If the fractional excretion of sodium (FEna) is less than 1, what does this indicate?

When the fractional excretion of sodium (FEna) is less than 1%, it primarily indicates prerenal disease. This finding suggests that the kidneys are conserving sodium in response to a decrease in renal perfusion or effective blood volume, which is often seen in conditions like dehydration, heart failure, or shock. In prerenal azotemia, the kidneys retain sodium to maintain intravascular volume and blood pressure, leading to a FEna less than 1% as the body strives to limit sodium loss.

In contrast, higher FEna values typically suggest intrinsic renal damage or intrarenal disease, where the kidneys may not properly reabsorb sodium due to damaged tubules. In postrenal disease, the FEna can be variable, depending on the extent and duration of the obstruction.

Therefore, a FEna of less than 1% is specifically indicative of the kidney's response to prerenal factors, emphasizing the importance of differentiating types of acute kidney injury based on such metrics.