How this instrument works
Light's Criteria sort pleural fluid into two broad categories. An exudate suggests something local is leaking protein and cells into the pleural space — infection, cancer, inflammation. A transudate suggests a systemic imbalance of pressure or oncotic pull is pushing fluid across an otherwise normal membrane — heart failure, cirrhosis, nephrotic syndrome. Getting that split right early points the whole workup in the right direction, toward chasing a local cause or toward managing the condition driving fluid across a normal membrane.
The rule needs only three numbers compared as ratios: pleural fluid protein against serum protein, pleural fluid LDH against serum LDH, and pleural LDH against two-thirds of the lab's own upper limit of normal for serum LDH. Any single ratio crossing its threshold — 0.5 for protein, 0.6 for LDH, or pleural LDH exceeding that two-thirds cutoff — is enough on its own to call the fluid an exudate. All three criteria do not need to agree; one is sufficient.
Richard Light and colleagues published the rule in 1972 after comparing it against the older, cruder method of reading pleural fluid protein alone. Five decades on it remains the first pass nearly every clinician runs, precisely because it needs nothing more exotic than protein and LDH — tests already sitting in a standard chemistry panel.
- Enter Pleural fluid protein and Serum protein in g/dL — these come from the same blood draw and thoracentesis sample.
- Enter Pleural fluid LDH and Serum LDH in U/L.
- Enter the Upper limit of normal for serum LDH at your own lab — this figure varies by lab and assay, so use the reference range printed on your report, not a generic number.
- Read the three ratios and the exudate/transudate call; the working block shows which specific criterion, or criteria, tripped the result.
Worked example — protein 4.0 vs 6.0 g/dL, LDH 300 vs 200 U/L
Pleural protein 4.0 g/dL against a serum protein of 6.0 g/dL gives a protein ratio of 4.0 ÷ 6.0 = 0.667, already past the 0.5 cutoff. Pleural LDH of 300 U/L against a serum LDH of 200 U/L gives an LDH ratio of 300 ÷ 200 = 1.5, well past 0.6. And with a serum LDH upper limit of normal of 200 U/L, two-thirds of that is 133.3 — the pleural LDH of 300 clears that too. All three criteria point the same way: exudate, though any one of them alone would have been enough to call it.
Compare that against a milder case: pleural protein 2.5 g/dL, serum protein 6.0 g/dL, pleural LDH 150 U/L, serum LDH 200 U/L, same 200 U/L upper limit of normal. The protein ratio comes to 2.5 ÷ 6.0 = 0.417, under the 0.5 line — no flag there. But the LDH ratio is 150 ÷ 200 = 0.75, past the 0.6 threshold. That single crossing is sufficient on its own; the fluid still reads as an exudate even though the protein ratio alone would have suggested otherwise.
Questions
What does calling an effusion an exudate actually change clinically?
It redirects the workup. An exudate points toward a local process worth chasing directly — pleural infection, malignancy, or an inflammatory condition — often prompting cytology, culture, or further imaging of the pleura itself. A transudate points the other way, toward a systemic driver like heart failure, cirrhosis, or nephrotic syndrome, where treating the underlying condition usually resolves the fluid without needing to investigate the pleura as the primary problem.
Do all three criteria need to be positive to call an exudate?
No — only one. The rule is deliberately built so that any single ratio crossing its threshold is sufficient, which is why it catches most true exudates even when one or two of the three measurements come back ambiguous or borderline.
Does Light's Criteria ever call a transudate an exudate by mistake?
Yes, most notably in heart failure patients already on diuretics. As diuretic therapy pulls volume out of the pleural space, the remaining fluid's protein and LDH concentrate, nudging the ratios past the exudate thresholds even though the underlying cause is still a systemic, transudative one. In that specific situation, clinicians sometimes lean on a secondary check — a serum-to-pleural albumin gradient — to catch the false call.
What upper limit of normal should I use for serum LDH?
Use the reference range printed on your own lab's report, not a generic textbook figure. LDH assays and their normal ranges vary meaningfully between labs, and the third criterion is defined relative to your specific lab's upper limit, not a universal constant.
Where does the rule come from?
Richard Light and colleagues published it in 1972 (Ann Intern Med. 1972;77(4):507-513) after finding that pleural fluid protein alone, the older standard, missed too many true exudates. Adding the LDH-based criteria closed most of that gap, and the three-part rule has stayed the standard first-line test ever since.
References
Read this first: This instrument computes a screening figure from population formulas — it is not a diagnosis, and it cannot see the whole picture a clinician can. Use it to inform a conversation, not to replace one.