What this calculator does
Fractional excretion of sodium asks what share of the sodium filtered by the kidneys ends up in the urine. A failing blood supply makes the kidney conserve sodium aggressively, so the fraction drops sharply.
The bands are narrow and low. Below 1 per cent points toward a prerenal cause, 1 to 4 per cent toward damage within the kidney, and above 4 per cent toward obstruction downstream.
The formula
Urine sodium times plasma creatinine is divided by plasma sodium times urine creatinine, then multiplied by 100. Using creatinine on both sides corrects for how concentrated the urine is.
| Term | Meaning |
|---|---|
| FeNa | The percentage of filtered sodium excreted in the urine. |
| Prerenal | Reduced blood flow to the kidney, which drives sodium conservation. |
| Intrinsic renal | Damage within the kidney itself, which impairs sodium reabsorption. |
The inputs explained
| Field | What to enter |
|---|---|
| Plasma sodium (mEq/L) | Plasma sodium in mEq/L. |
| Urine sodium (mEq/L) | Urine sodium in mEq/L. |
| Plasma creatinine (mg/dL) | Plasma creatinine in mg/dL. |
| Urine creatinine (mg/dL) | Urine creatinine in mg/dL, which corrects for urine concentration. |
When to use it
Classifying acute kidney injury
FeNa is one of the standard tests for separating prerenal from intrinsic causes.
Assessing volume status
A very low fraction indicates the kidney is conserving sodium hard.
Understanding a renal workup
Paired urine and plasma samples exist precisely to allow this comparison.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How does urine concentration change the result?
The same sodium values at four urine creatinine levels.
| Urine creatinine | FeNa | Reading |
|---|---|---|
| 100 mg/dL | 0.286% | Under 1%: suggests a prerenal cause (reduced blood flow to the kidneys) |
| 40 mg/dL | 0.714% | Under 1%: suggests a prerenal cause (reduced blood flow to the kidneys) |
| 20 mg/dL | 1.43% | 1% to 4%: suggests an intrinsic renal cause |
| 10 mg/dL | 2.86% | 1% to 4%: suggests an intrinsic renal cause |
Questions
Why does creatinine appear on both sides?
To correct for how concentrated the urine is. Without it, a dehydrated patient producing small volumes of concentrated urine would look quite different from the same patient producing dilute urine, when the underlying physiology is identical.
Why is it unreliable on diuretics?
Because diuretics force sodium excretion directly, which raises the fraction regardless of the underlying cause. In that situation fractional excretion of urea is often used instead, since urea handling is less affected.
Does it work in chronic kidney disease?
Poorly. FeNa was validated for acute kidney injury, and in chronic disease the baseline fraction is already altered, so the usual bands do not apply.
Is a low FeNa always prerenal?
No. It also occurs in some intrinsic conditions including contrast nephropathy and early glomerulonephritis. The test narrows the possibilities rather than settling them.
For estimated filtration rate, see the creatinine clearance calculator. For protein loss from a spot sample, see the urine protein-to-creatinine ratio calculator.