🫁 Light's Criteria Calculator

Light's criteria separate pleural effusions into exudates (local pleural disease) and transudates (systemic pressure or protein imbalance). An effusion is an exudate if any one of three is true: pleural-to-serum protein ratio above 0.5, pleural-to-serum LDH ratio above 0.6, or pleural LDH above two-thirds of the serum upper normal limit. Published by Richard Light in 1972, the criteria remain the first branch point in every pleural effusion workup.

Enter pleural and serum protein, pleural and serum LDH, and your lab's serum LDH upper normal. The calculator evaluates all three criteria and classifies the fluid, which decides whether to hunt for infection and malignancy or to treat heart failure, cirrhosis, or nephrosis.

How to Use This Calculator

Get the pleural fluid protein and LDH from the thoracentesis results and the same-day serum protein and LDH. Enter your laboratory's serum LDH upper reference limit. Read which criteria are met and the classification. Exudates need further fluid analysis (pH, glucose, cell count, cultures, cytology); transudates usually need treatment of the underlying systemic cause.

The Three Criteria

Protein and LDH leak into the pleural space when the pleura itself is diseased (infection, tumor, infarction), raising pleural levels toward or above serum levels: that is an exudate. When the pleura is healthy but hydrostatic pressure rises (heart failure) or oncotic pressure falls (cirrhosis, nephrotic syndrome), protein-poor fluid is pushed across: a transudate. The "any one" rule gives the criteria about 98% sensitivity for exudates at the cost of some specificity. The classic trap is diuretic-treated heart failure, which concentrates pleural protein and LDH and misclassifies a transudate as an exudate; the serum-to-pleural albumin gradient (>1.2 g/dL suggests transudate) rescues those cases.

Worked Example

Pleural protein 4.2 over serum 6.8 gives a ratio of 0.62 (>0.5 met); pleural LDH 420 over serum 220 gives 1.91 (>0.6 met); and 420 against two-thirds of the 280 upper limit gives 420 vs 187 (met). Result: EXUDATE, with 3 of 3 Light criteria met. One criterion is enough, and here all three point the same way, toward infection, malignancy, or embolism workup.

What Exudate vs Transudate Changes

Exudate means the pleura is the problem: send the fluid for pH, glucose, cell differential, Gram stain, cultures, and cytology, and consider chest imaging for pneumonia, malignancy, or pulmonary embolism. Transudate means the pleura is innocent: optimize heart failure treatment, evaluate liver and kidney function, and the effusion often resolves without further pleural procedures. Borderline cases, especially after diuretics, deserve the albumin gradient check before committing to an invasive workup.

Clinical Context Required

Light's criteria classify fluid; they do not diagnose the cause. An exudate still needs the full workup, and a transudate in a patient with fever and chest pain still needs clinical judgment. Thoracentesis itself carries risks including pneumothorax and is performed by trained clinicians with ultrasound guidance. For informational purposes only, not medical advice. Consult a healthcare professional.

Related tools: LRINEC score and mean arterial pressure.

For informational purposes only, not medical advice. Consult a healthcare professional.