D. Prerenal uraemia – Explanation
ATN or prerenal uraemia? In prerenal uraemia think of the kidneys holding on to sodium to preserve volume
The low urine sodium points towards prerenal uraemia, as does the urine:plasma osmolality and
urea ratio
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
