What is the Difference Between Transudate and Exudate Pleural Effusion

The main difference between transudate and exudate pleural effusion is that the transudate pleural effusion results in a filtrate of clear fluid with a low protein and cell content. It is due to the increased capillary hydrostatic pressure and/or decreased capillary oncotic pressure. But, on the other hand, the exudate pleural effusion results in a cloudy fluid with a high protein and cell content due to the increased capillary permeability by an inflammatory process. Furthermore, congestive heart failure, chronic kidney disease, protein-losing enteropathy, etc. result in transudate pleural effusion while infections, malignancies, pulmonary embolism, and autoimmune diseases result in exudate pleural effusion. 

In brief, transudate and exudate pleural effusion are the two pathophysiological conditions of pleural effusion, resulting in the accumulation of fluid in the pleural cavity. Importantly, Light’s criteria are the most prominent and accurate diagnostic method, which helps to distinguish between the two types of pleural effusion. 

Key Areas Covered 

1. What is Pleural Effusion
     – Definition, Characteristics, Diagnosis
2. What is the Transudate Pleural Effusion
     – Definition, Characteristics
3. What is the Exudate Pleural Effusion
     – Definition, Characteristics
4. What are the Similarities Between Transudate and Exudate Pleural Effusion
     – Outline of Common Features
5. What is the Difference Between Transudate and Exudate Pleural Effusion
     – Comparison of Key Differences  

Key Terms 

Exudate Pleural Effusion, Hydrostatic Pressure, Inflammation, Pleural Effusion, Transudate Pleural Effusion

What is Pleural Effusion 

Pleural effusion is a pathophysiological condition of lungs, resulting in the build-up of excess fluid in the pleural cavity. Generally, the pleural cavity is the fluid-filled space, which occurs between the visceral and parietal pleura, the lung linings. Typically, a small amount of fluid occurs in the pleural cavity for the lubrication and for facilitating breathing. 

Figure 1: Pleural Effusion

Moreover, the accumulation of excess fluid can impair breathing by limiting the expansion of the lungs. Basically, the amount of impairment of breathing and the effectiveness of the treatments are the two main factors, which determine the seriousness of the pleural effusion. Also, depending on the cause, there are two types of pleural effusion; protein-poor (transudate) and protein-rich (exudate).

Signs and Symptoms 

Looking at the signs and symptoms, some patients do not show any signs or symptoms. However, the common signs of pleural effusion include ascites, peripheral edema, pleural friction rub, elevated jugular venous pressure, unilateral lower extremity swelling, lymphedema, yellowish nails, etc. In addition to these, the symptoms of pleural effusion include fever, chest pain, dry, nonproductive cough, dyspnea (shortness of breath, or difficult, labored breathing), orthopnea (the inability to breathe easily unless the person is sitting up straight or standing erect), hemoptysis, weight loss, etc. 

Diagnosis 

In the process of diagnosis, the accumulation of around 300 ml of fluid in the pleural fluid produces detectable clinical signs. In general, the diagnosis of pleural effusion is based on both medical history and physical examination. Also, it is confirmed by a chest X-ray in which the pleural effusion appears as an area of whiteness on a standard posteroanterior chest X-ray. The other two diagnosis methods include chest CT (computed tomography) and lung ultrasound, which are more accurate than chest X-rays.

Figure 2: A Large Left Sided Pleural Effusion on an Upright Chest X-ray

Furthermore, thoracentesis is the diagnostic method, which helps to determine the cause of the pleural effusion. Also, it involves the removal of pleural fluid out for the evaluation of its chemical composition, for culturing, counting blood cells, and for determining the presence of cancer cells. 

Characteristics 

Generally, the regular pH of the pleural fluid is 7.6. A pleural fluid pH greater than 7.3 suggests that resolution is possible with medical therapy alone. However, a pH of less than 7.2 suggests that a more complicated effusion or empyema requiring surgical drainage has probably formed. On the other hand, the regular levels of glucose in the pleural fluid has to be less than 3.4 mmol/L. However, in conditions such as cancer, tuberculosis, empyema, and rheumatoid arthritis, the pleural fluid glucose/serum fluid glucose ratio is less than 0.5.

Figure 3: CT Scan of Chest Showing Loculated Pleural Effusion in Left Side

For instance, the normal levels of red blood cells in the pleural fluid are less than 10,000 RBCs/uL while the normal levels of white blood cells are less than 1000 WBC/uL. Basically, RBC counts greater than 100,000/uL occur in malignancy, trauma, or pulmonary infarct. Meanwhile, the elevated levels of WBC counts and percentage of neutrophils in the pleural fluid suggest either a bacteria infection or infectious diarrhea, active colitis, menstruation or ovulation, and pelvic inflammatory disease.  

Light’s Criteria 

Light’s criteria (Light et al., 1972) is the diagnostic method for the determination of transudate and exudate pleural effusion. When one of the following Light’s criteria is present in the fluid of the pleural cavity, the fluid becomes an exudate.  

• Effusion  protein/serum protein ratio is greater than 0.5;  
• Effusion  LDH (lactate dehydrogenase)/serum LDH ratio is greater than 0.6;  
• and Effusion  LDH level is greater than two-thirds of the upper limit of the  laboratory’s reference range of (ULN) serum LDH 

What is the Transudate Pleural Effusion 

Transudate pleural effusion is one of the two pathophysiological conditions of pleural effusion. Generally, it is characterized by the permeation or transudation of fluid into the pleural cavity through the walls of intact pulmonary vessels.  Also, the two main conditions, resulting in transudate pleural effusion are the increased hydrostatic pressure in congestive heart failure and/or decreased oncotic pressure in cirrhosis or nephrotic syndrome. Therefore, the filtrate is a clear fluid with a low protein and cell content.   

