What is the typical laboratory indication for pre-renal azotemia?

Pre-renal azotemia typically occurs when there is a decrease in renal perfusion, leading to inadequate blood flow to the kidneys without intrinsic kidney damage. In this context, the characteristic laboratory finding is an elevated blood urea nitrogen (BUN) level coupled with a low fractional excretion of sodium (FeNa).

Elevated BUN occurs in pre-renal azotemia due to factors that increase urea reabsorption, such as low intravascular volume, leading to dehydration or heart failure. The kidneys attempt to compensate for reduced blood flow by conserving sodium; this is reflected in a low FeNa, which indicates that the kidneys are functioning appropriately in response to the perceived hypovolemia. Therefore, this combination of elevated BUN and low FeNa is a typical laboratory indication of pre-renal azotemia.

This understanding of kidney function and response is essential because it helps differentiate pre-renal causes from intrinsic renal causes of azotemia, which would not typically show such low sodium excretion.