FENa Calculator – Fractional Excretion of Sodium
Calculate fractional excretion of sodium (FENa) to differentiate prerenal azotemia from intrinsic acute kidney injury using urine and serum values.
Enter urine sodium, serum sodium, urine creatinine, and serum creatinine to compute FENa and interpret acute kidney injury etiology.
FENa Calculator – Fractional Excretion of Sodium
Calculate fractional excretion of sodium (FENa) to differentiate prerenal azotemia from intrinsic acute kidney injury using urine and serum values.
FENa calculation examples
These examples illustrate prerenal, indeterminate, and intrinsic patterns.
| Lab values | FENa | Interpretation |
|---|---|---|
| UNa 15, SNa 140, UCr 200, SCr 2.5 mg/dL | FENa = 0.13% | Prerenal azotemia — avid sodium retention due to low renal perfusion. |
| UNa 80, SNa 138, UCr 60, SCr 3.0 mg/dL | FENa = 2.90% | Intrinsic renal injury (ATN) — tubular damage impairs sodium reabsorption. |
| UNa 30, SNa 140, UCr 100, SCr 2.0 mg/dL | FENa = 0.43% | Prerenal — borderline but still below 1% threshold, likely volume depletion. |
| UNa 50, SNa 135, UCr 80, SCr 2.5 mg/dL | FENa = 1.16% | Indeterminate zone — clinical context required; consider diuretic use or early ATN. |
About the FENa Calculator
Fractional Excretion of Sodium (FENa) is one of the most clinically useful formulas in nephrology and critical care medicine. It quantifies the proportion of filtered sodium that is excreted in the urine, expressed as a percentage. Because healthy kidneys can reabsorb more than 99% of the sodium filtered by the glomerulus, FENa provides a sensitive window into tubular function and sodium handling — both of which change dramatically in different forms of acute kidney injury (AKI).
The formula is: FENa (%) = (Urine Sodium × Serum Creatinine) / (Serum Sodium × Urine Creatinine) × 100. Using the creatinine ratio in the denominator normalizes for urine concentration and effectively compares sodium excretion to the simultaneous creatinine excretion as a surrogate for glomerular filtration. This elegant ratio requires only a spot urine sample and a simultaneous blood draw — no timed urine collection is needed.
The standard thresholds are: FENa < 1% indicates prerenal azotemia, 1–2% is the indeterminate zone, and > 2% suggests intrinsic renal injury (most commonly acute tubular necrosis, ATN). In prerenal states — dehydration, hemorrhage, heart failure, cirrhosis, or any condition reducing effective arterial blood volume — the kidneys respond by maximally retaining sodium, driving FENa well below 1%. In ATN, tubular cell damage impairs sodium reabsorption, causing FENa to rise above 2%.
Several important caveats limit the universal application of FENa. Diuretics, particularly loop diuretics, inhibit tubular sodium reabsorption and will falsely elevate FENa even in prerenal states. In such patients the Fractional Excretion of Urea (FEUrea) is a more reliable marker since urea handling is less directly affected by diuretics. Additionally, certain forms of intrinsic AKI — contrast-induced nephropathy, early myoglobinuric AKI, and some cases of glomerulonephritis — may preserve tubular sodium reabsorption and present with FENa below 1% despite tubular injury. Chronic kidney disease and normal aging reduce the kidney's ability to lower FENa maximally, so the 1% threshold may not apply to all patients.
FENa should always be interpreted in the context of the full clinical picture: the patient's volume status, urine output, urine microscopy, urinalysis, medication history, and the trajectory of creatinine. This calculator provides the mathematical result and standard interpretation to support clinical decision-making; it does not replace bedside assessment or specialist consultation.
How to use the FENa Calculator
- Collect a spot urine sample and a simultaneous blood sample — no timed urine collection is necessary.
- Enter the urine sodium and serum sodium values in mEq/L.
- Enter the urine creatinine and serum creatinine values in mg/dL.
- Click Calculate FENa to obtain the percentage and the standard clinical interpretation.
- Integrate the FENa result with the full clinical picture including volume status, urine microscopy, and medication history before reaching a diagnostic conclusion.
FENa Calculator FAQ
What does FENa measure?
FENa measures the fraction of sodium filtered by the glomerulus that is ultimately excreted in the urine. A very low FENa (< 1%) means the kidneys are retaining almost all filtered sodium, suggesting prerenal azotemia. A high FENa (> 2%) means the tubules are failing to reabsorb sodium normally, suggesting tubular injury.
Why is FENa < 1% in prerenal azotemia?
In prerenal states, reduced renal perfusion activates the renin-angiotensin-aldosterone system (RAAS) and the sympathetic nervous system, both of which powerfully stimulate tubular sodium reabsorption. The kidney attempts to restore circulating volume by retaining as much sodium — and with it water — as possible, driving FENa below 1% and often below 0.1%.
Can FENa be unreliable?
Yes. Loop and thiazide diuretics block tubular sodium transport and raise FENa even in prerenal states, making the test misleading when diuretics have been given. Contrast-induced nephropathy, myoglobinuric AKI, and some forms of acute glomerulonephritis can produce FENa < 1% despite tubular injury. In these scenarios, Fractional Excretion of Urea (FEUrea) or FE of uric acid may be more reliable.
What is the difference between FENa and FEUrea?
FENa uses sodium as the marker, while FEUrea uses urea nitrogen. Urea reabsorption is less directly affected by loop diuretics, making FEUrea useful when the patient has received diuretics. The FEUrea threshold for prerenal AKI is typically < 35% rather than the 1% used for FENa. Both can give overlapping results, so clinical judgment remains essential.
What units should I use for this calculator?
Urine sodium and serum sodium should be entered in milliequivalents per liter (mEq/L). Urine creatinine and serum creatinine should both be in the same units — this calculator uses mg/dL for both. If your lab reports creatinine in µmol/L, divide by 88.4 to convert to mg/dL.
Is FENa used in chronic kidney disease?
FENa is less reliable in chronic kidney disease (CKD) because the adapted nephrons in CKD may have altered baseline sodium handling. Patients with advanced CKD can have FENa > 2% at baseline without acute tubular injury. In CKD, clinical context, trend in creatinine, and urine microscopy become even more important than the FENa value alone.