FENa Calculator (Fractional Excretion of Sodium)

Enter paired urine and plasma sodium and creatinine values to calculate the fractional excretion of sodium (FENa) and see whether it points toward a prerenal or intrinsic renal cause of acute kidney injury.

Quick Facts

FENa formula
FENa = (UNa × PCr) / (PNa × UCr) × 100
A ratio of ratios — units cancel as long as urine and plasma creatinine share the same unit.
Prerenal cutoff
FENa < 1%
Suggests the kidneys are appropriately conserving sodium due to reduced perfusion.
Intrinsic renal cutoff
FENa > 2%
Suggests tubular damage (e.g., acute tubular necrosis) impairing sodium reabsorption.
Not diagnostic on diuretics
Use FEUrea instead
Recent diuretic use raises FENa independent of the underlying cause.

Your Results

Calculated
FENa
-
Fractional excretion of sodium
Urine/Plasma Sodium Ratio
-
UNa ÷ PNa
Urine/Plasma Creatinine Ratio
-
UCr ÷ PCr
Likely Category
-
Educational interpretation, not a diagnosis

Ready

Enter urine and plasma sodium and creatinine, then press Calculate. This tool is educational and does not replace clinical judgment.

Formula and Method for the FENa Calculator (Fractional Excretion of Sodium)

The fractional excretion of sodium (FENa) estimates what percentage of the sodium filtered by the kidneys actually ends up in the urine rather than being reabsorbed. It is calculated from a simple sodium clearance and creatinine clearance ratio: FENa (%) = (Urine Na × Plasma Cr) / (Plasma Na × Urine Cr) × 100. Clinicians use it primarily in the workup of acute kidney injury (AKI) to help distinguish a prerenal cause (reduced blood flow to otherwise healthy kidneys) from an intrinsic renal cause such as acute tubular necrosis (ATN).

How the calculation works

Enter urine sodium, plasma (serum) sodium, urine creatinine, and plasma (serum) creatinine, ideally drawn around the same time. The calculator forms two ratios — urine-to-plasma sodium (UNa ÷ PNa) and urine-to-plasma creatinine (UCr ÷ PCr) — then divides the sodium ratio by the creatinine ratio and multiplies by 100 to express FENa as a percentage. Creatinine is used as the reference marker because it is freely filtered and not reabsorbed, so its urine-to-plasma ratio reflects how concentrated the urine is overall; dividing sodium's ratio by that reference corrects for urine concentration.

Interpreting the FENa result

As a general clinical convention in oliguric AKI: FENa below 1% suggests a prerenal state — the kidneys are working normally and avidly reabsorbing sodium in response to reduced perfusion (dehydration, hemorrhage, heart failure, cirrhosis). FENa above 2% suggests intrinsic renal damage — the tubules are injured and can no longer reabsorb sodium efficiently, as in acute tubular necrosis. Values between 1% and 2% fall into an indeterminate zone and can occur with either process, so they should be interpreted alongside the rest of the clinical picture.

Limitations and when FENa does not apply

FENa is unreliable in several common situations: after loop or thiazide diuretics (which independently increase sodium excretion), in chronic kidney disease, with contrast-induced nephropathy, and in early or partial urinary obstruction — all of which can produce a FENa above 1% despite a prerenal-type physiology. When diuretics have been given recently, the fractional excretion of urea (FEUrea) is generally preferred instead, since urea handling is less affected by diuretic action. FENa is a supportive laboratory clue, not a standalone diagnosis, and should always be interpreted by a clinician alongside history, exam, and other labs.

Frequently Asked Questions

What is the FENa formula and what counts as a normal value?
FENa (%) = (Urine sodium × Plasma creatinine) / (Plasma sodium × Urine creatinine) × 100. It is not a single "normal" number so much as a threshold test used in acute kidney injury: values below 1% and above 2% point in different clinical directions, while 1-2% is an indeterminate zone.
What does a FENa below 1% versus above 2% suggest?
A FENa under 1% suggests prerenal azotemia — the kidneys are avidly retaining sodium in response to reduced perfusion, as with dehydration, blood loss, or heart failure. A FENa over 2% suggests intrinsic renal damage such as acute tubular necrosis, where the tubules can no longer reabsorb sodium normally. This is a supportive clue, not a standalone diagnosis.
Why doesn't FENa work well if the patient has taken diuretics?
Diuretics (especially loop diuretics) force sodium excretion regardless of the underlying cause of kidney injury, which can push FENa above 1% even in a true prerenal state and make the result misleading. In patients on recent diuretics, clinicians typically use the fractional excretion of urea (FEUrea) instead, since urea reabsorption is less affected by diuretic action.
What lab values do I need to calculate FENa?
You need four values drawn at roughly the same time: urine sodium, plasma (serum) sodium, urine creatinine, and plasma (serum) creatinine. Sodium can be entered in mEq/L or mmol/L (numerically equivalent) and creatinine in mg/dL, as long as the urine and plasma creatinine values use the same unit.

Related calculators