What is the Exudate Pleural Effusion 

Exudate pleural effusion is the other type of pleural effusion characterized by the escaping or exudation of fluid into the pleural cavity through lesions in blood and lymph vessels as caused by inflammation and tumors. Typically, these lesions allow larger molecules along with the solid matter to pass into the pleural cavity. Hence, the filtrate becomes cloudy while containing a high protein and cell content. 

Similarities Between Transudate and Exudate Pleural Effusion  

• Transudate and exudate pleural effusion are the two types of pathophysiological conditions of pleural effusion.  
• Both result due to the accumulation of fluid in the pleural cavity.    
• Also,  both arise due to various pathological conditions and can impair breathing.    
• Light’s criteria are the most prominent and accurate diagnostic method, which helps to distinguish between transudate and exudate pleural effusion. 

Difference Between Transudate and Exudate Pleural Effusion 

Definition 

Transudate pleural effusion refers to the type of pleural effusion in which fluid is pushed through the capillary due to high pressure within the capillary while exudate pleural effusion refers to the other type of pleural effusion in which fluid leaks around the cells of the capillaries caused by inflammation. 

Pathophysiology 

Transudate pleural effusion occurs due to the increased capillary hydrostatic pressure and/or decreased capillary oncotic pressure while exudate pleural effusion occurs due to the increased capillary permeability by an inflammatory process. 

The appearance of the Fluid 

Transudate pleural effusion results in a filtrate of clear or pale yellow color fluid with a low protein and cell content while exudate pleural effusion results in a cloudy or bloody fluid with a high protein and cell content, frothing when shook and forms clots when left standing. 

Common Causes 

Congestive heart failure, chronic kidney disease, protein-losing enteropathy, nephrotic syndrome, and hepatic cirrhosis result in transudate pleural effusion while infections, malignancies, pulmonary embolism, and autoimmune diseases result in exudate pleural effusion. 

Specific Gravity 

The specific gravity of transudate pleural effusion is less than 1.015 while the specific gravity of exudate pleural effusion is more than 1.015. 

Total Protein Content 

The total protein content of transudate pleural effusion is less than 2.5 g/dL while the total protein content of exudate pleural effusion is more than 2.9 g/dL. 

Fluid/Serum Protein  

The fluid/serum protein ratio is less than 0.5 in transudate pleural effusion while the fluid/serum protein ratio is more than 0.5 in exudate pleural effusion. 

SAAG (Serum-Ascites Albumin Gradient) 

The SAAG of transudate pleural effusion is more than 1.2 g/dL while the SAAG of exudate pleural effusion is less than 1.2 g/dL. 

Fluid/Serum LDH (Lactate Dehydrogenase) 

The fluid/serum LDH ratio is less than 0.6 in transudate pleural effusion, while the fluid/serum LDH is more than 0.6 in exudate pleural effusion. 

Fluid LDH 

The fluid LDH is less than 0.67 x ULN serum for transudate pleural effusion while the fluid LDH is more than 0.67 x ULN serum for exudate pleural effusion. 

Cholesterol Content 

The cholesterol content of the transudate pleural effusion is less than 1.2 mmol/l. While the cholesterol content of the exudate pleural effusion is 1.2 mmol/l or greater. 

Radiodensity on CT Scan  

The radiodensity on CT scan is 2-15 HU of transudate pleural effusion. In contrast, the radiodensity on CT scan is 4-33 HU of exudate pleural effusion. 

Conclusion 

Transudate pleural effusion is one of the two pathophysiological conditions resulting in the accumulation of fluid in the pleural cavity. Generally, this fluid is clear and contains mainly a low amount of protein and cell content. Also, LDH and cholesterol levels are less. Significantly, transudate pleural effusion occurs as a result of increased capillary hydrostatic pressure. On the other hand, exudate pleural effusion is the other pathophysiological condition of pleural effusion. It is characterized by the presence of high protein and cell content in the pleural accumulations. Therefore, the fluid in the pleural cavity can be either turbid or bloody. Similarly, LDH and cholesterol levels can be high. However, exudate pleural effusion results due to the increased capillary permeability by inflammation. Hence, the main difference between transudate and exudate pleural effusion is the origin and type of fluid results in each type of pleural effusion. 

References:

1. “Pleural Effusion.” AMBOSS. Available Here. 
2. Cooper, Angie. “Exeter Clinical Laboratory International.” Transudate or Exudate / Blood Sciences Test / Exeter Clinical Laboratory International, 19 Mar. 2019, Available Here.
3. “Pleural Effusion.” Wikipedia, Wikimedia Foundation, 26 Dec. 2019, Available Here.

Image Courtesy:

1. “Diagram showing a build up of fluid in the lining of the lungs (pleural effusion) CRUK 054” By Cancer Research UK – Original email from CRUK (CC BY-SA 4.0) via Commons Wikimedia   
2. “Effusionhalf” By James Heilman, MD – Own work (CC BY-SA 3.0) via Commons Wikimedia   
3. “Pleura effusion” By I, Drriad (CC BY-SA 3.0) via Commons Wikimedia