Fractional Excretion of Sodium & Transtubular Potassium Excretion
Serum (PNa): mmol/L
Urinary (UNa): mmol/L
Serum (PCr): mg/dl
Urinary (UCr): mg/dl
Fractional Excretion   
of Sodium (FENa) =
Renal Failure Index (RFI) = %
Serum (PK): mmol/L
Urinary (UK): mmol/L
Serum (POsm): mOsm/L
Urinary (UOsm): mOsm/L
Transtubular Potassium   
Gradient (TTKG) =

Fractional Excretion of Sodium (FENa) = (UNa x PCr) / (PNa x UCr) x 100

Renal Failure Index (RFI) = (UNa x PCr) / UCr

Transtubular Potassium Gradient (TTKG) = (UK x Posm) / (PK x Uosm)

Interpretation of the Urinary Indices

  Prerenal ATN Postrenal
UNa (mmol/L) < 20 > 40 > 40
FENa < 1% > 1% > 4%
RFI < 1% > 1% > 1%

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