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

Abstract

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.

Toward the optimal clinical use of the fraction excretion of solutes in oliguric azotemia

“…In a prospective study of 100 consecutive patients referred to our nephrology service for azotemic oliguria, we found that although the Fellrea is clearly related to the FeNa (Figure 4), the FeUrea was more accurate in patients treated with diuretics (p < 0.0001). 50,51 The FeNa, however, appeared to have an advantage in the presence of infection. Recent studies have shown that there is active transport of urea in the renal tubules.…”

“…More recently other solutes, including urea, uric acid, and lithium, have been investigated with the goal of improvement in the diagnostic ability to distinguish between PRA and ATN. [44][45][46][47][48][49][50][51][52][53][54] In an effort to evaluate these studies and the particular problems with each solute, we will briefly review the physiology of their renal handling in water deprivation states.…”

A Rat Model of Early Sepsis: Relationships between Gentamicin Pharmacokinetics and Systemic and Renal Effects of Bacterial Lipopolysaccharide Combined with Interleukin-2

“…The increased reabsorption of sodium, water and urea results in decreased values of FE Na and FE UN . 16,24,25) The gradual decrease in GFR observed in rats injected with LPSIL-2 during the treatment and follow-up periods was followed by a drop in FE UN during the latter period, which is a sign of a preserved tubular compensatory mechanism. In line with this, the renal histopathology did not reveal tubular necrosis or other significant sepsis-induced damage.…”

“…Experience with FE UN is, however, relatively limited. 8,24,25) Both indices increase in acute renal failure caused by tubular necrosis. Both renal hypoperfusion as well as septic hyperdynamic kidney injury cause a reduction in filtration pressure and GFR (in the latter case mainly due to efferent arteriolar vasodilatation).…”

Clearance and beyond: the complementary roles of GFR measurement and injury biomarkers in acute kidney injury (AKI)

“…RIFLE: risk, injury, failure, loss, end-stage. used to define prerenal AKI, although diuretics and sepsis make interpretation of the FE Na difficult (7) and aging and sepsis alter FE urea (31,75). While muddy brown or epithelial cell casts, renal tubular cells, and trace hematuria and pyuria are common in both septic and nonseptic AKI, and their presence in patients with an intermediate pretest probability of AKI is potentially helpful (88), these biomarkers are neither definitive of a type of AKI, nor ubiquitous in AKI (see Ref.…”