Acute kidney injury: acute tubular necrosis vs. prerenal uraemia

_ Pre-renal uraemia ('azotemia') _ Acute tubular necrosis| Urine sodium _ < 20 mmol/L _ > 40 mmol/L| Urine osmolality _ > 500 mOsm/kg _ < 350 mOsm/kg| Fractional sodium excretion* _ < 1% _ > 1%| Response to fluid challenge _ Good _ Poor| Serum urea:creatinine ratio _ Raised _ Normal | Fractional urea excretion** _ < 35% _ >35%| Urine:plasma osmolality _ > 1.5 _ < 1.1| Urine:plasma urea _ > 10:1 _ < 8:1| Specific gravity _ > 1020 _ < 1010| Urine _ Normal/ 'bland' sediment _ Brown granular casts|

Prerenal uraemia - kidneys hold on to sodium to preserve volume

*fractional sodium excretion = (urine sodium/plasma sodium) / (urine creatinine/plasma creatinine) x 100

**fractional urea excretion = (urine urea /blood urea ) / (urine creatinine/plasma creatinine) x 100