The renal failure index measures sodium conservation and concentrating ability. It is chiefly of use in oliguric patients for the early differentiation of prerenal azotemia from acute tubular necrosis.
Specimen Collection: Specimens should be obtained before onset of treatment.
renal failure index (RFI) in mEq/L =
= (urine sodium in mEq/L) / ((urine creatinine in mg/dL) / (plasma creatinine in mg/dL))
RFI
Interpretation
<= 1
prerenal azotemia
1.01 to 2.99
less definitive, but usually indicates tubular necrosis
>= 3
acute tubular necrosis
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