Comparison between fractional excretion of sodium and fractional excretion of urea in differentiating prerenal from renal azotemia in circulatory shock

BackgroundFractional excretion of sodium (FENa) is used to differentiate renal from prerenal azotemia. However, many drugs and medical conditions affect the sodium (Na+) handling in the kidney. But the fractional excretion of urea (FEurea) is dependent on passive forces and is less influenced by the diuretic therapy.ObjectiveComparison between FENa and FEurea in differentiating renal from prerenal azotemia in circulatory shock, and the effect of diuretics on their handling.MethodsBoth FENa and FEurea were measured in 40 patients (pts) with AKI complicating circulatory shock. The pts were divided into 26 pts with prerenal (group-1) and 14 pts with renal azotemia (group-2). Group-1 was subdivided into 12 pts who did not receive diuretics 24 h before the sampling process (group-1a) and 14 pts who received diuretics (group-1b).ResultsCompared to patients with renal azotemia (group-2), those with prerenal azotemia (group-1) showed significantly lower FENa (0.99 ± 0.66 and 2.57 ± 1.73, P < 0.05) respectively, and significantly lower FEurea (29.7 ± 7.6 and 43.7 ± 15.4, P < 0.001) respectively. For differentiating renal from prerenal azotemia, compared to FENa, FEurea showed better sensitivity (78.1% vs. 71.4%) and specificity (88.5% vs. 69.4%) respectively. Moreover, FEurea was not significantly affected by the use of diuretics; sensitivity (78% vs. 78%) and specificity (92% vs. 88%) respectively, compared to pts who did not receive diuretics. On the other hand, compared to pts who did not receive diuretics, the use of diuretics significantly affected FENa; sensitivity (64% vs. 71%) and specificity (58% vs. 70%) respectively.ConclusionsFEurea is more sensitive, specific and less affected by the use of diuretics than FENa in differentiating renal from prerenal azotemia in patients with AKI complicating circulatory shock.