What is the urine sodium level in postrenal failure?

In postrenal failure, the urine sodium level is typically greater than 40 mEq/L. This condition arises when there is an obstruction in the urinary tract that prevents urine from properly draining, causing a buildup of waste products in the blood. When the kidney function is compromised due to this obstruction, the kidneys often respond by excreting more sodium in an attempt to maintain proper blood volume and pressure.

As a result, the urine sodium concentration increases, often exceeding 40 mEq/L. This helps to differentiate postrenal failure from prerenal failure, where urine sodium levels are usually less than 20 mEq/L due to the kidneys' attempts to conserve sodium in the face of decreased perfusion. The understanding of these levels is critical for diagnosing the underlying cause of acute kidney injury and guiding subsequent management.