Formula
FENa (%) = (urine sodium × plasma creatinine) ÷ (plasma sodium × urine creatinine) × 100.
Fractional excretion of sodium estimates the proportion of filtered sodium that appears in urine. The creatinine ratio adjusts a spot urine sodium concentration for concentration or dilution of urine. All four measurements are needed: urine sodium alone is not a FENa result. This calculator allows the two creatinine samples to use different units, converts them to mg/dL internally, and then evaluates the ratio. Sodium values use the same selected unit; mEq/L and mmol/L are numerically equal for sodium.
Worked example
Using urine sodium 20 mEq/L, plasma sodium 140 mEq/L, plasma creatinine 1 mg/dL, and urine creatinine 100 mg/dL: FENa = (20 × 1) ÷ (140 × 100) × 100 = 0.142857%, displayed as 0.14%.
How to use this calculator
- Use paired urine and plasma measurements collected at a clinically appropriate time.
- Enter both sodium concentrations in the selected sodium unit.
- Enter each creatinine concentration and select its own unit. Check that urine and plasma values are not swapped.
- Select Calculate and read the percentage together with the limitations below.
When not to use this
Do not use FENa alone to determine the cause of an acute change in kidney function. Diuretics can increase sodium excretion; chronic kidney disease, sepsis, obstruction, and several intrinsic kidney disorders can also weaken conventional interpretations. Timing relative to fluids and other interventions matters. The original study involved a selected group of patients, so its observations do not transfer unchanged to every setting. FENa is particularly limited when measurements do not represent the same clinical state.
Reference ranges
| Measure | Context |
|---|---|
| Below about 1% | Often seen in prerenal states |
| About 1–2% | Substantial overlap; no standalone conclusion |
| Above about 2% | Often seen in intrinsic renal injury |
These categories are reference only and are not a diagnostic rule. The conventional greater-than-2% guide shown here is not the same as the greater-than-3% value described in Espinel’s original study. A systematic review found reduced diagnostic performance in populations with chronic kidney disease or diuretic exposure. Fractional excretion of urea, or FEUrea, is another measure sometimes considered in clinical assessment; it also has limitations and is not calculated here. This tool provides no treatment or fluid recommendations.
References
- Espinel CH. The FENa test. Use in the differential diagnosis of acute renal failure. JAMA. 1976;236:579–581.
- Abdelhafez M et al. Diagnostic Performance of Fractional Excretion of Sodium for the Differential Diagnosis of Acute Kidney Injury: A Systematic Review and Meta-Analysis. Clin J Am Soc Nephrol. 2022.
Frequently asked questions
Is a low FENa a diagnosis?
No. A low value describes sodium handling under the sampling conditions. It does not independently establish a cause or exclude intrinsic injury.
Can creatinine units differ between samples?
Yes, if each unit is selected correctly. The calculator uses 1 mg/dL = 88.4 µmol/L before evaluating the ratio.
What happens with a zero denominator?
Plasma sodium and urine creatinine must be greater than zero. Invalid entries are rejected rather than displaying infinity or a misleading percentage.
Why do diuretics matter?
They can alter urinary sodium excretion, making conventional FENa categories less reliable. The calculator cannot adjust for their effect.
Is FEUrea always a better alternative?
No. FEUrea has its own confounders. Choosing and interpreting either measure requires clinical context beyond this arithmetic tool.
Related calculators
Clinical reviewer: [REVIEWER NAME AND CREDENTIALS]
Last reviewed: [DATE — complete after qualified clinical review]. Clinical review is pending.