Pre-Renal Uraemia vs ATN: Urinary Indices

Key points

• Urine sodium: pre-renal < 20 mmol/L, ATN > 40 mmol/L
• Urine osmolality: pre-renal > 500 mOsm/kg, ATN < 350 mOsm/kg
• Fractional sodium excretion: pre-renal < 1%, ATN > 1%
• Response to fluid challenge: pre-renal good, ATN poor
• Rationale: in pre-renal uraemia the kidneys are structurally normal and actively conserve sodium/water to protect circulating volume