This Light criteria calculator can diagnose pleural effusions as exudates and provide the differentiation criteria from pleural transudates. Discover more about the rules involved, the score interpretation and the difference between these two types of pleural probes below the form.

Penetrating mechanism of injury

Total serum protein:*
Pleural fluid protein:*

LDH parameters

Serum LDH:*
Pleural fluid LDH:*
Upper limit of normal serum LDH (U/L):*

How does this Light criteria for pleural effusion calculator work?

This is a health tool that aims to help in the differential diagnosis of pleural exudates vs. transudates based on checking whether any of the Light criteria has been met. Pleural fluid determinations are analyzed along with serum protein and LDH (Lactate Hydrogenase).

This Light criteria for pleural effusion calculator divides the two input categories, two fields for protein parameters and three fields for LDH parameters, as follows:

Protein parameters

■ Total serum protein, measured in g/dL, the chemical test measuring proteins in plasma, meaning albumin and globulin. Helps in the determination of the cause of fluid collection in the lungs such as in cases of pulmonary edema. Normal range varies between 6.4-8.3 g/dL. 

■ Pleural fluid protein, measured in g/dL, normal values are between 1-2 g/dL, with no more than 2% protein.

LDH parameters

■ Serum Lactate dehydrogenase, measured in U/L, enzyme found in organs and tissues helping with metabolic processes, converting lactate to pyruvate.

■ Pleural fluid LDH, measured in U/L, should not be higher than ½ of plasma and allows the determination of the cause of pleural effusions.

■ Upper limit of normal serum LDH, measured in U/L and default set at 220 but customizable according to the specified determinations, usually between 200 and 300.

The only criticism of the model is that it only has a specificity of 83% due to the fact that it doesn’t count for serum and albumin pleural levels in detail and so around 25% of transudates are misinterpreted as exudates because it doesn’t differentiate for conditions that can produce transudate effusions in specific cases.

Light criteria and score interpretation

Once the five above fields have been completed, the calculator will check whether any of the three basic Light criteria has been met. In order for the diagnosis to be put as an exudate the probe should satisfy either of the following:

1. Pleural fluid protein / Total serum protein  >0.5;

2. Pleural fluid LDH / serum LDH >0.6;

3. Pleural fluid LDH >(2/3 *upper limit of normal serum LDH).

If the criteria has been met, the probe can usually be considered an exudate. The following investigational steps includeglucose, ADA, cell count, cytology and even pH and culture. If none of the criteria is met, the probe is likely to be a transudate.

Exudate vs. Transudate

With the help of the Light criteria, transudative (pressure filtration without capillary injury) and exudative (inflammatory fluid between the cells) pleural effusions are differentiated through the lactate dehydrogenase and protein levels in the fluid.

Normal pleural fluid characteristics:

■ Clear ultrafiltrate of plasma from the parietal pleura;

■ Less than 2% protein content (1-2 g/dL);

■ Lactate dehydrogenase (LDH) less than 1/2 of plasma;

■ Less than 1000WBC per mm3;

■ pH of 7.60-7.64.

Thoracentesis provides the imagistic means of analysis of the fluid, microscopic examination and quantification of cellular content. There are several types of pleural effusions based on their content and the mechanism of entry in the pleural cavity:

■ hydrothorax (serous fluid);

■ hemothorax (blood);

■ pyothorax (pus);

■ urinothorax (urine);

■ chylothorax (chyle).

Amongst the causes of fluid accumulating in the pleural space that impairs breathing in the case of exudative fluid are bacterial pneumonia, viral infections, PE, cancer (e.g. lung cancer, breast cancer, lymphoma).For transudate fluid, most common causes are congestive heart failure and cirrhosis with ascites.

The following table contains some of the differences between the two pleural effusion mechanisms:

Type Transudate Exudate
Aspect clear clouded
Gravity <1.012 >1.020
Protein content <2.5 g/dL >2.9 g/dL
Fluid/serum protein <0.5 >0.5
Albumin difference >1.2 g/dL <1.2 g/dL
Cholesterol content <45 mg/dL >45 mg/dL
Fluid LDH upper limit for serum <0.6 or <2/3 >0.6 or >2/3


1) Light RW. (2002) Clinical practice. Pleural effusion. N Engl J Med; 346(25):1971-7.

2) Paramothayan NS,  Barron J. (2002) New criteria for the differentiation between transudates and exudates. J Clin Pathol; 55(1): 69–71.

11 Sep, 2015