From the Guidelines
Pleural fluid lactate dehydrogenase (LDH) levels exceeding two-thirds of the standard upper limit for serum is a significant criterion for diagnosing an exudative effusion, as stated in Light's criteria. This criterion is part of a set of guidelines used to differentiate exudative from transudative pleural effusions, which also include the ratio of pleural fluid to serum protein and the ratio of pleural fluid to serum LDH 1. The normal range for serum LDH can vary between laboratories, but a pleural fluid LDH level above two-thirds of the upper limit of normal for serum LDH is considered indicative of an exudative effusion. For example, if the upper limit of normal for serum LDH is 280 U/L, a pleural fluid LDH level above 186.67 U/L (two-thirds of 280 U/L) would suggest an exudative effusion.
Key Points to Consider
- The LDH criterion is useful because it reflects increased cellular activity and breakdown in the pleural space, characteristic of exudative processes such as inflammation or malignancy.
- An effusion is considered exudative if it meets at least one of the criteria outlined in Light's criteria.
- While the LDH criterion is helpful, a comprehensive evaluation including clinical context and potentially other diagnostic tests is necessary for a definitive diagnosis of an exudative effusion 1.
- Alternative tests and criteria, such as the serum-effusion albumin gradient and N-terminal pro-brain natriuretic peptide (NT-BNP) levels, can be useful in certain contexts, especially when the diagnosis is ambiguous or when specific conditions like heart failure are suspected 1.
Clinical Application
In clinical practice, the presence of a pleural fluid LDH level exceeding two-thirds of the standard upper limit for serum should prompt further investigation to determine the underlying cause of the exudative effusion. This may involve additional laboratory tests, imaging studies, and potentially invasive procedures like pleural biopsy, depending on the clinical context and suspicion of underlying disease 1.
From the Research
Significance of Pleural Fluid Lactate Dehydrogenase (LDH) Levels
- Pleural fluid LDH levels exceeding two-thirds of the standard upper limit for serum is a significant indicator in diagnosing an exudative effusion 2, 3, 4.
- A pleural fluid/serum LDH ratio above 0.7 indicates that the fluid is an exudate, although in about 10% of inflammatory pleural effusions due to infection or cancer this ratio is comprised between 0.5 and 0.7 2.
- The enzyme profile of exudates is the reverse of the normal serum profile, with a decrease in isoLDH 1 and 2 and an increase in isoLDH 4 and 5, which provides little information on the origin of exudates 2.
- A more than 30% rise in isoLDH 2 is in favor of a malignancy (mesothelioma excluded) 2.
Diagnostic Criteria
- Light's criteria should be applied to measure the concentrations of protein and lactate dehydrogenase (LDH) in the pleural fluid and serum to determine whether a patient has a transudative or exudative pleural effusion 3, 5.
- A pleural fluid LDH of less than two thirds of the upper limit of normal for serum LDH is one of the criteria for transudative pleural effusion 5.
Differentiating the Etiology of Pleural Effusions
- The pleural fluid LDH-to-adenosine deaminase (ADA) ratio has been proposed as a biomarker to differentiate the etiology of pleural effusion, with a ratio <14.2 favoring tuberculous pleural effusion (TPE), a ratio >14.5 favoring parapneumonic pleural effusion (PPE), and a ratio >46.7 favoring malignant pleural effusion (MPE) 6.