The fractional excretion of urea in the differential diagnosis of prerenal failure and acute tubular necrosis in neonates.

BACKGROUND

Acute renal failure (ARF) in a newborn is a common problem. Fractional excretion of sodium (FENa) has been used to distinguish between the two main causes of ARF, prerenal failure and acute tubular necrosis (ATN). However, the clinical usefulness of FENa could be limited by furosemide diuretic that are commonly prescribed inARF patients. In contrast, urea is not reabsorbed significantly in the distal nephron, thus the fractional excretion of urea (FE UN) should not be affected by furosemide.

OBJECTIVE

To test the hypothesis that FE UN is not effected by furosemide and useful in differentiating between prerenal failure and ATN.

METHODS

Neonates admitted to the Department of Pediatrics, Thammasat University Hospital from August 2007-May 2009 were studies prospectively for ARF which is defined as urine output < 0.5 ml/kg/hr after the 1st day and serum creatinine > 1.5 mg/dl with normal maternal renal function. FENa and FEUN were performed on the initial time of diagnosis and were repeated on two consecutive days.

RESULTS

Neonates with ARF were classified as prerenal failure (n=38) and ATN (n=5). The prerenal failure neonates were divided into two groups: those prerenal failure without furosemide (n=27), those prerenal failure with furosemide (n=11). The FENa at the initial time of diagnosis and the two consecutive days in prerenal failure neonates (0.33 +/- 0.57, 10.1 +/- 2.73, 0.8 +/- 1.32%, respectively) were lower than ATN neonates (4.74 +/- 6.12, 5.05 +/- 4.03, 3.98 +/- 2.47%, respectively) significantly. Both FENa and FE UN were no statistical difference between the two prerenal failure groups and ATN neonates.

CONCLUSIONS

A FE Na in prerenal failure is significantly lower than ATN. A FE UN has no benefit in distinguishing between prerenal failure and ATN. Furosemide has no effect on both FENa and FE UN.