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

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

Background / References:

Analysis


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%

Equations


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.

References:


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.
Urinary Indices – Renal failure index / FENa