What is the urine sodium level in prerenal failure?

In prerenal failure, the kidneys respond to decreased perfusion by conserving sodium in an effort to maintain blood volume and pressure. This adaptive mechanism leads to a low urine sodium level. Typically, in cases of prerenal failure, the urine sodium concentration is less than 20 mEq/L.

This occurs because the body is attempting to retain as much sodium and water as possible to counteract the perceived low circulating volume due to factors like dehydration, heart failure, or any process that reduces renal perfusion. A higher urine sodium level would suggest that the kidneys are not adequately conserving sodium, which is more characteristic of intrinsic renal failure or post-renal obstructive processes where the kidneys lose their ability to concentrate urine effectively. The correct understanding of urine sodium levels in prerenal failure is essential for distinguishing between the different types of acute kidney injury during clinical assessments.