The Comparative Benefits of the Fractional Excretion of Urea and Sodium in Various Azotemic Oliguric States

Background: The fractional excretion of urea (FeUrea) may result in more reliable in the determination of renal function than sodium in the presence of oliguric azotemia; however, its usefulness remains controversial, perhaps due to an evolving understanding of urea transport within the kidney. Methods: This was a prospective observational study of 100 consecutive patients referred to the nephrology service for azotemic oliguria. Multiple clinical variables were analyzed to determine variables responsible for the differences between the FeUrea and fractional excretion of sodium (FeNa) in the ability to distinguish pre-renal azotemia from intrinsic renal disease. Results: Overall, the FeUrea was more accurate (95 vs. 54%, p < 0.0001), yet both tests accurately detected the presence of intrinsic renal disease (FeNa 75%, FeUrea 85%, p = NS). The FeUrea performed significantly better (98 to 49%, p < 0.0001) in detecting pre-renal azotemia, and that advantage came exclusively in patients taking diuretics (p < 0.0001); however, 4/5 cases incorrectly detected by the FeUrea were correctly detected by the FeNa. All 4 cases had infection. Conclusion: The FeUrea appears more accurate in patients receiving diuretics; however, the FeNa may have an advantage in patients with infection. Copyright (C) 2009 S. Karger AG, Basel