# What is the FENa?

Calculates the fractional excretion of sodium (FENa) from serum and urine sodium and creatinine, in US or SI units, and shows how the Espinel cut-offs read it in acute kidney injury.

- Page: https://www.acalculator.org/health/fena-calculator
- JSON spec: https://www.acalculator.org/health/fena-calculator.json
- Version: d043ef74209b

## Default answer

Example with the default inputs (Creatinine units mg/dL, Serum sodium (mmol/L) 140, Urine sodium (mmol/L) 20, Serum creatinine (mg/dL) 2, Urine creatinine (mg/dL) 100): The FENa is 0.29%. Under 1%: fits a prerenal cause.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| units | Creatinine units | US units (mg/dL) or SI units (µmol/L) for both creatinine values. |
| sNa | Serum sodium (mmol/L) | Sodium in the blood, in mmol/L (the same number as mEq/L). |
| uNa | Urine sodium (mmol/L) | Sodium in a spot urine sample, in mmol/L. |
| sCr | Serum creatinine (mg/dL) | Creatinine in the blood, in mg/dL. |
| uCr | Urine creatinine (mg/dL) | Creatinine in the same urine sample, in mg/dL. |
| sCrSi | Serum creatinine (µmol/L) | Creatinine in the blood, in µmol/L. |
| uCrSi | Urine creatinine (µmol/L) | Creatinine in the same urine sample, in µmol/L. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| fena | FENa | Fractional excretion of sodium, the percent of filtered sodium that reaches the urine, to 2 decimals. |
| reading | What the cut-offs say | How the Espinel cut-offs read this FENa in a person with acute kidney injury and low urine output. |

## Method

FENa (%) = (urine sodium × serum creatinine) ÷ (serum sodium × urine creatinine) × 100.

## Assumptions

- Serum and urine samples taken at about the same time, both creatinine values in the same unit.
- The FENa is rounded to 2 decimals and read from the rounded value: under 1% prerenal, 1% to 3% indeterminate, over 3% intrinsic kidney injury (Espinel 1976).
- The cut-offs were set for acute kidney injury with low urine output. Diuretics, chronic kidney disease, contrast dye, and early obstruction can make the FENa misleading.

## Worked examples

1. units = us, sNa = 140, uNa = 20, sCr = 2, uCr = 100 gives fena = 0.29%, reading = Under 1%: fits a prerenal cause. Source: Espinel 1976: FENa = (UNa × PCr) ÷ (PNa × UCr) × 100 = (20 × 2) ÷ (140 × 100) × 100 = 0.2857%, shown as 0.29%.
2. units = us, sNa = 135, uNa = 60, sCr = 4, uCr = 50 gives fena = 3.56%, reading = Over 3%: fits intrinsic kidney injury (such as ATN). Source: Espinel 1976 (over 3% in acute tubular necrosis): (60 × 4) ÷ (135 × 50) × 100 = 3.5556%, shown as 3.56%.
3. units = si, sNa = 138, uNa = 40, sCrSi = 221, uCrSi = 6,630 gives fena = 0.97%, reading = Under 1%: fits a prerenal cause.
4. units = us, sNa = 140, uNa = 28, sCr = 1, uCr = 20 gives fena = 1%, reading = 1% to 3%: indeterminate.

## FAQ

### What is FENa?

The fractional excretion of sodium (FENa) is the percent of the sodium filtered by the kidneys that ends up in the urine. Healthy kidneys keep almost all of it, so the FENa is usually under 1%. Clinicians use it in acute kidney injury to help tell a "prerenal" cause (too little blood flow to the kidneys) from damage inside the kidney.

### What do the FENa cut-offs mean?

In people in the oliguric phase of acute kidney failure (low urine output), Espinel (1976) found a FENa under 1% with a prerenal cause and over 3% with acute tubular necrosis (ATN). Miller and others (1978) also found a FENa under 1% in most prerenal cases. This page calls values from 1% to 3% indeterminate. Some teaching texts use 2% as the upper cut-off instead.

### When is FENa not reliable?

Diuretics such as furosemide raise the FENa, so a prerenal cause can look like kidney damage. Chronic kidney disease, contrast dye, early urinary blockage, and some cases of sepsis or burns can also mislead. For people on diuretics, clinicians often use the fractional excretion of urea instead.

### Do the creatinine units matter?

The serum and urine creatinine must be in the same unit, because the formula divides one by the other. If both are in mg/dL, or both in µmol/L, the FENa is the same. Sodium is in mmol/L, which is the same number as mEq/L.

## Sources

- Espinel CH. The FENa test: use in the differential diagnosis of acute renal failure. JAMA. 1976;236(6):579-581. https://doi.org/10.1001/jama.1976.03270060029022
- Miller TR, Anderson RJ, Linas SL, et al. Urinary diagnostic indices in acute renal failure: a prospective study. Ann Intern Med. 1978;89(1):47-50. https://doi.org/10.7326/0003-4819-89-1-47
- KDIGO 2012 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney Int Suppl. 2013;3(1):1-150. https://kdigo.org/guidelines/ckd-evaluation-and-management/ (1 mg/dL creatinine = 88.4 µmol/L)