Normal Limit of LDH in Serum for Calculating Transudative Effusions
According to Light's criteria, the upper limit of normal serum LDH to calculate transudative effusions is the laboratory-specific upper limit of normal, with pleural fluid LDH > 2/3 of this upper limit indicating an exudative effusion. 1, 2
Light's Criteria for Distinguishing Transudates from Exudates
Light's criteria are the gold standard for differentiating between transudative and exudative pleural effusions, with high sensitivity (98%) but moderate specificity (72%) for identifying exudates 1, 2.
According to these criteria, a pleural effusion is considered an exudate if it meets at least one of the following:
- Pleural fluid protein divided by serum protein ratio > 0.5
- Pleural fluid LDH divided by serum LDH ratio > 0.6
- Pleural fluid LDH > 2/3 the upper limit of normal for serum LDH 1, 2
Clinical Application
When evaluating a pleural effusion:
- Obtain both serum and pleural fluid samples for protein and LDH measurements
- Compare values using Light's criteria to determine if the effusion is transudative or exudative
- Use laboratory-specific reference ranges for serum LDH, as these can vary between institutions
Important Considerations
If serum samples are unavailable, an alternative approach is using an "or" rule of pleural fluid LDH >67% of the upper limit of normal serum LDH and cholesterol >55 mg/dL 1
For cases with results close to the cut-off points with high clinical suspicion of heart failure or cirrhosis (which typically cause transudates but may appear as exudates due to diuretic use), consider:
- Albumin gradient (serum albumin minus pleural fluid albumin) >1.2 g/dL, or
- Albumin ratio (pleural fluid albumin divided by serum albumin) <0.6 1
NT-proBNP measurements in pleural fluid or serum can help identify heart failure-related effusions, with levels >1500 μg/mL having high diagnostic accuracy 1
Pitfalls to Avoid
- Do not rely solely on a single parameter for classification
- Do not use pH litmus paper or pH meters for pleural fluid pH measurement; blood gas analyzers provide more accurate results 2
- Remember that Light's criteria are designed to maximize sensitivity for exudates at the expense of specificity, so some transudates may be misclassified as exudates 1, 2
The specific upper limit of normal for serum LDH varies between laboratories, so it's essential to refer to your institution's reference range when applying the 2/3 threshold for pleural fluid LDH.