1FENa Explained — Distinguishing AKI Types
FEUrea (%) = [(Urine Urea × Serum Cr) ÷ (Serum Urea × Urine Cr)] × 100
| Marker | <1% | 1–2% | >2% |
|---|---|---|---|
| FENa | Prerenal AKI | Indeterminate | Intrinsic renal / postrenal |
| FEUrea | <35% = Prerenal | 35–50% | >50% = Intrinsic renal |
When FENa Can Be Misleading
False low FENa (looks prerenal, but isn't):
- Early contrast nephropathy — tubules initially intact, FENa <1% for first 24 hours
- Myoglobinuria / haemoglobinuria — tubular toxin but early FENa low
- Early urinary obstruction
- Acute glomerulonephritis — sodium retention due to inflammation
False high FENa (looks intrinsic, but may be prerenal):
- Diuretics — loop and thiazide diuretics block tubular sodium reabsorption → use FEUrea instead
- Pre-existing CKD — tubular function already impaired
- Adrenal insufficiency
- Bicarbonaturia (high bicarbonate with sodium in urine)
AKI Management by Type
- Prerenal (FENa <1%): IV fluid resuscitation — 500 mL NS over 30 min (adult). Monitor urine output, creatinine. Identify and treat cause (bleeding, sepsis, heart failure, medications)
- Intrinsic renal (FENa >2%): Identify specific cause — ATN (most common), glomerulonephritis, interstitial nephritis, vasculitis. Nephrology referral. Avoid nephrotoxins. Optimise haemodynamics
- Postrenal (obstruction): FENa may be >2% or <1% early. Urinary ultrasound mandatory. Catheterise if urethral obstruction. Nephrostomy if ureteric obstruction
2Frequently asked questions
What is FENa and what does it measure?
Fractional Excretion of Sodium (FENa) measures what percentage of filtered sodium is excreted in the urine. Formula: FENa (%) = (Urine Na × Plasma Cr) / (Plasma Na × Urine Cr) × 100. It differentiates prerenal AKI (kidney conserving sodium) from intrinsic renal AKI (kidney unable to conserve sodium).
How is FENa interpreted?
FENa <1%: prerenal AKI — kidneys are functioning normally but underperfused; respond to volume resuscitation. FENa >2%: intrinsic renal AKI (ATN) — tubular damage prevents sodium reabsorption. FENa 1–2%: indeterminate — consider clinical context. In oliguric patients on diuretics, use FEUrea instead (FEUrea <35% = prerenal).
When is FENa unreliable?
FENa is unreliable in: patients on diuretics (artificially raises FENa even in prerenal states — use FEUrea instead), contrast nephropathy (can show low FENa despite ATN), myoglobinuria/haemoglobinuria, acute glomerulonephritis, and early urinary obstruction. Always interpret FENa alongside clinical context, urine microscopy, and response to fluids.
What is FEUrea and when is it used?
FEUrea = (Urine Urea × Plasma Cr) / (Plasma Urea × Urine Cr) × 100. FEUrea <35%: prerenal. FEUrea >50%: intrinsic renal (ATN). FEUrea is preferred over FENa in patients on diuretics because diuretics do not significantly affect urea reabsorption. It requires urine and plasma urea levels alongside creatinine.
What is the treatment of prerenal AKI?
Prerenal AKI: IV fluid resuscitation (crystalloid — Ringer's Lactate preferred over normal saline for large volumes), identify and treat the underlying cause (bleeding, sepsis, dehydration, cardiac failure, hepatorenal syndrome). Response to fluids (rising urine output, falling creatinine) confirms prerenal aetiology. Avoid nephrotoxins (NSAIDs, aminoglycosides, contrast) during AKI.
What urine findings suggest ATN vs prerenal AKI?
Prerenal AKI: urine osmolality >500 mOsm/kg, urine Na <20 mEq/L, FENa <1%, urine specific gravity >1.020, urine microscopy — normal or hyaline casts only. ATN (intrinsic): urine osmolality <350 mOsm/kg, urine Na >40 mEq/L, FENa >2%, urine specific gravity ~1.010, urine microscopy — muddy brown granular casts (pathognomonic of ATN).
Medical disclaimer: This calculator is for educational and clinical decision-support purposes only. It does not replace clinical judgment or specialist consultation. RxMedCalc is not liable for clinical decisions made solely on this tool.