What change in urine would indicate a prerenal issue rather than an intrinsic problem?

A low urine sodium level is indicative of a prerenal issue, as it suggests that the kidneys are responding to decreased perfusion by conserving sodium. In cases of prerenal azotemia, the body is often attempting to maintain blood volume and pressure due to conditions such as dehydration or heart failure. When the kidneys sense low blood flow, they will reabsorb sodium effectively, resulting in lower levels of sodium in the urine.

In contrast, intrinsic renal problems, such as acute tubular necrosis or glomerulonephritis, often lead to increased urine sodium due to the kidneys' impaired ability to reabsorb sodium. Consequently, other factors such as increased proteinuria and hematuria are more characteristic of intrinsic issues, as they indicate damage or inflammation to the renal structures. High creatinine levels can occur in both prerenal and intrinsic issues, as they are both markers of renal function decline. However, the urine sodium level specifically highlights the kidneys' response to hemodynamic changes, making it a strong indicator of a prerenal problem.