How it's calculated
FENa (%) = (Urine sodium × Plasma creatinine) ÷ (Plasma sodium × Urine creatinine) × 100. As long as urine and plasma creatinine are both entered in the same units (e.g. both mg/dL), and urine/plasma sodium in the same units, the ratio is unit-independent.
Interpretation
| FENa | Interpretation |
|---|---|
| < 1% | Suggests prerenal cause (volume depletion, poor renal perfusion) |
| 1 – 2% | Indeterminate |
| > 2% | Suggests intrinsic renal cause (e.g. acute tubular necrosis) |
Clinical use
- FENa helps distinguish prerenal azotaemia (where the kidneys are appropriately retaining sodium in response to poor perfusion) from intrinsic acute kidney injury (where tubular damage impairs sodium reabsorption).
- FENa is not valid if the patient has received diuretics recently — diuretics increase urinary sodium excretion regardless of the underlying cause, invalidating the test. Use fractional excretion of urea (FEUrea) instead in that situation.
- A low FENa in established (rather than early) acute tubular necrosis can still occur in some cases (particularly contrast-induced or pigment nephropathy), so a low value doesn't completely exclude intrinsic injury.
- FENa should be interpreted alongside the full clinical picture (history, examination, urinalysis, and trend in kidney function) rather than in isolation.
Frequently asked questions
What is FENa used for?
It helps distinguish prerenal causes of acute kidney injury (like dehydration or heart failure reducing kidney perfusion) from intrinsic causes (like acute tubular necrosis), which are managed very differently.
Why doesn't FENa work if the patient has taken a diuretic?
Diuretics directly increase urinary sodium excretion regardless of the underlying cause of kidney injury, which artificially raises FENa and makes the result unreliable — fractional excretion of urea (FEUrea) is used instead in patients on diuretics.
What does a FENa below 1% mean?
It suggests a prerenal cause of acute kidney injury — the kidneys are functioning normally but retaining sodium appropriately in response to reduced blood flow or volume, as would happen with dehydration or heart failure.
Can FENa be falsely low in intrinsic kidney injury?
Yes — in some causes of intrinsic AKI, particularly early contrast-induced nephropathy or pigment nephropathy (rhabdomyolysis), FENa can be misleadingly low, so it should be interpreted alongside the full clinical picture rather than in isolation.
References
- Espinel CH. The FENa test. Use in the differential diagnosis of acute renal failure. JAMA. 1976;236:579-581.
- Carvounis CP, et al. Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure. Kidney Int. 2002;62:2223-2229.
- KDIGO Clinical Practice Guideline for Acute Kidney Injury. Kidney Int Suppl. 2012;2:1-138.