What is the FENa formula?

FENa (%) = (UNa × PCr) / (PNa × UCr) × 100, where UNa and PNa are urine and serum sodium and UCr and PCr are urine and serum creatinine. Because the creatinine units cancel, both creatinine values only need to share a unit.

How do I interpret the FENa result?

In oliguric acute kidney injury, FENa below 1% suggests pre-renal azotaemia where the kidney avidly reabsorbs sodium, while FENa above 2% suggests intrinsic renal failure such as acute tubular necrosis. Values between 1 and 2% are indeterminate.

When is FENa unreliable?

FENa is invalid after loop diuretics, which force sodium loss and falsely raise it — use the fractional excretion of urea instead. It is also unreliable in chronic kidney disease and non-oliguric states.

What units should I use?

Enter both sodium values in mmol/L. Enter both creatinine values in the same unit as each other — either mg/dL or µmol/L — since they appear as a ratio and the units cancel out.

What is the Fractional Excretion of Sodium (FENa) Calculator?

Free FENa calculator. Enter urine and serum sodium and creatinine to compute the fractional excretion of sodium and differentiate pre-renal azotaemia ( 2%). Runs entirely in your browser. It runs free in your browser on Gera Tools, with nothing uploaded.

Fractional Excretion of Sodium (FENa) Calculator

Name: Fractional Excretion of Sodium (FENa) Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

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The fractional excretion of sodium (FENa) measures what percentage of the sodium filtered by the glomerulus ends up in the urine. It is one of the most useful bedside tests for working out why a patient has acute kidney injury (AKI): is the kidney itself damaged, or is it simply starved of blood flow and holding onto sodium as hard as it can?

How it works

The calculator implements the standard formula:

FENa (%) = (UNa × PCr) / (PNa × UCr) × 100

where UNa and PNa are urine and serum sodium (mmol/L), and UCr and PCr are urine and serum creatinine. Creatinine acts as a marker of how much filtrate has been concentrated. Because creatinine appears in both the numerator and the denominator, its units cancel — so you can enter both creatinine values in mg/dL or both in µmol/L without converting.

A healthy or volume-depleted kidney reabsorbs almost all filtered sodium, giving a very low FENa. A kidney with damaged tubules (acute tubular necrosis) loses the ability to reclaim sodium, so a larger fraction is excreted.

Worked example

A patient presents with oliguria and rising creatinine after two days of vomiting. Labs show:

Urine sodium (UNa): 12 mmol/L
Serum sodium (PNa): 142 mmol/L
Urine creatinine (UCr): 8,800 µmol/L
Serum creatinine (PCr): 220 µmol/L

FENa = (12 × 220) / (142 × 8,800) × 100 = 2,640 / 1,249,600 × 100 ≈ 0.21%

This very low FENa suggests pre-renal azotaemia — the kidney is avidly conserving sodium because of the volume depletion from vomiting. A cautious IV fluid challenge is the appropriate next step, while checking that no diuretics have been given recently.

Interpretation

In oliguric AKI with no recent diuretics:

FENa < 1% — pre-renal azotaemia. The kidney is sodium-avid; restore perfusion (fluids, treat the cause).
FENa 1–2% — indeterminate; correlate clinically.
FENa > 2% — intrinsic renal failure, classically acute tubular necrosis.

When to use FEUrea instead

FENa becomes unreliable whenever tubular sodium handling is disrupted for reasons unrelated to the underlying AKI. The main offenders:

Loop diuretics (furosemide, bumetanide) — block Na/K/2Cl co-transport and push FENa up regardless of pre-renal state
Thiazide diuretics — similar but milder effect
Recent contrast — contrast nephropathy early on can still show FENa below 1%

In these situations, use the fractional excretion of urea (FEUrea), calculated the same way substituting urea for sodium. FEUrea below approximately 35% suggests pre-renal; above 50% suggests intrinsic injury. Urea reabsorption is less sensitive to diuretics, making it a more robust marker when tubular sodium handling is pharmacologically altered.

Notes and pitfalls

FENa is only meaningful in oliguric AKI. A low FENa can also be seen in intrinsic injury from contrast nephropathy, early sepsis, rhabdomyolysis, acute glomerulonephritis, and hepatorenal syndrome. Treat the number as one data point alongside the history, volume status, and urine microscopy. All calculation runs locally in your browser; nothing is sent to a server. This tool is for educational use only and does not constitute medical advice.