What is the FENa?
Enter sodium and creatinine from a blood sample and a spot urine sample to calculate the fractional excretion of sodium (FENa). For information only. This is not medical advice or a diagnosis. Do not use it to make treatment decisions without a clinician. No licensed clinician has reviewed this page.
- FENa
- 0.29%
The FENa is 0.29%. Under 1%: fits a prerenal cause.
Under 1%: fits a prerenal cause
- Under 1%: fits a prerenal cause
- 1% to 3%: indeterminate
- Over 3%: fits intrinsic kidney injury (such as ATN)
- What the cut-offs say
- Under 1%: fits a prerenal cause
FENa: 0.29%. The FENa is 0.29%. Under 1%: fits a prerenal cause.
Where does the FENa sit?
This calculator gives general information only. It is not medical advice and does not replace a doctor or other health care provider. Talk with a professional about your results. Terms of use
How to calculate
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.
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.
Method: FENa (%) = (urine sodium × serum creatinine) ÷ (serum sodium × urine creatinine) × 100.
- 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
Each example is checked against the calculator on every build.
- Creatinine units mg/dL, Serum sodium (mmol/L) 140, Urine sodium (mmol/L) 20, Serum creatinine (mg/dL) 2, Urine creatinine (mg/dL) 100 gives FENa 0.29%, What the cut-offs say 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%
- Creatinine units mg/dL, Serum sodium (mmol/L) 135, Urine sodium (mmol/L) 60, Serum creatinine (mg/dL) 4, Urine creatinine (mg/dL) 50 gives FENa 3.56%, What the cut-offs say 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%
- Creatinine units µmol/L, Serum sodium (mmol/L) 138, Urine sodium (mmol/L) 40, Serum creatinine (µmol/L) 221, Urine creatinine (µmol/L) 6,630 gives FENa 0.97%, What the cut-offs say Under 1%: fits a prerenal cause.
- Creatinine units mg/dL, Serum sodium (mmol/L) 140, Urine sodium (mmol/L) 28, Serum creatinine (mg/dL) 1, Urine creatinine (mg/dL) 20 gives FENa 1%, What the cut-offs say 1% to 3%: indeterminate.
Medical disclaimer
This calculator is for information and education only. It is not medical advice, and its result is not a diagnosis. Do not use it to start, stop, or change a treatment without a clinician who knows your health history. No licensed clinician has reviewed this page. This site's editors checked the formulas and worked examples against published sources. If you think you have a medical emergency, call your local emergency number.
When this does not apply. Espinel (1976) studied adults in the oliguric phase of acute kidney failure (low urine output). The cut-offs do not apply to people without acute kidney injury. Diuretics, chronic kidney disease, urinary blockage, and contrast dye can give a misleading FENa.
How the FENa is worked out
FENa (%) = (urine sodium × serum creatinine) ÷ (serum sodium × urine creatinine) × 100
- Sodium is in mmol/L (the same number as mEq/L).
- Both creatinine values are in the same unit: mg/dL, or µmol/L with the SI setting. The unit cancels, so no conversion is needed.
- The FENa is rounded to 2 decimal places (halves round up, read from the decimal value: a result within 1e-9 (relative) of a half counts as the half, so 2.005 rounds to 2.01). A result over 100% is impossible, so the page asks you to check the numbers.
Cut-offs
Read from the rounded FENa, for a person with acute kidney injury and low urine output (Espinel 1976):
| FENa | Reading |
|---|---|
| under 1% | fits a prerenal cause |
| 1% to 3% | indeterminate |
| over 3% | fits intrinsic kidney injury, such as acute tubular necrosis |
For example, serum Na 100, urine Na 300.5, serum creatinine 2, urine creatinine 200 give exactly 3.005%, shown as 3.01%, which is over 3%.
Inputs the calculator accepts
Serum sodium 100 to 180 mmol/L, urine sodium 1 to 400 mmol/L, serum creatinine 0.1 to 20 mg/dL (8.84 to 1,768 µmol/L), urine creatinine 1 to 1,000 mg/dL (88.4 to 88,400 µmol/L).
Worked examples by hand
The default. Serum Na 140, urine Na 20, serum creatinine 2.0 mg/dL, urine creatinine 100 mg/dL: (20 × 2) ÷ (140 × 100) × 100 = 40 ÷ 14,000 × 100 = 0.2857%, shown as 0.29%, under 1%.
High FENa. Serum Na 135, urine Na 60, serum creatinine 4.0, urine creatinine 50: (60 × 4) ÷ (135 × 50) × 100 = 240 ÷ 6,750 × 100 = 3.56%, over 3%.
SI units. Serum Na 138, urine Na 40, serum creatinine 221 µmol/L, urine creatinine 6,630 µmol/L: (40 × 221) ÷ (138 × 6,630) × 100 = 8,840 ÷ 914,940 × 100 = 0.97%, under 1%.
On the cut-off. Serum Na 140, urine Na 28, serum creatinine 1.0, urine creatinine 20: 28 ÷ 2,800 × 100 = 1.00%, indeterminate.
Other questions people ask
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.