Tests of renal tubular function used to differentiate pre-renal from tubular injury (acute tubular necrosis) test urinary concentrating ability. The normal tubular response to pre-renal conditions is to concentrate the urine for the perceived dehydration, thus maximizing water reabsorption. Urine osmolality is concentrated to over 450 mOs/kg compared to normal serum levels of 280-300 mOs/kg. Prerenal syndromes are therefore indicated by urine-to-plasma osmolar ratios of greater than 1.5.
Tubular sodium handling can be evaluated by urine sodium which is actively reabsorbed in pre-renal states to therefore reabsorb water. In tubular damage sodium reabsorbing ability is lost. In pre-renal states urine sodium values are less than 20 mEq/L. Fractional Excretion of Sodium and Fractional Excretion of Urea Nitrogen examine sodium and urea clearance as a percentage of creatinine clearance. Excretion of both is decreased in pre-renal states (to <1% for FENa, and < 35% for FEUN).
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