Light’s Criteria Calculator for Pleural Effusion

Written by Great Medico

Updated September 24, 2026 •2 min read

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A lung or oxygenation value can be easy to misread when the formula is hidden. Light’s Criteria Calculator for Pleural Effusion makes the calculation explicit, so you can see what the result represents—and what it does not.

What this calculator does

Light’s Criteria Calculator for Pleural Effusion uses Pleural-fluid protein, Serum protein, Pleural-fluid LDH, Serum LDH, and Serum LDH upper limit of normal to produce the exudative by light’s criteria used by this calculator. It only uses the fields shown in this calculator; it does not silently add tests, diagnoses, symptoms, or clinical variables that are absent from the interface. Uses standard health-calculator relationships where a stable equation exists; complex or policy-dependent tools expose assumptions/points so the result can be checked against the current clinical or public-health source.

How to use it

Enter or select Pleural-fluid protein, Serum protein, Pleural-fluid LDH, Serum LDH, and Serum LDH upper limit of normal. Use measurements from the same respiratory assessment and confirm whether oxygen values are percentages, fractions, or pressures. If an input is unknown, it is better to leave the calculation unresolved than to invent a value.

How the calculation works

Three comparisons are tested: pleural/serum protein ratio >0.5, pleural/serum LDH ratio >0.6, and pleural LDH > two-thirds of the serum LDH upper limit of normal. If any one is positive, the tool labels the fluid exudative by Light’s criteria.

Example

With pleural protein 3 g/dL and serum protein 6 g/dL, the protein ratio is 0.50—not above the criterion. With pleural LDH 250, serum LDH 300, and serum LDH ULN 250, the LDH ratio and two-thirds-ULN comparison are positive, so the tool reports an exudative result.

How to interpret the result

One positive Light criterion classifies the effusion as exudative by the rule. That classification narrows the differential; it does not identify the cause, and some treated heart-failure effusions can be misclassified.

Limitations and notes

Respiratory measurements depend on technique, calibration, oxygen/ventilator settings, altitude, timing, and the patient population in which a rule was validated. A single calculated value should be interpreted with symptoms, examination, serial measurements, imaging, and the clinical question that prompted the test.