Light's Criteria

Background

Light's criteria classify a pleural effusion as either an exudate or a transudate, which directs the subsequent diagnostic work-up. They compare pleural fluid and serum protein and lactate dehydrogenase (LDH): an effusion is an exudate if any one of three conditions is met — pleural/serum protein ratio greater than 0.5, pleural/serum LDH ratio greater than 0.6, or pleural LDH greater than two-thirds of the upper limit of normal serum LDH. Transudates arise from systemic factors such as heart failure, whereas exudates result from local processes such as infection, malignancy or pulmonary embolism. The criteria are deliberately sensitive for exudates.

Interpreting the result

If none of the three criteria is met, the effusion is a transudate, pointing to systemic causes such as heart failure, cirrhosis or nephrotic syndrome. If one or more criteria are met, it is an exudate, prompting investigation for local pathology including infection, malignancy and pulmonary embolism. Because the criteria are highly sensitive, they occasionally misclassify a transudate as an exudate — particularly in patients on diuretics — so a borderline exudate in a clinically transudative setting may warrant further testing such as the serum-effusion albumin gradient.

Advice

If exudative, investigate for causes such as infection, malignancy or PE. Light's criteria can occasionally misclassify a transudate as exudate (e.g. on diuretics).

Worked example

A pleural effusion has a pleural/serum protein ratio of 0.6, a pleural/serum LDH ratio of 0.4 and pleural LDH below two-thirds of the serum upper limit: one criterion (the protein ratio) is met, so the effusion is classified as an exudate and investigated for a local cause.

Pearls / pitfalls

• Only one of the three criteria needs to be met for the effusion to be classified as an exudate — they are applied as an OR rule.
• Diuretic therapy can raise pleural protein and LDH and falsely label a cardiac transudate as an exudate; use the serum-effusion albumin gradient if the clinical picture suggests a transudate.
• Send paired serum protein and LDH at the same time as the pleural sample, otherwise the ratios cannot be calculated.
• Classification directs investigation but is not a diagnosis — an exudate still requires a search for its underlying cause.

Evidence & validation

Described by Light and colleagues, the criteria remain the standard initial method for separating exudative from transudative effusions and are recommended in pleural disease guidance, including from the British Thoracic Society.

Frequently asked questions

References

1. Light RW, Macgregor MI, Luchsinger PC, Ball WC. Pleural effusions: the diagnostic separation of transudates and exudates. Ann Intern Med. 1972;77(4):507–513.
2. Roberts ME, et al. British Thoracic Society Guideline for pleural disease. Thorax. 2023;78(Suppl 3).

About the creator

Richard Light

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