FENa Calculator

What FENa is

The fractional excretion of sodium, or FENa, is the percentage of the sodium filtered by the kidneys that ends up in the urine rather than being reabsorbed. It is used in the workup of acute kidney injury to help tell apart two broad causes: a prerenal picture, where the kidneys are underperfused but structurally intact and clinging to sodium, versus intrinsic damage such as acute tubular necrosis, where the tubules can no longer hold sodium back.

The logic is simple. Kidneys short of blood flow conserve sodium hard, so very little escapes into the urine, and FENa is low. Damaged tubules cannot do that, so more sodium spills through, and FENa is higher. Below are the formula, how to read the number, and the several situations where it quietly stops being reliable, which is where most of the care is needed.

The formula this calculator uses

FENa compares sodium handling to creatinine handling, using paired urine and plasma values:

FENa (%) = ((U_Na × P_Cr) ÷ (P_Na × U_Cr)) × 100

where U is urine and P is plasma, for sodium (Na) and creatinine (Cr). A convenient feature falls out of the maths: because sodium appears as a ratio and creatinine as a ratio, the units cancel, so as long as your two sodium values share units and your two creatinine values share units, the result is the same. Sodium is conventionally in mEq/L and creatinine in mg/dL.

A worked example

Take urine sodium of 20 mEq/L, plasma creatinine of 2.0 mg/dL, plasma sodium of 140 mEq/L, and urine creatinine of 40 mg/dL:

FENa = ((20 × 2.0) ÷ (140 × 40)) × 100
FENa = (40 ÷ 5600) × 100
FENa ≈ 0.71%

At 0.71 percent, well under 1, the kidneys are holding onto sodium avidly, which fits a prerenal picture such as volume depletion, provided nothing in the clinical situation is distorting the result. That last caveat is doing real work, as the interpretation and pitfalls below explain.

How to interpret the result

The classic thresholds, in the setting FENa was designed for, are:

These bands are guides, not rules. No single FENa value proves a cause on its own, and the test performs best in patients who are oliguric and free of the confounders below. It is one piece of a picture that also includes the history, the examination, the urine sediment, and the rest of the labs.

Where FENa misleads

FENa has real blind spots, and knowing them is the difference between using it well and being fooled by it:

• Diuretics. This is the big one. Diuretics force sodium into the urine, so they push FENa up and can make a prerenal patient look intrinsic. In anyone recently given a diuretic, FENa is unreliable, and the fractional excretion of urea (FEUrea), which is less affected, is the better test, with a value under about 35 percent suggesting prerenal.
• Sodium-avid states. Cirrhosis, heart failure and nephrotic syndrome can drive FENa below 1 percent even when intrinsic injury is present, because the body is retaining sodium for other reasons.
• Chronic kidney disease and other causes. Baseline tubular dysfunction, urinary obstruction, contrast, and pigment nephropathy can all shift FENa away from the simple prerenal-versus-ATN split.
• Non-oliguric states. The thresholds were validated mainly in oliguric acute kidney injury, and are less dependable outside it.

To be clear about what this page is: a calculation and an educational aid, not a diagnosis. FENa supports a clinical assessment of acute kidney injury; it does not stand in for one. Interpret it alongside everything else, and let the whole picture, not a single percentage, drive the decision.

Questions people ask

What units should I use?

It does not matter much, because the units cancel. As long as your urine and plasma sodium are in the same units as each other, and your urine and plasma creatinine are in the same units as each other, the result is correct. By convention that is mEq/L for sodium and mg/dL for creatinine.

My patient is on furosemide. Can I still use FENa?

Not reliably. Diuretics increase sodium excretion and inflate FENa, so a prerenal patient on a diuretic can show a falsely high value. In that setting the fractional excretion of urea is the more dependable test, since urea handling is less affected by diuretics.

Does a low FENa confirm dehydration?

No. A FENa under 1 percent is consistent with a prerenal state, but several other conditions can produce it, and volume status is a clinical judgement made from the whole picture. Treat the number as supporting evidence, not proof.

References

Where the figures come from. FENa and the 1 percent threshold trace to Espinel's original description of the test. The limitation in patients on diuretics, and the role of the fractional excretion of urea in that setting, draw on the work of Carvounis and colleagues.

1. Espinel CH. The FeNa test: use in the differential diagnosis of acute renal failure. JAMA. 1976;236(6):579-581.
2. Carvounis CP, Nisar S, Guro-Razuman S. Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure. Kidney International. 2002;62(6):2223-2229.