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Urinary indices - Renal Failure Index (RFI) and Fractional excretion of sodium (FE-Na)


Urine Sodium (UNa): meq/L
Plasma Sodium (PNa): meq/L
Plasma creatinine (PCr): mg/dl
Urine Creatinine (UCr): mg/dl

Background / References:


Suggestive of prerenal azotemia or glomerulonephritis
Urinary Na+ <20 mEq/L
Urine/plasma Cr ratio >30
Renal failure index (RFI) <1
urine/plasma osmolality ratio: >1.4
Fe-Na < 1%

Suggestive of ATN or postrenal azotemia:
Urinary Na+ >40 mEq/L
Urine/plasma Creat ratio <20
Renal failure index (RFI) >1
urine/plasma osmolality ratio: 1.0
Fe-Na > 2-3%


RFI = Urine_Na * Plasma_Cr / Urine_Cr

Fractional excretion of sodium (FE-Na) =
100 * ((Urine_Na in mEq/L) / (plasma_Na in mEq/L)) / ((Urine Cr in mg/dL) / (plasma Cr in mg/dL))

(FE-Na < 1%) seen in: pre-renal azotemia and acute glomerulonephritis. Also seen in 5-10% of nonoliguric or x-ray contrast induced ATN, and early sepsis.

(FE-Na > 1% ) Seen in most cases of ATN. Also seen in pre-existing chronic renal failure, and after diuretic administration.


Durakovic Z, Durakovic A, Durakovic S. The lack of clinical value of laboratory parameters in predicting outcome in acute renal failure. Ren Fail 1989-90;11(4):213-9. 

Porush JG. New concepts in acute renal failure. Am Fam Physician 1986 Mar;33(3):109-18. 

Myers BD, Hilberman M, Spencer RJ, Jamison RL. Glomerular and tubular function in non-oliguric acute renal failure. Am J Med 1982 Apr; 72(4): 642-9.

Miller TR, Anderson RJ, Linas SL, Henrich WL, Berns AS, Gabow PA, Schrier RW. Urinary diagnostic indices in acute renal failure: a prospective study. Ann Intern Med 1978 Jul;89(1):47-50.


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