FENa & FEUrea Calculator — Fractional Excretion

Enter Urine & Serum Values

Sodium

Urine Sodium (mEq/L or mmol/L)

Units: mEq/L = mmol/L (equivalent for sodium)

Plasma Sodium (mEq/L or mmol/L)

Normal: 135–145 mEq/L

Creatinine (mg/dL)

Urine Creatinine

Enter in mg/dL. Both creatinine values must use the same unit.

Plasma/Serum Creatinine

Enter in mg/dL. Creatinine units cancel in the FENa ratio.

Urea (mmol/L)

Urine Urea

Enter in mmol/L. Both urea values must use the same unit.

Plasma/Serum Urea (BUN)

Enter in mmol/L. BUN in mg/dL ÷ 2.8 = mmol/L

Results

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FENa (%)

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FEUrea (%)

Formulae

FENa (%) = (U_Na / P_Na) × (P_Cr / U_Cr) × 100
FEUrea (%) = (U_Urea / P_Urea) × (P_Cr / U_Cr) × 100

U = Urine | P = Plasma/Serum | Na = Sodium | Cr = Creatinine
Note: creatinine units cancel in the ratio, so mg/dL or μmol/L may be used — but both urine and plasma creatinine must be in the same unit.
Similarly, both urea values must be in the same unit (mmol/L or mg/dL).

FENa Interpretation

FENa	Interpretation	Mechanism
< 1%	Pre-renal AKI	Avid sodium reabsorption — intact tubular function responding to reduced effective circulating volume
1 – 2%	Indeterminate / Mixed	May represent early ATN, contrast nephropathy, hepatorenal syndrome, or diuretic effect
> 2%	Intrinsic Renal (ATN)	Tubular damage impairs sodium reabsorption; sodium spills into urine

FEUrea Interpretation

FEUrea	Interpretation
< 35%	Pre-renal AKI — avid urea reabsorption consistent with volume depletion or reduced renal perfusion
≥ 35%	Intrinsic Renal AKI (ATN) — tubular dysfunction reduces urea reabsorption capacity

When to Use FEUrea Instead of FENa

Critical Clinical Note: FENa is unreliable in patients receiving diuretics. Diuretics promote urinary sodium excretion, artificially elevating FENa even in pre-renal states. Use FEUrea in these patients — urea reabsorption is not significantly affected by loop or thiazide diuretics.

Conditions where FENa may be falsely elevated (>1%) despite pre-renal physiology:

Diuretic use: Loop diuretics (furosemide), thiazides — most common cause of false-positive FENa
Contrast-induced nephropathy (CIN): Early CIN may show FENa <1% despite tubular injury
Myoglobinuria / rhabdomyolysis: Myoglobin inhibits tubular sodium reabsorption; FENa may exceed 1% with pre-renal physiology
Acute glomerulonephritis: Sodium retention may occur despite intrinsic renal disease
Hepatorenal syndrome: FENa typically very low (<1%) despite severe renal impairment — resembles pre-renal
Early ATN / partial obstruction: FENa may be in the indeterminate range (1–2%)

Clinical Application

FENa and FEUrea are used in the evaluation of acute kidney injury (AKI) to guide initial differentiation and management:

Pre-renal AKI: Respond to volume resuscitation; no further tubular damage if treated promptly
ATN: Require supportive care, avoidance of nephrotoxins, and potential renal replacement therapy; volume loading alone is insufficient and may be harmful
Combined use: Using both FENa and FEUrea together increases diagnostic accuracy, particularly in ambiguous cases or when diuretics complicate interpretation
Limitations: Both indices are static measurements and do not capture dynamic changes in renal function; interpret alongside urine osmolality, urine microscopy (muddy brown casts suggest ATN), and clinical context

References

Espinel CH. The FENa test: use in the differential diagnosis of acute renal failure. JAMA. 1976;236(6):579–581.
Carvounis CP, Nisar S, Guro-Razuman S. Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure. Kidney Int. 2002;62(6):2223–2229.
Miller TR, Anderson RJ, Linas SL, et al. Urinary diagnostic indices in acute renal failure: a prospective study. Ann Intern Med. 1978;89(1):47–50.
Perazella MA, Coca SG. Traditional urinary biomarkers in the assessment of hospital-acquired AKI. Clin J Am Soc Nephrol. 2012;7(1):167–174.

FENa & FEUrea