Light's Criteria: One Positive Condition Is Enough
Editorial record
This source-linked educational article has been updated through AI-assisted evidence checks and editorial review. Verify information against current guidance and local protocols before applying it to patient care.
Citation methodologyRetrospective educational series: 17 September to 16 October 2026. This coverage window is not the publication date.
Light's criteria are not a majority vote. The biochemical classification is exudative when any one of the three conditions is positive. Counting how many are positive can help check the calculation, but it does not turn the rule into a two-out-of-three threshold or a severity scale.
A Case With Only the Third Condition Positive
Consider fictional appropriately paired pleural-fluid and serum results:
| Measurement | Value |
|---|---|
| Pleural protein | 28 g/L |
| Serum protein | 60 g/L |
| Pleural LDH | 150 U/L |
| Serum LDH | 300 U/L |
| Laboratory serum LDH upper limit | 210 U/L |
The protein ratio is 28/60 = 0.467, not above 0.5. The LDH ratio is 150/300 = 0.50, not above 0.6. Two-thirds of the laboratory upper limit is 140 U/L; pleural LDH 150 is above 140. Thus one condition is positive, sufficient for an exudative biochemical pattern. The serum LDH measurement of 300 and its laboratory upper limit of 210 serve different roles; do not exchange them.
The three strict greater-than conditions and the OR relationship are set out in the ICS/NCCP pleural-effusion guideline. The 1972 Light publication is the historical source, but its full text was not accessed for this draft.
Check the Boundary Without Rounding Across It
A ratio exactly 0.5 does not exceed 0.5. A ratio exactly 0.6 does not exceed 0.6. A pleural LDH exactly equal to two-thirds of the upper limit does not exceed that limit. A different condition could still be positive in the same sample.
Use the unrounded result to assess a boundary. A display rounded to 0.50 may hide a value slightly above or below 0.5. Record laboratory precision and avoid claiming that an equality inferred from a rounded display is exact.
The local Light's criteria tool accepts the three already assessed conditions, not these five raw laboratory numbers. Perform and verify every comparison first; do not enter a concentration into a condition field. An unticked box is treated as negative by the widget. A comparison that has not been performed is unknown, not a negative result; an incomplete assessment must not be called transudative because its boxes are unticked.
Classification Does Not Identify the Cause
An exudative pattern is not itself a diagnosis of infection, malignancy or a need for drainage. The guideline also notes that diuresis can cause a heart-failure-associated effusion to meet exudative criteria. Discordance requires clinical interpretation, not automatic relabelling. Interpretive limitations.
Handover question: Are all three criteria needed? No. In this example, waiting for two positive criteria would misapply the rule. Document the positive comparison and the clinical question still to be resolved, rather than treating the number of positives as a measure of how serious the effusion is.
Reading label: 26 September 2026. Day 10 of the retrospective educational series. This is a reading-sequence date, not a publication or clinical-review date.
Related calculators
- Light's Criteria Calculator - Pulmonology