Transudate and exudate are two different types of fluids that are secreted in the body in reaction to different pathologies.
Transudates result from an imbalance in oncotic and hydrostatic pressures. These are usually ultrafiltrates of plasma.
Exudates are the result of inflammation. These are produced by a variety of inflammatory conditions and often require more extensive evaluation and treatment than transudates.
Normal Fluid Production and Balance in Body Cavities
The serous body cavities surrounding the lungs, heart and abdomen normally contain a small amount of fluid. This fluid is formed when plasma exits from a capillary bed in the tissue and enters the interstitial space. The rate of fluid formation depends on the balance between plasma and tissue oncotic and hydrodynamic pressures.
The forces promoting fluid filtration out of the blood vessels (capillaries on the arteriolar side) into the interstitium (plasma hydrodynamic pressure and tissue oncotic pressure) are slightly higher than the forces promoting fluid absorption from the tissue back into the blood vessels ( capillaries on the venule side) (plasma oncotic pressure and tissue hydrodynamic pressure).
This leads to a net accumulation of small amounts of fluid in the interstitium that enters the body cavities. Excess fluid is normally removed by the lymphatic system. In case there is an imbalance between fluid production and reabsorption, excess fluid may accumulate, resulting in an effusion.
Effusions can occur in various body cavities. Pleural effusion is the excess fluid that accumulates in the pleural cavity, the fluid-filled space that surrounds the lungs. Ascites refers to an abnormal accumulation of fluid in the abdominal (peritoneal) cavity. Pericardial effusion is excess fluid between the heart and the sac surrounding the heart, known as the pericardium. Synovial effusion refers to excess fluid accumulation around the knee joint.
Causes
Transudates
- Increased Hydrostatic Pressure or Decreased Plasma Oncotic Pressure
- Congestive heart failure
- Hepatic cirrhosis
- Hypoproteinemia (e.g., nephrotic syndrome)
Exudates
- Increased Capillary Permeability or Decreased Lymphatic Resorption
- Infections: Bacterial, Tuberculosis, Viral
- Neoplasms: Bronchogenic/ovarian/prostate carcinoma, Lymphoma, Hepatoma, Mesothelioma, Metastatic carcinoma
- Noninfectious inflammatory disease: Rheumatoid disease, Systemic lupus erythematosus
- Trauma
- Pancreatitis
- Bile peritonitis (e.g., ruptured gallbladder)
Differentiation of Transudate and Exudate
Transudates are usually bilateral. They occur as a result of either increased capillary hydrostatic pressure or decreased plasma oncotic pressure.
Exudates are usually unilateral and result from increased capillary permeability or decreased lymphatic resorption associated with infection, connective tissue disease, pancreatitis or cancer.
The first step in the evaluation of effusions is to differentiate between transudate and exudate effusions.
To separate the two, several chemical parameters have been proposed, although none is 100% accurate.
Gross Findings
Transudates are typically clear, pale yellow to straw-colored, odorless and do not clot.
Due to the presence of leukocytes, tumor cells, or increased protein levels, exudates usually show some degree of cloudiness or turbidity, and they often clot.
A bloody effusion suggests trauma, malignancy, or pulmonary infarction. A traumatic tap is suggested by uneven blood distribution, fluid clearing with continued aspiration, or formation of small blood clots.
Total Protein Concentration
Values above 3.0 g/dL are found in exudates while concentration below 3.0 g/dl is found in transudates.
However, using total protein alone misclassifies both exudates and transudates by about 30%.
A battery of tests when carried out together increase sensitivity and accuracy.
Exudates typically have higher protein concentration and LD activity and lower pH and glucose values than transudates.
Light’s Criteria
| Lab Test | Transudate | Exudate |
| Appearance | clear, pale yellow or straw cloured | Turbid or cloudy |
| Fluid total protein | < 3.0 g/dL | ≥3.0 g/dL |
| Fluid/serum protein | <0.5 | ≥0.5 |
| Fluid/serum LD | <0.6 | ≥0.6 |
| Fluid LD | <2/3 upper limit of normal serum LD | ≥2/3 upper limit of normal serum LD |
| Cholesterol | <45 mg/dL
|
≥45 mg/dL |
| Fluid/serum cholesterol ratio
|
< 0.30
|
≥0.30
|
| Serum–fluid albumin gradient
|
>1.2 g/dl
|
1.2 g/dl
|
| Fluid/serum bilirubin ratio
|
< 0.60 | ≥ 0.60
|
| Specific gravity
|
<1.015 | ≥ 1.015 |
| LD, Lactate dehydrogenase | ||
In ascites, it is more difficult to differentiate a transudate or an exudate. Infected or malignancyrelated samples can have protein concentrations in the transudative range (i.e., <3.0 g/dL), and many patients with cirrhotic or heart failure ascites have protein values in the exudative range (> 3.0 g/dL)
However, the serum–ascites albumin gradient, defined as the serum albumin concentration minus the ascitic fluid albumin concentration, is widely considered as the most reliable method to differentiate ascitic transudates from exudates.
Further analysis of exudates is directed toward ruling out malignancy and infection.
Other investigations in the diagnostic work-up of effusions are as follows.
Leukocyte counts
Transudates usually have low leukocyte counts (<1000/µL) as compared to exudates (>1000/µL).
Cytological examination
This is done by examining a Romanowsky stained smear under the microscope. This can provide useful information regarding the various types and numbers of leukocytes, mesothelial cells and also any malignant cells, if present. Cytological examination is mandatory in case of clinical suspicion of malignancy.
Microbiological examination
- Culture and sensitivity testing and Gram stain: for bacterial infections
- Acid fast bacilli stain: direct staining of tuberculous effusions for acid fast bacteria
ADA (adenosine deaminase)
Increased levels of this enzyme serve are seen in tubercular effusions.
Tumor markers
Although not recommended as a routine test, various tumor markers are often a useful adjunct in noninflammatory exudates with negative cytology.
Immunological studies
Rheumatoid factor, Anti-nuclear antibody, and complement levels have limited role in routine diagnosis.
