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Last updated: June 4, 2026

FENa Calculator

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FENa Calculator — Inputs, Formula, and Result Interpretation

I am Nina Calhoun. This tool computes the Fractional Excretion of Sodium (FENa) from paired urine and plasma measurements to support renal function assessment. Results are for estimation and planning; not a diagnosis.

Quick Start

  • Enter urine sodium (mEq/L), plasma sodium (mEq/L), urine creatinine (mg/dL), and plasma creatinine (mg/dL).
  • All inputs must be positive and in the stated units.
  • Press Calculate to get FENa as a percent (%), rounded to two decimals.

How It Works / Inputs → Output

  • Inputs: Urine Na, Plasma Na, Urine Cr, Plasma Cr.
  • Output: FENa (%) = fraction of filtered sodium excreted in urine.
  • Unit consistency matters; do not mix mmol/L with mEq/L unless equivalent for sodium (they are numerically equal for Na), and use mg/dL specifically for creatinine as shown.

Formula / Method

Equation (per spec):

fena_percent = 100 * (urine_sodium * plasma_creatinine) / (plasma_sodium * urine_creatinine)

Variables:

  • urine_sodium: urine sodium concentration (mEq/L)
  • plasma_sodium: plasma sodium concentration (mEq/L)
  • urine_creatinine: urine creatinine (mg/dL)
  • plasma_creatinine: plasma creatinine (mg/dL)

Worked Example

Given: urine Na = 50 mEq/L; plasma Na = 140 mEq/L; urine Cr = 100 mg/dL; plasma Cr = 2 mg/dL.

fena_percent = 100 * (50 * 2) / (140 * 100)
             = 100 * 100 / 14,000
             = 100 * 0.007142857...
             = 0.71 %

Result: 0.71% (displayed as 0.71 with en-US formatting).

Applications / Use Cases

  • Differentiate prerenal azotemia vs intrinsic renal causes in acute settings when diuretics are not confounding.
  • Track changes in sodium handling alongside creatinine during fluid and medication adjustments.

Assumptions & Limitations

  • Intended for adult use; pediatric values may vary with maturation and collection accuracy.
  • Diuretics, advanced CKD, contrast exposure, and acute tubular injury can alter FENa and reduce specificity.
  • Sampling should be near-simultaneous; time gaps increase error.
  • Lab variability and urine collection errors can shift results; avoid rounding inputs prematurely.

Tips / Common Mistakes

  • Do not mix units (e.g., µmol/L for creatinine) without proper conversion.
  • Check for diluted urine samples; very low urine creatinine inflates FENa.
  • Avoid using FENa after recent diuretic dosing; consider alternative indices in that scenario.
  • Recompute if any input is zero or negative; the calculator requires positive values.

Note: FENa is a supportive metric. Clinical judgment and full context are required.

Frequently Asked Questions

What does FENa measure?

FENa estimates the percentage of filtered sodium excreted in urine, reflecting renal tubular sodium handling.

What inputs do I need?

Urine sodium (mEq/L), plasma sodium (mEq/L), urine creatinine (mg/dL), and plasma creatinine (mg/dL).

What is the exact formula used here?

fena_percent = 100 * (urine_sodium * plasma_creatinine) / (plasma_sodium * urine_creatinine).

Can I enter mmol/L for sodium?

For sodium, mEq/L and mmol/L are numerically equivalent; enter the numeric value as shown in mEq/L.

How precise is the result?

The tool displays up to two decimals; real-world variability arises from sampling timing and lab assay error.

When should I avoid interpreting FENa?

Be cautious with diuretic use, advanced CKD, contrast injury, or non-simultaneous samples; results may be misleading.

Does this calculator provide medical advice?

No. It offers an estimate for planning and education; clinical decisions require professional evaluation.

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