Pleural effusions occur when excessive fluid such as blood or pus accumulates between the visceral pleura and parietal, known as the pleural cavity. Evaluating patients for pleural effusions involves certain challenges. However, prompt diagnosis and treatment are essential to reduce the risk of lung damage while addressing the underlying cause of the effusion.
To achieve this, medical professionals may classify a pleural effusion as exudate or transudate. When these classifications are inaccurate, they are known as pseudoexudative pleural effusions.
In this article, we learn more about pleural effusions, their different types, and the potential misidentification of transudative effusions as exudative effusions.
What Is a Pleural Effusion?
There is usually a small amount of liquid in the pleura, or in the thin membranes that line the lungs and chest cavity. This substance helps with lubrication during respiration.
However, sometimes there may be excessive fluid, including blood or pus in the pleural cavity—the area outside the lungs—within the chest cavity.
An abnormal amount of fluid can cause chest pains, difficulty breathing, and lung damage.
Treatment for pleural effusions involves draining the fluid from the pleural cavity, antibiotics to treat bacterial infections, pleurodesis to prevent future fluid build-up, and surgery.
What Are the Types of Pleural Effusions?
There are two main types of pleural effusions:
Transudative Pleural Effusion
This type of pleural effusion occurs when an increase in pressure in the blood vessels makes them leak protein-filled fluid into the pleural cavity.
It usually occurs when a patient:
- Has cirrhosis
- Has just begun peritoneal dialysis
- Is suffering from malnutrition due to low albumin in the blood
- Has congestive heart failure
- Is suffering from kidney disease
- Has coronary heart disease
This type of effusion can also be due to a low blood protein count, with heart failure being the most common cause.
An exudative effusion is a result of fluid buildup caused by blocked blood or lymph vessels as a result of:
- Lung injury
These can be due to conditions such as:
- Heart surgery complications
- Inflammatory conditions like lupus, rheumatoid arthritis, or pancreatitis
Often, patients who have transudative pleural effusion are wrongly classified as having exudative effusion. If the misclassification is later identified using certain testing criteria, it is known as a pseudoexudative pleural effusion.
Differentiating Between Transudative and Exudative Pleural Effusions: Light’s Criteria and Alternative Tests
Light’s criteria is a test that has been historically used to determine whether a pleural effusion is exudative or transudative.
Dr. Richard Light came up with the criteria in 1972. It offers higher sensitivity that enables clinicians to distinguish between transudative and exudative effusions based on their biochemistry. While they are less specific, they can identify exudates with greater accuracy compared to transudates, which tend to be misclassified as exudates in 15% to 25% of cases.
According to these criteria:
- Pleural fluid LDH is categorized as transudates if it is less than ⅔ ULN serum LDH, while it is more than 2/3 ULN serum LDH in exudates.
- The ratio of pleural fluid/serum LDH is less than 0.6 in transudates and equal to or more than 0.6 in exudates.
- Lastly, the ratio of pleural fluid and serum protein is less than 0.5 in transudates, and equal to or more than 0.5 in exudates.
If any one of these criteria is met, it means that the effusion is exudative, which is why it has a lower specificity compared to other criteria. Patients are thus often misidentified as having pseudoexudative pleural effusions when they actually have a transudative pleural effusion.
The misclassification often occurs among patients who are taking chronic diuretics and are experiencing heart failure-related pleural effusions.
The pseudoexudative pleural effusions caused by congestive heart failure can be correctly reclassified with a serum-effusion albumin gradient (serum album minus pleural effusion albumin).
Meanwhile, those with hepatic hydrothorax can be reclassified with pleural effusion/serum albumin ratio.
Dr. Light recommends a serum albumin (pleural albumin ratio of less than 1.2 mg/dl) to confirm whether the effusion is indeed exudative.
For transudates, the pleural effusion/serum albumin gradient should be lower than 0.6 while the serum-effusion albumin gradient should be more than 1.2 gm/dL.
Medical practitioners can also evaluate the following to determine a possible pseudoexudative pleural effusion:
- Effusion/serum albumin ratio of less than 0.6 for transudates and equal to or more than 0.6 for exudates
- Pleural fluid NT-pro-BNP equal to or more than 1,300 pg/mL for transudates and less than 1,300 pg/mL for exudates
- Pleural fluid cholesterol levels lower than 55 mg/L in transudates and equal to or higher than 55 mg/L for exudates
- Effusion/serum cholesterol ratio lower than 0.3 for transudates and equal to or higher than 0.3 for exudates
These alternative tests can be applied if a Light’s criteria diagnosis points to an exudate that is suspected to be a pseudoexudative pleural effusion. Conducting these tests can thus help accurately determine if the effusion is transudate or exudate.
The correct diagnosis of a pleural effusion is crucial for proper treatment and enhancing patient outcomes. By using Light’s criteria, doctors can more accurately distinguish between exudative, transudative, and pseudoexudative pleural effusions.
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