Upper Limit of Normal for Pleural Fluid LDH
The upper limit of normal for pleural fluid LDH is defined as 67% (or two-thirds) of the upper limit of normal for serum LDH in your laboratory. 1, 2
Practical Application
When no serum sample is available, use pleural fluid LDH >67% of the upper limit of normal serum LDH as a standalone criterion to identify exudates. 1, 2
The absolute value varies by laboratory because each facility establishes its own reference range for serum LDH based on the specific assay methodology employed. 3
As a practical threshold, pleural fluid LDH >200 U/L has been validated as a cutoff for identifying exudates in many clinical settings, though this represents approximately two-thirds of a typical serum LDH upper limit. 4, 5
Light's Criteria Context
Pleural fluid is classified as an exudate when it meets one or more of the following: pleural fluid protein/serum protein ratio >0.5, pleural fluid LDH/serum LDH ratio >0.6, or pleural fluid LDH >2/3 the upper limit of normal for serum LDH. 1, 2, 6
Light's criteria demonstrate 98% sensitivity and 72% specificity for identifying exudates. 2
The modified Light's criteria suggest using a cutoff of pleural fluid LDH >0.66 (rather than 0.6) times the upper limit of laboratory normal as a better discriminator. 1
Alternative Approach Without Serum Sample
If serum is unavailable, combine pleural fluid LDH >67% of the upper limit of normal serum LDH OR pleural fluid cholesterol >55 mg/dL to identify exudates. 1, 2
This "OR" rule maintains diagnostic accuracy comparable to full Light's criteria while avoiding the need for simultaneous blood sampling. 7
Important Caveats
In cardiac effusions classified as exudates by Light's criteria, pleural fluid LDH <250 U/L (combined with albumin gradient >1.2 g/dL and bilateral effusion) suggests the effusion is actually cardiac in origin despite meeting exudate criteria. 1, 2
Approximately 10% of inflammatory pleural effusions from infection or malignancy may have pleural fluid/serum LDH ratios between 0.5 and 0.7, falling in an intermediate zone. 8
When Light's criteria classify an effusion as exudate but there is high pre-test probability for heart failure or cirrhosis, use albumin gradient >1.2 g/dL or albumin ratio <0.6 to correctly reclassify approximately 80% of these pseudoexudates. 1, 2