Pleural Effusion Light’s Criteria

Helps with the differential diagnosis between pleural effusions of exudate and transudate type.

In the text below the calculator there is more information on the criteria, its interpretation and more differences between exudative and transudative effusions.

The Light’s criteria calculator helps with pleural effusion diagnosis. It consists of five criteria, the combination of which can diagnose exudative and rule out transudative effusion.

There are two types of parameters involved: protein (from total serum and pleural fluid) and LDH (serum and pleural fluid lactate dehydrogenase and upper limit of normal serum LDH).

When either of the three is satisfied, diagnosis of exudative pleural effusion is likely:

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).


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)
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Steps on how to print your input & results:

1. Fill in the calculator/tool with your values and/or your answer choices and press Calculate.

2. Then you can click on the Print button to open a PDF in a separate window with the inputs and results. You can further save the PDF or print it.

Please note that once you have closed the PDF you need to click on the Calculate button before you try opening it again, otherwise the input and/or results may not appear in the pdf.


Light’s criteria explained

The Light’s criteria consists in an evaluation of protein and LDH parameters to see whether at least one of three rules is being met. If so, the sample provided is likely to be a pleural exudate.

The five parameters to be input are described in the table below:

Protein parameters
Total serum protein Measures total albumin and globulin. This offers information on the cause of fluid collection (i.e. in pulmonary edema).
Normal values: 6.4 - 8.3 g/dL.
Pleural fluid protein No more than 2% of fluid should be protein.
LDH parameters
Serum Lactate dehydrogenase Enzyme found in organs and tissues, which helps with metabolic processes by converting lactate to pyruvate.
Pleural fluid LDH Should not be higher than ½ of plasma. This offers information on the cause of pleural effusion.
Upper limit of normal serum LDH Is set default at 220 U/L.

The main limitation of the model is the fact that it does not account for serum and pleural levels of albumin, thus it only reaches a specificity of 83%.

It was found that actually, 25% of determined exudates are in fact transudates.



There are three rules in the Light’s criteria:

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

2. Pleural fluid LDH / serum LDH >0.6;

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

The parameters input are checked against the above three rules by performing the necessary determinations. For positive exudative effusion diagnosis, at least one of the three rules should be satisfied.

If so, the next investigations should be: glucose, ADA, cytology, cell count, pH and in some cases culture.

If none of the rules is met, the sample is likely to be a transudate, the other type of pleural effusion.


About the study

The Light criteria was created in 1972 following a study on pleural fluid from 150 patients. The examination consisted in: pleural fluid cell counts, protein levels and lactic dehydrogenase.

The criteria was associated with more than 70% of exudates but in some cases, diagnosis was missed.

The following table introduces the sensitivities and specificities of the various criteria:

Light’s Criteria Sensitivity (%) Specificity (%)
Light’s Criteria, 1 or more of the following: 98 83
Pleural fluid protein / Serum protein >0.5 86 84
Pleural fluid LDH / Serum LDH >0.6 90 82
Pleural fluid LDH > 2/3 x Serum LDH upper limit of normal 82 89

Richard W. Light, the creator of the criteria, is a professor in the Division of Allergy, Pulmonary, and Critical Care Medicine at Vanderbilt University School of Medicine.


Pleural exudate vs. transudate

Depending on the type of fluid and the mechanism of entry in the pleural cavity, there is hydrothorax (serous fluid), hemothorax (blood), pyothorax (pus), urinothorax (urine) or chylothorax (chyle).

The characteristics of normal pleural fluid are:

■ Clear ultrafiltrate of plasma from the parietal pleura;

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

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

■ Less than 1000WBC per mm3;

■ pH of 7.60 - 7.64.

There are two types of pleural effusion, differentiated through the levels of lactate dehydrogenase and protein in the pleural fluid:

■ Transudates are defined as pressure filtration without capillary injury;

■ Exudates are defined as inflammatory fluid between the cells.

The table below shows the differences between the two:

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

Transudative effusions occur due to imbalances between the hydrostatic and oncotic pressure. Most common causes include congestive heart failure and ascites from cirrhosis.

Exudative effusions are caused by bacterial pneumonia, viral infections, pulmonary embolism or malignancy of several types (lung, breast or lymphoma).


Original source

Light RW, Macgregor MI, Luchsinger PC, Ball WC Jr. Pleural effusions: the diagnostic separation of transudates and exudates. Ann Intern Med. 1972; 77(4):507-13.


Light RW, Erozan YS, Ball WC Jr. Cells in pleural fluid. Their value in differential diagnosis. Arch Intern Med. 1973; 132(6):854-60.

Other references

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

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

3. Ferreiro L, Toubes ME, Valdés L. Contribution of pleural fluid analysis to the diagnosis of pleural effusion. Med Clin (Barc). 2015; 145(4):171-7.

Specialty: Pulmonology

System: Respiratory

Objective: Diagnosis

Type: Criteria

No. Of Criteria: 5

Year Of Study: 1972

Article By: Denise Nedea

Published On: June 1, 2017

Last Checked: June 1, 2017

Next Review: June 1, 2023