FENa calculator

Author: Henrick Yau
- 1 October 2026
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Calculators

Calculate the Fractional Excretion of Sodium (FENa) to help diagnose the cause of acute kidney injury (AKI). This calculator assists physicians, nephrologists, and medical professionals in differentiating between prerenal and intrinsic renal causes of AKI.

Patient Laboratory Values

Normal range: 135-145 mEq/L
Spot urine concentration
Normal range: 0.7-1.3 mg/dL
Spot urine concentration

Patient Information

About Fractional Excretion of Sodium (FENa)

The Fractional Excretion of Sodium (FENa) is a calculation used to determine what fraction of the sodium filtered by the kidneys is excreted in the urine. It's particularly useful in the differential diagnosis of acute kidney injury (AKI).

Clinical Significance
  • FENa < 1%: Typically indicates prerenal azotemia, suggesting that the kidneys are functioning normally but receiving inadequate blood flow. Common causes include volume depletion, heart failure, or liver failure.
  • FENa > 2%: Suggests intrinsic renal failure such as acute tubular necrosis (ATN), where the kidney tubules are damaged and unable to reabsorb sodium properly.
  • FENa 1-2%: May be indeterminate and requires clinical correlation.
Limitations and Considerations
  • Diuretics: Can artificially increase FENa by promoting sodium excretion, potentially leading to misinterpretation.
  • Chronic Kidney Disease: Patients with CKD may have altered baseline FENa values.
  • Other Factors: Certain conditions like glomerulonephritis, advanced age, and sepsis can affect FENa interpretation.
  • Timing: Most valuable when calculated early in the course of AKI before significant adaptation occurs.
Alternative Calculations
  • Fractional Excretion of Urea (FEUrea): May be more reliable in patients on diuretics.
  • Urine Sodium Concentration: Values <20 mEq/L suggest prerenal azotemia, while values >40 mEq/L suggest intrinsic renal failure.
  • Renal Failure Index (RFI): Another calculation used to differentiate causes of AKI.

Disclaimer: This calculator is designed to assist healthcare professionals and should not replace clinical judgment. The interpretation of FENa values must be considered in the context of the patient's complete clinical picture, including history, physical examination, and other laboratory findings. Always consult with a nephrologist or appropriate specialist for complex cases.

What the FENa calculator does for AKI assessment

The FENa Calculator is a clinical tool that helps assess the cause of acute kidney injury (AKI). By analysing key laboratory values, it estimates the Fractional Excretion of Sodium (FENa), which helps medical professionals distinguish between prerenal azotaemia and intrinsic renal failure such as acute tubular necrosis.

Much like a GFR Calculator estimates kidney function or a Corrected Calcium Calculator adjusts for protein levels, this calculator interprets how the kidneys handle sodium to guide treatment decisions.

FENa (%) = [(Urine Sodium ร— Plasma Creatinine) / (Plasma Sodium ร— Urine Creatinine)] ร— 100

Enter sodium, creatinine, age and diuretic use

  • Enter the patientโ€™s serum sodium and urine sodium values in mEq/L.
  • Input serum creatinine and urine creatinine in mg/dL.
  • Add the patientโ€™s age and select a relevant clinical scenario (e.g., acute kidney injury, CKD).
  • If the patient is on diuretics, tick the appropriate box to account for their effect on sodium excretion.
  • Click โ€œCalculate FENaโ€ to see the results and interpretation.

Why quantify fractional excretion of sodium

This calculator provides valuable insights for managing kidney-related conditions. Here's why it is useful:

  • Rapid clinical insight: Quickly categorise the cause of AKI as prerenal or intrinsic.
  • Supports diagnosis: Works alongside tools like the Mentzer Index Calculator or Body Surface Area Calculator for a broader clinical picture.
  • Improves accuracy: Reduces the risk of misdiagnosis by quantifying how the kidneys handle sodium.
  • Contextual interpretation: Offers guidance based on factors such as age, clinical scenario, and diuretic use.

Clinical Interpretation

  • FENa < 1%: Suggests prerenal azotaemia โ€“ the kidneys are conserving sodium due to low perfusion (e.g., dehydration or heart failure).
  • FENa > 2%: Indicates intrinsic renal failure such as acute tubular necrosis where sodium reabsorption is impaired.
  • FENa 1โ€“2%: An indeterminate range โ€“ requires additional clinical data.
  • On diuretics: FENa may be unreliable. In such cases, consider using the Fractional Excretion of Urea instead.

Questions about FENa and kidney failure

Can this calculator be used if the patient is on diuretics?

Yes, but with caution. Diuretics increase sodium excretion and can make the FENa misleading. The calculator includes an option to indicate diuretic use, and it will adjust the interpretation accordingly.

What if the FENa result is between 1โ€“2%?

This is considered a grey zone. You may need to rely on other data, such as urine output, history, and other tests like a GFR estimation or urinalysis.

Is this tool only useful in acute kidney injury?

FENa is most useful in AKI but may have some role in other contexts. Always consider the broader clinical picture, similar to how a Child-Pugh Calculator helps in liver assessment or a Blood Pressure Calculator helps monitor hypertension.

Are there alternatives to FENa?

Yes. Alternatives include:

  • Fractional Excretion of Urea (FEUrea): More reliable in patients on diuretics.
  • Urine Sodium Concentration: Can also hint at prerenal or intrinsic causes.
  • Renal Failure Index (RFI): Another option to analyse kidney function.

Clinical disclaimer for this diagnostic tool

This calculator is intended for use by healthcare professionals. It does not replace clinical judgment or comprehensive patient evaluation. The FENa value should be interpreted alongside other factors, including patient history, physical findings, and additional lab data.