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.

Fractional Excretion of Sodium (FENa) Calculator

Q: Can FENa be low in intrinsic kidney injury?

Yes. A FENa below 1% can occur despite intrinsic injury in contrast nephropathy, early sepsis, rhabdomyolysis, acute glomerulonephritis, and hepatorenal syndrome, so always interpret it within the full clinical picture.

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.

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.

Notes and pitfalls

FENa is only meaningful in oliguric AKI. It is invalid after loop diuretics — furosemide blocks sodium reabsorption and pushes FENa up regardless of cause; in that situation use the fractional excretion of urea (FEUrea < 35% favours pre-renal). 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.