Fractional excretion of urea in the follow-up of acute renal failure due to prerenal azotemia

Fractional excretion of urea (FEU) is a major issue to discriminate between prerenal azotemia and acute tubular necrosis in acute renal failure (ARF). Its role in the course of ARF remains unclear. The aim of this study was to evaluate FEU in the follow-up of ARF due to prerenal azotemia in order to predict the necessity of renal replacement therapy (RRT).

The prospective study took place at the ICU of Stadtspital Waid, Zürich. All patients admitted starting from 19 February 2006 were evaluated for ARF according to the RIFLE classification. ARF due to prerenal azotemia was defined as ARF combined with FEU of less than or equal to 35%. FEU was calculated as [(urine urea/blood urea)/(urine creatinine/plasma creatinine)] × 100. Urine specimens were taken and FEU was calculated daily until complete or partial renal recovery was reached or the criteria for RRT were met. The goal of therapy was reconstitution of renal function by treatment of the underlying condition. RRT was initiated according to the usual criteria. Statistics were determined using Fisher's exact test.

By 7 December 2006, 15 patients met the inclusion criteria for ARF due to prerenal azotemia (nine males, six females). The mean age was 71 ± 11 (SD) years for male patients and 58 ± 31 years for female patients. Twelve out of the 15 patients responded to conservative management and had complete or partial renal recovery. Three patients needed RRT. Two of them refused RRT and died during the course of the disease. During the first 48 hours after initiation of conservative therapy, FEU remains less than or equal to 35% in all three patients who needed RRT. By contrast, nine out of 12 patients in whom renal function recovered without RRT showed a FEU of more than 35% within the first 48 hours (P < 0.05) (Figure 1).

Figure 1

Fractional excretion of urea (FEU) in the follow-up of acute renal failure (ARF) due to prerenal azotemia. Data presented as mean ± SD. n = number of patients.

In patients presenting with ARF due to prerenal azotemia, an increase of FEU above 35% within the first 48 hours after initiation of conservative therapy for ARF is a valuable parameter to predict renal recovery. After initiation of conservative therapy, measurement of FEU is of no value concerning discrimination of prerenal azotemia and acute tubular necrosis in ARF.