Light’s Criteria Calculator (pleural effusion: exudate or transudate)

Understanding the Light's criteria calculator — concept and principle

Light's criteria sort a pleural effusion (fluid around the lung) into an exudate or a transudate using protein and LDH measured in the fluid and the blood. Doctors use the result to narrow the cause: exudates come from inflamed, infected or invaded pleura, while transudates come from pressure or protein imbalance.

Any one criterion = exudate: with the default inputs the result is 3 of 3 met (exudate).

How it is calculated

The fluid is an exudate if at least one of these is true:

Pleural fluid protein ÷ serum protein above 0.5
Pleural fluid LDH ÷ serum LDH above 0.6
Pleural fluid LDH above two-thirds of the upper limit of normal for serum LDH

If none is met, it is a transudate. The calculator shows how many criteria are met and each ratio. Protein units cancel, so g/dL or g/L both work if used for both values.

The principle

In a transudate the pleura is intact and fluid is pushed out by higher pressure in the vessels (heart failure) or pulled out by low blood protein (cirrhosis, nephrotic syndrome). Large molecules like protein and the enzyme LDH stay mostly in the blood, so the fluid has little of them. When the pleura is inflamed or invaded by infection or cancer, its capillaries leak and damaged cells release LDH, so protein and LDH in the fluid rise towards blood levels. Light and colleagues chose the cut-offs in 1972 to catch nearly all exudates.

Worked example

Default inputs: fluid protein 3.5, serum protein 6.8 g/dL, fluid LDH 180, serum LDH 250, upper limit of serum LDH 250 U/L.

Protein ratio = 3.5 ÷ 6.8 = 0.51 → above 0.5, met.
LDH ratio = 180 ÷ 250 = 0.72 → above 0.6, met.
Fluid LDH ÷ upper limit = 180 ÷ 250 = 0.72; two-thirds of 250 is 166.7, so met.
Criteria met = 3 → Exudate.

How to read the result

An exudate leads to further tests for infection, cancer, pulmonary embolism and inflammatory disease. A transudate points to heart, liver or kidney causes. The criteria are very sensitive but label about one in four heart-failure effusions as exudates, especially after diuretics; then a serum-minus-fluid albumin gradient above 1.2 g/dL supports a transudate.

Common mistakes

Using serum and fluid samples taken on different days.
Using the patient's serum LDH instead of the laboratory's upper limit for the third criterion.
Mixing protein units between fluid and serum.

Keep in mind: This is a decision aid for clinicians, not a diagnosis. The cause of an effusion is decided with the full clinical picture and further tests.

Frequently asked questions

Why are exudates over-diagnosed in heart failure?

Diuretics remove water from the effusion faster than protein, raising its protein and LDH.

Is one criterion enough for an exudate?

Yes. Meeting any one of the three classifies the fluid as an exudate.

What else is checked in pleural fluid?

Often cell count, glucose, pH, Gram stain and culture, and cytology for cancer cells.

Sources: 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

Checked 9 Nov 2026. 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 — the three criteria as standard values; albumin gradient 1.2 g/dL as a standard value.

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