FENa Calculator

This FENa calculator works out the fractional excretion of sodium from four lab values: serum sodium, urine sodium, serum creatinine and urine creatinine. Add serum and urine urea and it calculates FEUrea too, the version that holds up better after diuretics. Each creatinine and urea value has its own unit menu, so a serum result in µmol/L and a urine result in mmol/L can go straight in without converting by hand.

It’s built for students, nurses and clinicians checking their arithmetic. It doesn’t diagnose anything, and the sections below explain why the number always needs the rest of the clinical picture.

About the FENa calculator

FENa is one of the first calculations taught for acute kidney injury (AKI), and one of the easiest to get wrong. The formula is simple. The trap is units: many labs report serum creatinine in µmol/L or mg/dL and urine creatinine in mmol/L. Divide one by the other without converting and the answer is out by a factor of 1,000.

• Mixed units handled. Pick the unit for each creatinine and urea value separately: mg/dL, µmol/L or mmol/L. The FENa calculator converts before dividing.
• Unit mistakes caught. Values far outside the plausible range are refused with a message, and a urine creatinine at or below the serum level is flagged, because that almost always means a unit is wrong.
• FEUrea alongside. Enter urea values and you get FEUrea next to FENa, with a warning when the two disagree.
• Diuretic awareness. Tick the box if a diuretic has been given, and the result reminds you why FENa is less reliable in that case.
• Working shown. Every step, including the converted values, is listed.

Nothing you enter leaves your browser, and nothing is saved.

How to use the FENa calculator

1. Enter serum and urine sodium in mmol/L. For sodium, mmol/L and mEq/L are the same number.
2. Enter serum and urine creatinine and set each unit to match the lab report.
3. Optionally, enter urea. Serum urea or BUN, and urine urea or urine urea nitrogen (UUN), each with its unit. Leave both blank to skip FEUrea.
4. Tick the diuretic box if a loop or thiazide diuretic has been given.
5. Read the result. FENa is compared with 1% and FEUrea with 35%. Open “Show the working” to see the converted values.

The urine and blood samples should be taken at about the same time. A spot urine sample is standard; you don’t need a 24-hour collection.

Where FENa fits in acute kidney injury

Acute kidney injury means kidney function falls over hours or days, usually noticed as a rising creatinine or falling urine output. Its causes are grouped into three:

• Prerenal: not enough blood reaching the kidneys, from dehydration, bleeding, heart failure or low blood pressure. The kidneys themselves are working.
• Intrinsic: damage inside the kidney, most often acute tubular necrosis after prolonged low blood flow or a toxin.
• Postrenal: a blockage to urine flow, anywhere from the ureters to the urethra.

The distinction matters because a prerenal cause often recovers quickly once blood flow is restored, while intrinsic injury needs different management. FENa was designed to help separate the first two. A FENa calculator can’t see the blockage behind a postrenal cause; imaging does that.

Why not just use urine sodium?

A low urine sodium on its own also suggests the kidneys are conserving salt. The problem is that urine sodium depends on how concentrated the urine is. A patient who is holding on to water will have a higher urine sodium concentration even if the kidneys are holding on to sodium too.

FENa corrects for that by dividing by creatinine’s urine-to-serum ratio, which rises and falls with urine concentration in the same way. What’s left is the share of filtered sodium that escaped, whatever the urine volume. That’s why a FENa calculator needs all four values, not just the urine sodium.

The FENa formula

FENa (%) = (urine Na x serum creatinine) / (serum Na x urine creatinine) x 100

The idea is to compare how much sodium leaves in the urine with how much the kidneys filtered. Creatinine stands in for filtration, because the kidneys filter it and barely reabsorb it. Dividing sodium’s urine-to-serum ratio by creatinine’s gives the fraction of filtered sodium that escaped reabsorption.

Healthy tubules reabsorb almost all the sodium the kidneys filter. When blood flow to the kidneys falls but the tubules still work, they hold on to sodium even harder, and FENa drops. When the tubules themselves are damaged, they can’t reabsorb it, and FENa rises.

The FEUrea formula

FEUrea (%) = (urine urea x serum creatinine) / (serum urea x urine creatinine) x 100

It’s the same calculation with urea in place of sodium. Urea is reabsorbed mainly in the proximal tubule, upstream of where loop and thiazide diuretics act, so diuretics distort it less than they distort FENa. That’s why the FENa calculator offers it as a second opinion.

How to read the result

These cut-offs come from a 2023 Kidney360 review of FENa and FEUrea, which summarises the original studies. The calculator words every result as a pattern, not a diagnosis, because neither test decides the cause of AKI on its own.

You may see calculators that label a FENa above 4% as “post-renal” (obstruction). The review we used doesn’t describe a separate cut-off for obstruction, and urinary obstruction is one of the situations where FENa is considered unreliable. We don’t show one.

Where the numbers come from

• 1976: Espinel described FENa in 17 patients with oliguric acute renal failure.
• 1978: Miller and colleagues evaluated urinary indices, including FENa, prospectively in 102 patients.
• 1992: Kaplan and Kohn first looked at FEUrea, in six patients treated with diuretics.
• 2002: Carvounis and colleagues studied FEUrea in 102 episodes of AKI. 92% of prerenal patients not on diuretics had a FENa below 1%, but only 48% of those on diuretics did. In that diuretic group, 89% had a FEUrea below 35%, and a FEUrea of 35% or less had a positive predictive value of 98% for a prerenal cause.

