Fractional Excretion of Sodium (FENa) Calculator

About the Fractional Excretion of Sodium (FENa) Calculator

The Fractional Excretion of Sodium (FENa) calculator estimates the percentage of filtered sodium that is being excreted in the urine. It is commonly used as one clue in the acute kidney injury workup because low values often fit sodium-avid states such as volume depletion, while higher values can fit tubular injury or other intrinsic renal processes.

The useful part of FENa is that it puts urine sodium and creatinine together with plasma sodium and creatinine in one ratio, so a spot urine sample can be used without a timed collection. The less useful part is that the number is easy to overinterpret. Diuretics, chronic kidney disease, glomerulonephritis, contrast nephropathy, obstruction, sepsis, and other settings can all make the usual thresholds less reliable. This page is therefore meant as a structured AKI clue, not a stand-alone sorter.

When This Page Helps

FENa is most useful when it is read beside volume status, medication exposure, and urine sediment rather than by itself. This page keeps the ratio, the usual threshold bands, and the common failure modes together so the result supports the AKI differential instead of pretending to settle it.

How to Use the Inputs

1. Enter urine sodium (UNa) and select the unit (mEq/L or mmol/L).
2. Enter plasma sodium (PNa) — typically 135–145 mEq/L.
3. Enter urine creatinine (UCr) and select the unit (mg/dL or µmol/L).
4. Enter plasma creatinine (PCr) — note that elevated values indicate renal dysfunction.
5. Indicate diuretic use so the page can flag one of the main limitations of FENa.
6. Review FENa result, AKI classification, and complementary diagnostic indices.

Example Calculation

Tips & Best Practices

• Obtain urine and blood samples simultaneously — delays between specimens reduce accuracy.
• Always check for diuretic use in the prior 24–48 hours — even a single dose of furosemide can elevate FENa in a prerenal patient.
• In diuretic-treated patients, use FEUrea instead (FEUrea <35% = prerenal, >50% = intrinsic).
• FENa < 1% does NOT rule out intrinsic disease — contrast nephropathy, rhabdomyolysis, and acute GN can all have low FENa.
• In CKD patients, baseline FENa may already be >1% due to chronic tubular adaptation — interpret with caution.
• Urine microscopy (looking for muddy brown casts) is often more specific than FENa for diagnosing ATN.

FENa Works Best as a Supporting Clue

FENa is one piece of the AKI evaluation, not the whole answer. The ratio is most helpful when it is read together with the urine sediment, medication exposure, volume status, ultrasound findings, and the broader hemodynamic picture.

Why the Ratio Fails in Real Life

Diuretics, chronic kidney disease, contrast-associated injury, rhabdomyolysis, sepsis, glomerular disease, and obstruction can all make the classic FENa thresholds less dependable. That is why the page keeps the usual exceptions visible instead of treating the cutoffs as absolute.

What to Pair With It

When the result is ambiguous, FEUrea, urine microscopy, repeat chemistry, and the clinical course often do more to sort out the cause of AKI than one FENa value alone.

Frequently Asked Questions

• What does FENa measure?

  FENa estimates the percentage of filtered sodium that ends up in the urine. Low values often fit sodium retention, while higher values can fit impaired tubular sodium reabsorption. The main value of the number is as one part of the AKI picture rather than as a diagnosis by itself.

• Why is FENa unreliable with diuretics?

  Diuretics directly increase sodium excretion, so they can push FENa upward even when the patient is actually sodium avid. That is why FEUrea is often preferred when diuretics have already been given.

• When can FENa be low despite intrinsic renal disease?

  Several intrinsic renal problems can still show a low FENa early on, including contrast-associated AKI, rhabdomyolysis, some glomerular diseases, early sepsis, and early obstruction. A low value therefore does not automatically rule out intrinsic disease.

• What other tests help differentiate prerenal from intrinsic AKI?

  Urine microscopy, FEUrea, urine osmolality, BUN/creatinine ratio, urine sediment, and the overall hemodynamic picture often add more context than FENa alone. In many cases the urine microscopy and the clinical course are more informative than the ratio by itself.

• What is a typical FENa range?

  In the AKI context, values below about 1% are commonly discussed as sodium-avid, values above about 2% are more compatible with impaired tubular reabsorption, and the 1–2% range is a gray zone. Those cutoffs are useful shorthand, not a perfect rule.

• How useful is FENa in chronic kidney disease?

  Its value is more limited in CKD because baseline tubular handling may already be altered. In acute-on-chronic kidney injury, the ratio should be interpreted cautiously and always alongside the broader renal workup.