E. Urinary sodium – Explanation
Acute renal failure: acute tubular necrosis vs. Prerenal uraemia
Prerenal uraemia – kidneys hold on to sodium to preserve volume
| Pre-renal uraemia | Acute tubular necrosis | |
| Urine sodium | < 20 mmol/L | > 30 mmol/L |
| Fractional sodium excretion* | < 1% | > 1% |
| Fractional urea excretion** | < 35% | >35% |
| Urine:plasma osmolality | > 1.5 | < 1.1 |
| Urine:plasma urea | > 10:1 | < 8:1 |
| Specific gravity | > 1020 | < 1010 |
| Urine | ‘bland’ sediment | brown granular casts |
| Response to fluid challenge | Yes | No |
*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
