FEUrea Calculator (Fractional Excretion of Urea)

A FEUrea calculator estimates fractional excretion of urea, a percentage that may help clinicians understand how the kidneys are handling urea during an acute kidney injury evaluation. It is often discussed when FENa may be harder to interpret, especially after diuretic use.

The Medicalic FEUrea calculator is designed to make the calculation clear, accessible, and easier to review. A FEUrea result cannot diagnose kidney failure, identify the cause of acute kidney injury, or decide treatment by itself. Interpretation depends on symptoms, urine output, fluid status, blood pressure, medications, kidney history, and other lab results. In acute kidney injury, no single urine marker should be used alone; clinical context is essential. ^[1]

This tool uses serum urea or BUN, urine urea or BUN, serum creatinine, and urine creatinine. It supports serum and urine urea/BUN in mg/dL or mmol/L, plus creatinine in mg/dL or µmol/L, so it can work with many U.S. and international lab reports.

What Is FEUrea?

FEUrea stands for fractional excretion of urea. You may also see it written as FEUN, or fractional excretion of urea nitrogen. It estimates the percentage of filtered urea that leaves the body in urine instead of being reabsorbed by the kidneys.

In simple terms, FEUrea compares urea in blood and urine while using creatinine to adjust for urine concentration. This makes it more informative than looking at urine urea alone.

Urea forms in the liver during protein metabolism and leaves the body mainly through the kidneys. Serum urea or BUN can rise when kidney clearance decreases. Dehydration, gastrointestinal bleeding, high protein intake, catabolic states, corticosteroid use, and liver-related factors can also affect it. ^[2]

FEUrea is mainly used as an interpretation aid in acute kidney injury evaluation, not as a general kidney screening test.

How to Use the FEUrea Calculator

To use the FEUrea calculator, enter values from the same lab evaluation whenever possible.

1. Enter serum urea or BUN.
   Choose the unit shown on the lab report.
2. Enter urine urea or urine BUN.
   Use the same urea/BUN convention when available.
3. Enter serum creatinine.
   Choose mg/dL or µmol/L.
4. Enter urine creatinine.
   Choose the correct urine creatinine unit.
5. Review the FEUrea percentage.
   The calculator should return FEUrea as a percentage with a general interpretation.

Use blood and urine values collected close together. FEUrea works best when the samples reflect the same clinical moment. Do not combine an old serum creatinine with a new urine sample unless a clinician specifically asks for that comparison.

FEUrea Calculator Formula

The standard FEUrea calculator formula is:

It may also be written as:

Where:

• UUrea = urine urea or urine BUN
• SUrea = serum urea or serum BUN
• UCr = urine creatinine
• SCr = serum creatinine

The urea or BUN values should use the same convention after conversion. Creatinine units should also match after conversion. Published FEUrea studies commonly use the same structure: urine urea multiplied by plasma creatinine, divided by plasma urea multiplied by urine creatinine, then multiplied by 100. ^[3]

FEUrea Example Calculation

Imagine the lab values are:

• Serum urea/BUN: 20 mg/dL
• Urine urea/BUN: 300 mg/dL
• Serum creatinine: 1.2 mg/dL
• Urine creatinine: 100 mg/dL

Place the values into the formula:

A FEUrea of 18% is below 35%. In adult AKI evaluation, that can support a prerenal pattern. It does not prove dehydration, volume depletion, heart failure, or any single diagnosis. It simply adds one clue to the broader clinical picture.

How to Interpret Your FEUrea Result

In many adult acute kidney injury discussions, FEUrea is commonly interpreted using a 35% cutoff, but the result should still be read with the full clinical picture.

Some discussions further describe 35–50% as less specific or mixed, and values above 50% as more consistent with intrinsic tubular injury, but the main practical cutoff is commonly discussed around 35%.

A classic study found that low FEUN, commonly discussed around the 35% cutoff, helped distinguish prerenal azotemia from acute tubular necrosis, including in patients who had received diuretics.^[4] However, cutoffs are not diagnoses. A low value may support prerenal physiology, while a higher value may fit intrinsic tubular injury, but the result still needs clinical interpretation.

What FEUrea Below and Above 35% May Suggest

A FEUrea below 35% suggests the kidneys are reabsorbing more urea. Clinicians often discuss this pattern with prerenal AKI, where kidney perfusion or effective circulating volume may be reduced. Possible contexts include dehydration, low effective circulating volume, heart failure, cirrhosis-related fluid imbalance, or other conditions where the kidneys conserve water and solutes.

A FEUrea of 35% or higher may fit an intrinsic renal pattern, especially tubular injury where the kidneys cannot reabsorb urea normally. Acute tubular injury or acute tubular necrosis is often discussed in this setting. Some clinicians may view 35–50% as less specific than higher values, but the result should not be used by itself to label the cause of AKI.

When FEUrea Is Useful

FEUrea may be useful when clinicians evaluate acute kidney injury and want another urine index beyond FENa. It is especially discussed when recent diuretic use may make sodium-based interpretation less reliable.

It may also provide context when blood and urine samples were collected close together, urine output is reduced, and the care team is comparing prerenal and intrinsic renal patterns. A FEUrea result works best as part of a broader assessment that may include creatinine trends, urine microscopy, fluid balance, blood pressure, medication exposure, kidney ultrasound when needed, and the patient’s symptoms. ^[1]

FEUrea vs FENa

FENa measures fractional excretion of sodium. FEUrea measures fractional excretion of urea. Both are calculated urine indices used during AKI evaluation, but they focus on different substances.

FENa can be affected by diuretics because diuretics directly increase sodium excretion. FEUrea is often discussed as an alternative clue in that setting. However, FEUrea is not perfect. Fractional excretion markers, including sodium and urea, can be affected by drugs and are not always diagnostic. ^[1]

When FEUrea Can Be Misleading

FEUrea can be misleading in complex illness. Use caution with chronic kidney disease, sepsis, critical illness, non-oliguric AKI, urinary obstruction, mixed AKI causes, liver disease, high protein intake, gastrointestinal bleeding, catabolic states, and very young infants or neonates.

Critically ill patients deserve extra caution. One ICU study concluded that FEUrea may be of little help in distinguishing transient AKI from persistent AKI, including in patients receiving diuretics. [5] A 2024 systematic review also found that FEUrea has limitations when distinguishing intrinsic from prerenal AKI and should be interpreted carefully rather than treated as a standalone answer. ^[6]

This evidence does not make FEUrea useless. It means FEUrea should guide discussion, not replace clinician judgment.

Common Mistakes to Avoid

Common mistakes include entering serum urea in the urine field, reversing serum and urine creatinine, choosing the wrong creatinine unit, or mixing BUN and urea conventions incorrectly. Always follow the unit and label shown on the lab report.

Do not mix BUN and urea conventions unless the calculator or clinician clearly supports that unit conversion. Avoid using values from different dates, ignoring diuretic use, or treating FEUrea as a diagnosis. Adult cutoffs should also be used carefully in children and neonates because pediatric interpretation can differ.

FEUrea Calculator FAQs