Fractional excretion of urea as a diagnostic index in acute kidney injury in intensive care patients

Purpose: Acute kidney injury (AKI) is a dynamic process that evolves from an early reversible condition to an established disease. Value of urine indices in the event of AKI is uncertain in critically ill patients. The aim of this study was to evaluate the performance of fractional excretion of urea (FeU) for differentiating persistent from transient AKI in patients admitted to the intensive care unit.

Methods: This was an observational study. Forty-seven patients with AKI according to the RIFLE classification were included. Transient AKI was defined as AKI resolved within 3 days after inclusion. Persistent AKI was defined as persistent serum creatinine elevation or oliguria.

Results: Fractional excretion of urea was lower in case of transient, 33% (25-39), than persistent AKI, 47% (36-61) (P = .001). Areas under the receiver operating characteristic curve for FeU in case of transient AKI were better than those for other urinary indexes, 0.78 (95% confidence interval, 0.63-0.92). Optimal cutoff point according to the receiver operating characteristic curve was 40%. In patients treated with diuretics, FeU was the only predictive index of transient AKI. Fractional excretion of urea gradually increased from days 1 to 7 in transient AKI, whereas plasma creatinine decreased.

Conclusions: Fractional excretion of urea less than 40% was found to be a sensitive and specific index in differentiating transient from persistent AKI in intensive care unit patients especially if diuretics had been administered.