Acute tubular necrosis is a kidney disorder involving damage to the tubule cells of the kidneys, resulting in acute kidney failure. ATN presents with AcuteRenal Failure(ARF) and is one of the most common causes of ARF. This online calculator helps you to calculate the ARF index. Formula: RFI = UNa x SCr/UCr Where, RFI = Renal Failure Index UNa = Urine Sodium in mEq/L SCr = Serum Creatinine in mg/dL UCr = Urine Creatinine in mg/dL Acute Tubular Necrosis is classified as a renal cause of ARF. Free online Medical Calculators.