Light’s Criteria for Pleural Effusion

Enter pleural and serum protein and LDH values to classify effusions and guide further diagnostics.

Pleural fluid protein

Serum protein

Pleural fluid LDH (U/L)

Serum LDH (U/L)

Serum LDH upper normal limit (U/L)

Typically ~200–250 U/L; use lab-specific reference.

Results

Protein ratio

0.62

Threshold: > 0.5

Criterion met

LDH ratio

1.56

Threshold: > 0.6

Criterion met

Pleural LDH vs ULN

280

Threshold: > 2/3 upper limit

Criterion met

Interpretation

Exudative effusion

Exudative effusion – investigate malignancy, infection, inflammatory or pulmonary embolic causes.

Remember:

Light’s criteria maximize sensitivity for exudates; false positives occur when diuretics concentrate pleural fluid.
Consider serum-pleural albumin gradient >1.2 g/dL to reclassify pseudo-exudates.

How to Use This Calculator

Concurrently measure total protein and LDH in pleural fluid and serum. Use lab-specific reference limits. Enter protein in mg/dL or g/L and LDH in U/L. The calculator standardizes units automatically. Combine Light’s criteria with clinical, imaging, and histologic data to determine etiology and need for further testing.

Formula

Exudate if any of the following:

Frequently Asked Questions

Diuretics can concentrate pleural fluid protein, giving false exudate classification. Consider serum-pleural albumin gradient or NT-proBNP. Light’s criteria use total protein. Albumin gradient is a supportive test when results conflict with clinical suspicion. Hemolysis elevates LDH. Re-collect samples if hemolysis is suspected to avoid misclassification. Yes, but chylous effusions are usually exudative; confirm with triglyceride and chylomicron testing. Ideally measure pleural and serum samples concurrently; significant time gaps may distort ratios.