The FeNa calculator is a powerful clinical tool used to assess the cause of acute kidney injury (AKI). By calculating the fractional excretion of sodium (FeNa), clinicians can rapidly differentiate between prerenal and intrinsic renal causes of AKI.
In this guide, you’ll learn how to calculate FeNa, interpret results accurately, and confidently use this calculation in clinical practice.
What is the FeNa Calculator?
The FeNa calculator determines the fractional excretion of sodium, which estimates the percentage of filtered sodium excreted in the urine.
This FeNa calculation helps differentiate:
- Prerenal AKI → due to hypoperfusion (e.g., dehydration, heart failure)
- Intrinsic AKI → usually acute tubular necrosis (ATN)
It’s a quick, bedside tool that guides management decisions in AKI patients.
How to Calculate FeNa: Formula and Step-by-Step
To calculate FeNa, you need 4 lab values:
- Urine Sodium (UNa) in mEq/L
- Plasma Sodium (PNa) in mEq/L
- Urine Creatinine (UCr) in mg/dL
- Plasma Creatinine (PCr) in mg/dL
FeNa Calculation Formula
FeNa (%)=(PNa×UCr)(UNa×PCr)×100
Example FeNa Calculation
If
Urine Na = 20 mEq/L
Plasma Na = 140 mEq/L
Urine Cr = 100 mg/dL
Plasma Cr = 2 mg/dL
FeNa=(140×100)(20×2)×100=0.29%
This indicates prerenal AKI (explained next).
FeNa Interpretation: How to Analyze Results
Understanding FeNa interpretation is key to applying this test in real life.
| FeNa Values | Interpretation | Likely Cause |
|---|---|---|
| < 1% | Sodium retention → Prerenal AKI | Hypovolemia, low perfusion |
| > 2% | Sodium wasting → Intrinsic AKI | Acute Tubular Necrosis (ATN) |
| 1–2% | Indeterminate (mixed causes) | Consider other tests |
Tip: In patients on diuretics, FeUrea is often more reliable.
FeNa Normal Range
Technically, there’s no “normal range” — FeNa values are interpreted in the context of AKI:
- < 1% → Suggests prerenal AKI
- > 2% → Suggests intrinsic (ATN) AKI
In healthy individuals, FeNa varies widely and is not used routinely outside AKI settings.
FeNa Calculator vs MdCalc: Which One Should You Use?
Many clinicians use the FeNa calculator mdcalc online tool for convenience.
Our FeNa calculator provides the same formula-based accuracy — designed for fast bedside application or desktop/mobile use.
Why use our FeNa calculator?
- Fast & user-friendly
- Accurate FeNa calculation
- Instant interpretation (prerenal vs intrinsic)
- Mobile-optimized
- No login required
FeNa in Prerenal AKI: Key Insights
A FeNa < 1% suggests prerenal AKI, where the kidneys conserve sodium to maintain perfusion.
Common causes of prerenal FeNa low:
- Dehydration
- Heart failure
- Cirrhosis
- Hypovolemia
Immediate management = restore volume (fluids).
FeNa in AKI: When is it Most Useful?
Use the FeNa calculator in AKI patients when you need to:
- Differentiate prerenal vs intrinsic AKI
- Decide whether IV fluids or other interventions are needed
- Rule out acute tubular necrosis (ATN)
FAQs: FeNa Calculator & FeNa Calculation
Q1: What is the purpose of FeNa calculation?
To differentiate prerenal vs intrinsic AKI by assessing sodium handling.
Q2: Can I use FeNa in patients on diuretics?
Caution — diuretics can falsely elevate FeNa. Consider FeUrea instead.
Q3: What are the FeNa values indicating prerenal AKI?
FeNa < 1% strongly suggests prerenal AKI.
Q4: How reliable is FeNa calculator mdcalc?
It’s reliable — but our FeNa calculator offers the same functionality with faster interpretation.
Conclusion: Master AKI Diagnosis With FeNa Calculator
The FeNa calculator remains a trusted, fast diagnostic tool in nephrology and critical care.
By knowing how to calculate FeNa, interpret FeNa values, and apply results in AKI, you can dramatically boost diagnostic accuracy and improve patient outcomes.
Ready to assess renal function fast?
👉 Use our FeNa calculator now and make confident clinical decisions!