Pleural Effusions: transudate vs exudate

An important part of evaluating a pleural effusion includes categorising the fluid as a Transudate or an Exudate. We use Light's Criteria to guide us here. If one or more of the following is positive, then we are dealing with an exudate:

1. pleural fluid protein/serum protein is greater than 0.5

2. pleural fluid LDH/serum LDH is greater than 0.6

3. Pleural fluid LDH is more than two-thirds of the serum normal upper

Exudate: these include infections (bacterial, viral, tuberculous, fungal and parasitic). Remember tuberculous infectious will have predominantly lymphocytes. Malignancy is an unfortunate cause of an exudative pleural effusion and may be seen in primary lung tumors or metastatic disease such as breast cancer. It is also common in lymphoma. Gastrointestinal causes include pancreatitis and esophageal rupture. Exudative pleural effusions are seen in Connective Tissue Diseases like pleuritis in lupus, rheumatoid pleuracy, and more rarely Churg Strauss syndrome and Wegeners Granulomatosis. A pulmonary embolism can cause an an exudative or transudative effusion however exudative effusions are more common. Other things to consider include chylothorax and drug-induced exudative effusions (amiodarone, nitrofurantoin).

Transudate: These are most commonly seen in in patients with congestive heart failure. Cirrhotic patients often have right-sided transudative effusions. The nephrotic syndrome, pulmonary embolism, and rarely constrictive pericarditis are other causes.

Here is a great diagnositic approach to pleural effusions

A link for complicated pleural effusions - thanks to the CMR at Toronto General Hospital.

Here is the NEJM video on how to perform a thoracentesis