Expansion
Below 1 per cent suggests prerenal; above 2 per cent suggests tubular injury
Expansion
FENa = (urine sodium x plasma creatinine) / (plasma sodium x urine creatinine) x 100
| FENa | Interpretation |
|---|---|
| Under 1 per cent | Prerenal: tubules intact and avidly conserving sodium |
| Over 2 per cent | Intrinsic (acute tubular necrosis): damaged tubules cannot reabsorb sodium |
Supporting features of a prerenal picture: urine osmolality above 500 mOsm/kg, urine sodium under 20 mmol/l, a urea to creatinine ratio above 100:1, and bland urinary sediment.
Exceptions matter:
- Diuretics invalidate the test entirely
- A low FENa can occur in contrast nephropathy, rhabdomyolysis, early sepsis, hepatorenal syndrome and glomerulonephritis, all of which involve intense vasoconstriction
- Chronic kidney disease raises baseline FENa
When a diuretic has been given, the fractional excretion of urea is more reliable, with a value under 35 per cent suggesting a prerenal state.