From the Guidelines
Transudative pleural effusions are primarily caused by systemic conditions such as congestive heart failure, cirrhosis, and renal failure, which alter the balance of hydrostatic and oncotic pressures, leading to fluid accumulation in the pleural space. The most common causes of transudative pleural effusions include:
- Congestive heart failure, where increased pulmonary venous pressure forces fluid into the pleural space 1
- Cirrhosis with ascites, where fluid crosses the diaphragm through lymphatic channels 1
- Nephrotic syndrome, which decreases serum albumin and reduces oncotic pressure 1
- Hypoalbuminemia from any cause, which also reduces oncotic pressure 1
- Pulmonary embolism, which can produce a transudative effusion in its early stages 1 Less common causes include peritoneal dialysis, myxedema from hypothyroidism, and superior vena cava obstruction. According to the most recent study by Morris et al. 1, these effusions are characterized by low protein content and a pleural fluid-to-serum protein ratio less than 0.5, with lactate dehydrogenase (LDH) levels below 200 IU/L or a pleural fluid-to-serum LDH ratio less than 0.6. Treatment should focus on addressing the underlying condition rather than the effusion itself, as the fluid will resolve once the primary cause is corrected 1.
The European Respiratory Journal study 1 also highlights that more than 80% of transudates are due to heart failure, followed by liver cirrhosis, hypoalbuminemia, nephrotic syndrome, and atelectasis. Most transudates can be successfully treated with diuretics, making further investigations unnecessary. However, it is essential to note that categorization of a pleural effusion as transudate or exudate is not always indicative of a particular aetiology or group of aetiologies, and misclassification can occur 1.
In clinical practice, the most frequent causes of pleural effusion include heart failure, malignancy, pneumonia, tuberculosis, post-surgery, pericardial diseases, and cirrhosis 1. Aetiologies will vary according to whether the effusion is unilateral or bilateral. The primary goal is to identify and treat the underlying cause of the transudative pleural effusion, which will ultimately improve patient outcomes and quality of life.
From the Research
Causes of Transudative Pleural Effusion
The causes of transudative pleural effusion can be attributed to various systemic factors that alter the formation or absorption of pleural fluid, without involving the pleural surfaces directly 2, 3. Some of the common causes include:
- Congestive heart failure, which is the most common cause of transudative effusion 2, 3, 4, 5
- Pulmonary embolism 2, 3, 4, 5
- Cirrhosis of the liver with ascites 2, 5
- Nephrotic syndrome 2, 3, 4
- Hepatic hydrothorax 3, 5
Characteristics of Transudative Pleural Effusion
Transudative pleural effusions are characterized by a low cell and protein content, and can be diagnosed by examining the characteristics of the pleural fluid 2, 3. The diagnosis of transudative effusion is established by the following criteria:
- The ratio of the pleural fluid to the serum protein is less than 0.5
- The ratio of the pleural fluid to the serum LDH is less than 0.6
- The pleural fluid LDH is less than two thirds the upper limit of normal for the serum LDH 2