In the setting FENa was designed for, oliguric patients without chronic kidney disease who haven’t had diuretics, the Kidney360 review reports a sensitivity of 95% and a specificity of 91% for telling prerenal injury from ATN.

When FENa is unreliable

Knowing the exceptions matters more than knowing the cut-off.

High FENa in a prerenal state:

• Diuretics, which make the kidneys excrete sodium on purpose. This is the classic case for FEUrea instead.
• Chronic kidney disease, where damaged tubules already struggle to conserve sodium.

Low FENa despite intrinsic injury:

• Pigment nephropathy, from myoglobin or haemoglobin.
• Contrast-associated kidney injury, early in its course.
• Some early acute tubular or interstitial injury, and cardiorenal states.

Also limited in: urinary tract obstruction and acute glomerular disease. In all these situations the FENa calculator still does the arithmetic correctly, but the result says less about the cause.

Unit conversions

The calculator converts every creatinine value to µmol/L and every urea value to mmol/L before it divides. Because FENa is a ratio, it only matters that each pair ends up in the same unit, but converting everything to one unit also lets it check the values make sense.

FENa calculator example: a prerenal pattern

Serum sodium 140 mmol/L, urine sodium 20 mmol/L, serum creatinine 2 mg/dL, urine creatinine 100 mg/dL.

1. Urine sodium x serum creatinine: 20 x 2 = 40.
2. Serum sodium x urine creatinine: 140 x 100 = 14,000.
3. 40 / 14,000 x 100 = 0.29%.

A FENa of 0.29% is well below 1%, the pattern seen when the kidneys are underperfused. The urine creatinine is 50 times the serum creatinine, a sensible ratio, so no unit warning appears.

Example: an intrinsic pattern

Serum sodium 135 mmol/L, urine sodium 60 mmol/L, serum creatinine 3.5 mg/dL, urine creatinine 50 mg/dL. The FENa calculator gives (60 x 3.5) / (135 x 50) x 100 = 3.11%. That’s above 1%, in the range often seen in acute tubular necrosis.

Example: mixed units

A lab reports serum creatinine as 176.8 µmol/L and urine creatinine as 8.84 mmol/L, with the same sodium values as the first example. Set the serum unit to µmol/L and the urine unit to mmol/L. The calculator converts the urine value to 8,840 µmol/L and gives exactly the same 0.29%.

Leave the urine unit on µmol/L by mistake and 8.84 µmol/L is far below any real urine creatinine. The calculator refuses it and asks you to check the unit, instead of reporting a FENa of 286%.

Example: after a diuretic

Same patient as the first example, but after furosemide the urine sodium is 147 mmol/L. Serum urea nitrogen is 40 mg/dL and urine urea nitrogen 400 mg/dL.

• FENa: (147 x 2) / (140 x 100) x 100 = 2.1%, above the 1% cut-off.
• FEUrea: (400 x 2) / (40 x 100) x 100 = 20%, below the 35% cut-off.

The two disagree, and the calculator says so. After a diuretic, FEUrea is usually the more reliable of the two, and here it keeps pointing to a prerenal pattern even though FENa no longer does.

Common calculation errors

• Mixed creatinine units. Serum in µmol/L, urine in mmol/L, divided as if they matched. The result is 1,000 times too high. Set each unit in the FENa calculator and it converts for you.
• Swapping serum and urine. Putting serum creatinine where urine creatinine goes flips the ratio. The calculator warns when urine creatinine isn’t higher than serum.
• Forgetting the x 100. Without it, 0.0029 looks like a tiny FENa when it’s really 0.29%.
• Using BUN against urine urea in mmol/L. Urea nitrogen and urea are different quantities. Choose the matching unit for each and the calculator handles the 2.8014 factor.
• Reading the number without the context. A FENa below 1% after contrast, or above 1% after furosemide, can point the wrong way. Check the limitations above before acting on it.

Tips for using a FENa calculator

1. Take the samples together. Serum and urine values from different days describe different kidneys.
2. Check the units on the report. Urine creatinine in particular is often reported in mmol/L.
3. Note any diuretic. If one was given before the urine sample, lean on FEUrea rather than FENa.
4. Look at the whole picture. Volume status, urine output, urine microscopy and the trend in creatinine all count for more than one ratio.
5. Don’t enter identifying details. The calculator only needs the numbers.

More calculators

A patient’s age on the day of a test is quick to find with the age calculator, and the date duration calculator counts the days between two results when you’re tracking a creatinine trend. Like this FENa calculator, our DOTS calculator is built on a published formula and shows every step of its working.

Sources

• “Fractional Excretion of Sodium and Urea are Useful Tools in the Evaluation of AKI: PRO”, Kidney360, 2023: thresholds, study history and performance figures.
• Carvounis CP et al., “Significance of the fractional excretion of urea in the differential diagnosis of acute renal failure”, Kidney International, 2002.
• MDCalc, FENa calculator: commonly cited limitations of FENa.