suspected chylothorax

Pleural fluid creatinine in suspected urinothorax


A pH < 7.2 suggests that a more complicated effusion or empyema requiring surgical drainage has probably formed. Pleural fluid pH should be measured with a blood gas analyzer and not with litmus paper or a pH meter, because both of the latter methods result in falsely elevated values. A pleural fluid glucose < 3.4 mmol/L or a pleural fluid: serum glucose ratio < 0.5 may be seen in effusions caused by cancer, tuberculosis, empyema and rheumatoid arthritis.

Chylous Pleural Effusions

Chylous pleural effusions usually result from disruption or obstruction of the thoracic duct and are typically described as exudative lymphocytic pleural effusions with a milky appearance. Identifying chylothorax is important in determining the etiology of pleural effusion, but the biochemical parameters of chylous effusions have never been thoroughly analyzed. The criteria published in most medical textbooks were based on a small study published more than 30 years ago. Recently, investigators from the Mayo Clinic published their biochemical analysis of the pleural fluid obtained from 74 adults with a diagnosis of chylothorax (Mayo Clin Proc. Feb 2009;84(2)129-33). Gross appearance of the fluid was not a sensitive diagnostic criterion in identifying chylothorax. Only 44% of cases had the classic milky appearance attributed to chylothorax. A nonmilky appearance should not be used as a criterion to rule out a chylous effusion.

Most chylous effusions (86%) were classified as exudative effusions. Only 10% of the chylous effusions had lactate dehydrogenase levels in the exudative range. The traditional biochemical criterion for chylothorax is a pleural fluid triglyceride level greater than 1.3 mmol/L. The Mayo study validated this criterion.

Peritoneal Fluid

The criteria for classifying transudates and exudates in pleural and pericardial fluids are often misleading when applied to peritoneal fluid (ascites). Transudative processes may produce a peritoneal fluid protein level in the exudate range. Calculation of a serum to ascites albumin gradient (SAAG) is a more physiologically appropriate test. It is calculated as the serum albumin concentration minus the peritoneal fluid albumin. A high gradient (>11 g/L) indicates ascites related to portal hypertension, usually due to cirrhosis. A low gradient < 11 g/L is usually associated with peritoneal seeding by cancer, tuberculosis and nephrotic syndrome.

Ascites fluid with an amylase level more than 3 times the serum value is usually caused by pancreatitis, pancreatic pseudocyst or trauma. Elevated bilirubin may indicate biliary tract injury. Elevated cholesterol in ascites fluid has been associated with malignancy. Elevated alkaline phosphatase has been associated with bowel injury.

Spontaneous bacterial peritonitis (SBP) is identified in 10 to 30% of patients hospitalized with ascites and mortality can approach 30%. Many patients do not manifest symptoms such as abdominal pain, fever or encephalopathy at the time of presentation. Therefore, it is recommended that all patients with cirrhosis and ascites undergo a paracentesis at the time of admission to assess for SBP. The diagnosis is based on a polymorphonuclear (PMN) leukocyte count of 250/uL or greater. An elevated PMN count alone is sufficient to establish the diagnosis as ascitic and blood cultures are often negative.

Cell Counts

Total leukocyte and RBC counts are of limited value in body fluid analysis except when diagnostic peritoneal lavage is performed. Normally, less than 10,000 RBCs/uL are present in pleural, pericardial and peritoneal fluids. Pleural and pericardial fluids usually have less than 1000 WBC/uL while peritoneal fluid has less than 500 WBC/uL. RBC counts >100,000/uL in pleural fluid are suggestive of malignancy, trauma, or pulmonary infarct. Lavage fluids usually have less than 50,000 to 100,000 RBCs/uL and less than 200 WBC/uL. Higher RBC and WBC cell counts in lavage fluids are consistent with hollow organ perforation.


RBC Normal Range

WBC Normal Range













According to the literature related specifically to nephrology and dialysis, normal peritoneal dialysis fluid should contain less than 50 WBC/uL and have fewer than 15% neutrophils. A fluid should be considered suspicious for bacterial infection, when the total WBC count is >100/uL and the percentage of neutrophils exceeds 50%, Common nonbacterial causes of neutrophilia in dialysis fluids include infectious diarrhea, active colitis, menstruation or ovulation, and pelvic inflammatory disease. A Gram stain should also be performed, but the sensitivity for bacterial peritonitis is only 50%. However, a positive gram stain is predictive of culture results in 85% of cases.

Differential count is performed to determine the predominant cell type present in the fluid, which can suggest certain diseases. Reference ranges have not been established. Increased numbers of neutrophils are seen with exudates caused by bacterial infection, infarction, cancer or pancreatitis. Increased numbers of lymphocytes are associated with viral infections, tuberculosis, lymphoproliferative disorders, congestive heart failure, and cirrhosis. Eosinophils are increased in infections, neoplasms, chronic renal failure, pneumothorax, pulmonary infarction and parasitic infestations. Plasma cells are present in rheumatoid arthritis, cancer, tuberculosis, and multiple myeloma.

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