FENa & Urine Indices Calculator
Fractional excretion of sodium, urine anion gap, and renal failure index for AKI workup.
About this calculator
This calculator computes three classic bedside urine indices used to work up acute kidney injury (AKI): fractional excretion of sodium (FENa), urine anion gap, and the renal failure index (RFI). FENa comes from Dr. Carlos Espinel's formula, introduced in a 1976 JAMA paper on 17 patients with acute renal failure, and compares how much filtered sodium ends up in urine relative to how much filtered creatinine does; it is the most widely used index for distinguishing pre-renal azotemia (FENa under 1%, where the kidneys are avidly reabsorbing sodium in response to reduced perfusion) from intrinsic renal injury such as acute tubular necrosis (FENa over 2%, where tubular damage impairs sodium reabsorption); 1-2% is a mixed or indeterminate zone. The renal failure index (urine sodium divided by the urine-to-plasma creatinine ratio) draws the same pre-renal/intrinsic distinction using only urine sodium and the creatinine ratio, with a cutoff of 1 rather than a percentage.
Urine anion gap (urine sodium plus urine potassium minus urine chloride) is a separate tool used in non-anion-gap metabolic acidosis: a negative value suggests gastrointestinal bicarbonate loss (such as diarrhea), while a positive value suggests renal tubular acidosis, because it indirectly estimates unmeasured urinary ammonium excretion. All three indices lose reliability in patients on diuretics (which directly increase urinary sodium excretion regardless of renal perfusion status) -- fractional excretion of urea is the standard alternative index in that setting, but it is not computed by this calculator. These are adjunctive diagnostic clues, not standalone diagnoses, and should be interpreted alongside the clinical history, exam, and other labs.
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
Results
FENa
0.43%
Figures current as of 1976. Source: Espinel CH. The FeNa test: use in the differential diagnosis of acute renal failure. JAMA. 1976;236(6):579-581.
How to Use This Calculator
- Enter spot urine and plasma sodium (mEq/L) and creatinine (mg/dL) values.
- Enter spot urine potassium (mEq/L) and chloride (mEq/L) for the urine anion gap.
- Read the FENa (%): values <1% suggest pre-renal, 1-2% mixed, >2% intrinsic renal disease.
- Review the urine anion gap: negative values suggest GI bicarbonate loss, positive values suggest renal tubular acidosis.
- Use the renal failure index and UCr/PCr ratio to further differentiate acute kidney injury etiology.
How the result changes with Plasma Sodium (mEq/L)
| Plasma Sodium (mEq/L) | FENa |
|---|---|
| 100 | 0.6% |
| 105 | 0.57% |
| 180 | 0.33% |
What each input means
- Urine Sodium (mEq/L)
- Spot urine sodium concentration.
- Plasma Sodium (mEq/L)
- Serum sodium concentration.
- Urine Creatinine (mg/dL)
- Spot urine creatinine concentration.
- Plasma Creatinine (mg/dL)
- Serum creatinine concentration.
- Urine Potassium (mEq/L)
- Spot urine potassium (for urine anion gap).
- Urine Chloride (mEq/L)
- Spot urine chloride (for urine anion gap).
What each result means
- FENa
- <1% suggests pre-renal; 1-2% mixed; >2% intrinsic renal disease.
- Urine Anion Gap
- Negative = GI bicarb loss; Positive = renal tubular acidosis.
- Renal Failure Index
- RFI <1 suggests pre-renal; >1 suggests intrinsic renal.
- UCr/PCr Ratio
- Urine-to-plasma creatinine concentration ratio.
How this is calculated
Worked example, using the default values
- Identify Input Parameters4 parametersUrine Sodium (mEq/L) = 40, Plasma Sodium (mEq/L) = 140, Urine Creatinine (mg/dL) = 80, Plasma Creatinine (mg/dL) = 1.2 = 6 input(s) provided
- Calculate FENaFENa = (urineNa * plasmaCr) / (plasmaNa * urineCr) * 1000.429 = 0.429
- Calculate Urine Anion GapUrine Anion Gap = urineNa + urineK - urineCl20 = 20
- Calculate Renal Failure IndexRenal Failure Index = (urineNa * plasmaCr) / urineCr0.6 = 0.6
Figures and sources
- Espinel's FENa formula for AKI differential diagnosis (1976) — Espinel CH. The FeNa test: use in the differential diagnosis of acute renal failure. JAMA. 1976;236(6):579-581.
Engine last updated . Checked against 2 independently-derived tests — how we verify calculators. Built by Paul Gunder, a software engineer, not a licensed financial, medical, or legal professional.
Frequently Asked Questions
What does a FENa below 1% mean?
A FENa under 1% suggests pre-renal azotemia -- the kidneys are structurally intact and are avidly reabsorbing filtered sodium in response to reduced renal perfusion (from volume depletion, heart failure, or similar causes). Above 2% suggests intrinsic renal injury such as acute tubular necrosis, where damaged tubules can no longer reabsorb sodium normally; 1-2% falls into a mixed or indeterminate zone.
Why do urine potassium and urine chloride not affect the FENa result?
FENa is calculated purely from urine and plasma sodium and creatinine (Espinel's formula) -- potassium and chloride play no role in that specific ratio. They matter for a different output on this calculator, the urine anion gap, which does depend on both of them alongside urine sodium.
How is the renal failure index different from FENa?
Both distinguish pre-renal from intrinsic renal causes of AKI using the same underlying urine sodium and creatinine-ratio logic, but they're scaled differently: FENa expresses the result as a percentage with a 1-2% threshold zone, while the renal failure index expresses it as a plain ratio with a single cutoff of 1 -- below 1 suggests pre-renal, above 1 suggests intrinsic renal. Renal failure index does not use plasma sodium at all, only urine sodium and the urine-to-plasma creatinine ratio.
Why does a positive urine anion gap suggest renal tubular acidosis rather than diarrhea?
Urine anion gap is used as an indirect estimate of urinary ammonium excretion, which itself isn't routinely measured. In diarrhea-related bicarbonate loss, the kidneys respond appropriately by excreting more ammonium, which carries unmeasured chloride with it and drives the calculated gap negative. In renal tubular acidosis, the kidneys can't mount that ammonium response, so chloride excretion stays low relative to sodium and potassium, pushing the gap positive.
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