"The aetiology of urinothorax can be dichotomised as either due to obstructive uropathy or due to trauma (including iatrogenic post-surgical trauma) to the urinary system 1-3. Urinothoraces are most often seen alongside urinoma, whereby the urine is thought to traverse the diaphragm into the pleural space 3. Although the pathophysiology of this remains unclear, there are two leading theories: either urine travels through lymphatic drainage into the pleural space, or retroperitoneal urine moves into the peritoneal cavity and then travels directly into the pleural space via a direct transdiaphragmatic passage along a pressure gradient 3."
"The fluid in a urinothorax is usually a transudate but biochemically, often has a low pH and a high LDH and hence may be misclassified as being exudative as per Light's criteria 2-4. However, the most important biochemical feature is the pleural fluid creatinine-to-serum creatinine ratio which is >1, with an average of 1.09–19.80 1-3."