FENa Calculator
Calculate fractional excretion of sodium (FENa) from urine and serum values to help assess kidney function.
How is this calculated?
Formula:
FENa = (UNa × SCr) / (SNa × UCr) × 100
Interpretation:
- < 1% → Prerenal Azotemia
- 1% – 2% → Indeterminate / Gray Zone
- > 2% → Intrinsic Renal Failure (ATN)
This tool is for educational and clinical decision support only. It is not a substitute for professional clinical judgment.
What Is Fractional Excretion of Sodium (FENa)?
Fractional excretion of sodium (FENa) is a calculation used to evaluate the cause of acute kidney injury (AKI). It measures the percentage of filtered sodium that is excreted in the urine, helping clinicians distinguish between prerenal azotemia (reduced blood flow to the kidneys) and intrinsic renal damage, such as acute tubular necrosis (ATN).
A low FENa suggests the kidneys are retaining sodium appropriately in response to decreased perfusion, while a high FENa indicates impaired tubular reabsorption, a hallmark of intrinsic kidney injury.
How the FENa Calculation Works
The FENa formula compares sodium and creatinine concentrations in both urine and serum:
FENa (%) = (Urine Sodium × Serum Creatinine) / (Serum Sodium × Urine Creatinine) × 100
This ratio normalizes sodium excretion against creatinine excretion, accounting for variations in urine concentration. The result is expressed as a percentage of the filtered sodium load that appears in the final urine.
Key Assumptions
- The patient has stable kidney function at the time of measurement.
- Spot urine samples are used; the calculation assumes they reflect steady-state conditions.
- Diuretic use can alter sodium handling and may affect interpretation.
How to Use the FENa Calculator
- Enter the urine sodium concentration (mEq/L or mmol/L).
- Enter the serum sodium concentration (mEq/L or mmol/L).
- Enter the urine creatinine concentration (mg/dL or µmol/L).
- Enter the serum creatinine concentration (mg/dL or µmol/L).
- The calculator will automatically compute the FENa percentage.
Ensure all units are consistent. Most clinical labs report sodium in mEq/L and creatinine in mg/dL. If using different units, convert before entering values.
Interpreting FENa Results
FENa values are typically interpreted within the context of acute kidney injury:
| FENa Value | Interpretation |
|---|---|
| < 1% | Suggests prerenal azotemia (kidneys are conserving sodium appropriately). |
| 1% – 2% | Borderline; may require additional clinical context or alternative indices. |
| > 2% | Suggests intrinsic renal injury, most commonly acute tubular necrosis (ATN). |
These thresholds are general guidelines. Interpretation should always consider the full clinical picture, including medication history, volume status, and other lab findings.
Limitations of FENa
FENa is a useful diagnostic tool but has important limitations:
- Diuretic use: Loop diuretics increase sodium excretion, potentially elevating FENa even in prerenal states.
- Chronic kidney disease: Baseline tubular dysfunction can alter sodium handling, making FENa less reliable.
- Early AKI: FENa may be misleading if measured very early in the course of injury before tubular function is fully impaired.
- Contrast-induced nephropathy: FENa may remain low despite intrinsic injury in some cases.
In situations where FENa is unreliable, alternative indices such as fractional excretion of urea (FEUrea) may provide better diagnostic accuracy.
Practical Use Cases
- Emergency department evaluation: Rapid differentiation of prerenal AKI from ATN in patients presenting with elevated creatinine.
- ICU monitoring: Serial FENa measurements can help track the progression or resolution of acute kidney injury.
- Pre-renal vs. intrinsic AKI: FENa is most valuable when the clinical history is unclear and volume status is difficult to assess.
FAQ
What is a normal FENa value?
In healthy individuals with normal kidney function and adequate hydration, FENa is typically less than 1%. This reflects efficient sodium reabsorption by the renal tubules.
Can FENa be used in patients on diuretics?
FENa is less reliable in patients taking diuretics, especially loop diuretics, because these medications directly increase sodium excretion. In such cases, fractional excretion of urea (FEUrea) may be a better alternative.
What is the difference between FENa and FEUrea?
FENa measures sodium excretion, while FEUrea measures urea excretion. FEUrea is less affected by diuretic use and may be more accurate in distinguishing prerenal from intrinsic AKI when FENa is unreliable.
Does FENa work for chronic kidney disease?
FENa is less useful in chronic kidney disease because baseline tubular function is often impaired, leading to altered sodium handling. It is primarily designed for evaluating acute changes in kidney function.
What units should I use for the FENa calculation?
Sodium should be entered in mEq/L (or mmol/L, which is numerically equivalent). Creatinine should be entered in mg/dL. If your lab reports creatinine in µmol/L, convert to mg/dL by dividing by 88.4 before entering the value.