Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure.

Fractional excretion of sodium (FE_Na) has been used in the diagnosis of acute renal failure (ARF) to distinguish between the two main causes of ARF, prerenal state and acute tubular necrosis (ATN). However, many patients with prerenal disorders receive diuretics, which decrease sodium reabsorption and thus increase FE_Na. In contrast, the fractional excretion of urea nitrogen (FE_UN) is primarily dependent on passive forces and is therefore less influenced by diuretic therapy.

To test the hypothesis that FE_UN might be more useful in evaluating ARF, we prospectively compared FE_UN with FE_Na during 102 episodes of ARF due to either prerenal azotemia or ATN.

Patients were divided into three groups: those with prerenal azotemia (N = 50), those with prerenal azotemia treated with diuretics (N = 27), and those with ATN (N = 25). FE_Na was low only in the patients with untreated plain prerenal azotemia while it was high in both the prerenal with diuretics and the ATN groups. FE_UN was essentially identical in the two pre-renal groups (27.9 ± 2.4% vs. 24.5 ± 2.3%), and very different from the FE_UN found in ATN (58.6 ± 3.6%, P < 0.0001). While 92% of the patients with prerenal azotemia had a FE_Na <1%, only 48% of those patients with prerenal and diuretic therapy had such a low FE_Na. By contrast 89% of this latter group had a FE_UN <35%.

Low FE_UN (≤35%) was found to be a more sensitive and specific index than FE_Na in differentiating between ARF due to prerenal azotemia and that due to ATN, especially if diuretics have been administered.