Urinary Sodium/Potassium Ratio in Acute Kidney Injury Accurately Differentiates Prerenal Azotemia from Acute Tubular Necrosis

Abstract Objective : To develop a more accurate, cost effective, non-invasive test to differentiate between pre-renal renal failure (PRA) and acute tubular necrosis (ATN) in acute kidney injury (AKI). Methods : Urine sodium/potassium (Na/K) ratios were compared with fractional excretion of sodium (FeNa) and renal failure index (RFI) as well as other commonly used indices to differentiate patients with PRA from ATN. Patients with a rise in serum creatinine > 0.5 mg/d identified from medical records for a six- to eighteen-month period, were reviewed and categorized either as PRA or ATN based on presenting findings, course in hospital or renal biopsy. All patients had urinary sodium and potassium, creatinine, and serum creatinine done. Results : The Na/K was < 1 in PRA and > 1 in ATN, correctly identifying all 42 cases of PRA and all 28 patients with ATN. The FeNa was >1 and misdiagnosed 9 of 42 patients with PRA and was >1 and correctly diagnosed all patients with ATN. The RFI was >1 and misdiagnosed 11 of 42 patients with PRA but was >1 and correctly diagnosed all patients with ATN. The BUN/creatinine ratio, urine sodium concentration and U/P creatinine ratio all had a very poor correlation with the correct diagnosis. Conclusion : The Na/K ratio correctly diagnosed all 42 cases of PRA and all 28 cases of ATN. It is easy to do, is cost effective, non-invasive, and is useful for following patients with PRA to see if and when they develop ATN.