Utility of fractional excretion of urea in the differential diagnosis of acute kidney injury in children

Introduction: The fractional excretion of sodium (FE_Na) has been used as an index for the differential diagnosis of acute tubular necrosis (ATN) and prerenal acute kidney injury (AKI). The reliability of this index, however, decreases with the use of the diuretic agent furosemide. The fractional excretion of urea nitrogen (FE_UN) has been shown to be useful in such settings in adults. The objective of this study was to examine whether FE_UN is also useful in these settings in children. Methods: We assessed 102 episodes of AKI in 74 children, classifying these into three groups based on history, physical examination, urine examination and subsequent clinical course: (1) prerenal AKI without furosemide (N = 37), (2) prerenal AKI with furosemide (N = 32) and (3) ATN (N = 33). Results: Of the 37 prerenal AKI episodes without furosemide, 35 showed low FE_Na of <1 %, with an overall average of 0.35 ± 0.11 %, whereas prerenal AKI with furosemide (1.63 ± 0.37 %) and ATN (8.76 ± 2.11 %) were associated with a higher FE_Na. FE_UN in the clinical setting of prerenal AKI was lower than that in ATN (27.9 ± 2.1 vs. 51.6 ± 3.8 %, respectively) and, in contrast to FE_Na, not significantly different between the categories of prerenal AKI with and without furosemide (29.2 ± 3.1 vs. 25.1 ± 2.9, respectively). The sensitivity of FE_UN <35 % was 75 % in prerenal AKI with furosemide, whereas that of FE_Na was 53 %. Conclusions: FE_UN is useful in detecting prerenal AKI in children administered furosemide.