Fractional Excretion of Sodium (FENa) Calculator
Calculate the fractional excretion of sodium to differentiate prerenal from intrinsic renal causes of acute kidney injury.
Medical Disclaimer
This calculator is for informational and educational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider before making decisions about your health. Never disregard professional medical advice or delay seeking it because of results from this tool.
Inputs
FENa <1% (urine Na typically <20): prerenal azotemia; FENa >2% (urine Na >40): ATN/intrinsic renal disease
Normal Na: 136–145 mEq/L; hyponatremia <136; hypernatremia >145; critical <120 or >160 mEq/L
Results
FENa
0.21%
Classification
1(1=Prerenal, 2=Indeterminate, 3=Intrinsic)
How to Use This Calculator
- Enter urine sodium, plasma sodium, urine creatinine, and plasma creatinine (all in mEq/L or mg/dL).
- Review FENa (%) — < 1% suggests pre-renal azotemia; > 2% suggests intrinsic renal or post-renal cause.
- FENa is unreliable in patients on diuretics — use FEUrea in that setting.
How the result changes with Urine Sodium
| Urine Sodium | FENa | Classification |
|---|---|---|
| 30 | 0.32% | 1(1=Prerenal, 2=Indeterminate, 3=Intrinsic) |
| 105 | 1.13% | 2(1=Prerenal, 2=Indeterminate, 3=Intrinsic) |
| 195 | 2.09% | 3(1=Prerenal, 2=Indeterminate, 3=Intrinsic) |
| 270 | 2.89% | 3(1=Prerenal, 2=Indeterminate, 3=Intrinsic) |
What each input means
- Urine Sodium
- Urine sodium concentration in mEq/L. FENa interpretation: <1% = prerenal (low urine Na expected, often <20 mEq/L); >2% = intrinsic renal (ATN); 1–2% = indeterminate. FENa is unreliable with diuretics — use FEUrea instead.
- Plasma Sodium
- Serum/plasma sodium concentration in mEq/L. Normal range: 136–145 mEq/L. Hyponatremia: <136; hypernatremia: >145. Critical values: <120 or >160 mEq/L require urgent correction.
- Urine Creatinine
- Urine creatinine concentration in mg/dL.
- Plasma Creatinine
- Serum/plasma creatinine concentration in mg/dL.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersUrine Sodium = 20, Plasma Sodium = 140, Urine Creatinine = 80, Plasma Creatinine = 1.2 = 4 input(s) provided
- Calculate FENaFENa0.214 = 0.214%
- Calculate ClassificationClassification1 = 1
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