Fractional Excretion of Urea (FEUrea) Calculator

About the Fractional Excretion of Urea (FEUrea) Calculator

The Fractional Excretion of Urea (FEUrea) Calculator estimates what percentage of filtered urea is excreted in the urine. It is mainly used as a bedside AKI aid when diuretics make FENa harder to interpret, because loop and thiazide diuretics directly change urinary sodium handling but have less direct effect on the urea-based calculation.

This page converts the entered urea and creatinine measurements into a common unit system, applies the standard FEUrea formula, and places the result into the usual prerenal, indeterminate, or intrinsic-renal bands. It also keeps the FEUrea-versus-FENa context visible so the result is not read in isolation.

The output is still only one part of AKI assessment. Volume status, urine sediment, medication exposure, obstruction, CKD, sepsis, and the rest of the renal workup can change what the number means in practice.

When This Page Helps

When AKI is being evaluated in a patient on diuretics, the usual sodium-based indices are harder to interpret. This calculator keeps the urea and creatinine inputs together, applies the same fractional-excretion structure every time, and makes the prerenal-versus-intrinsic split easier to review alongside the rest of the urine studies already being collected.

How to Use the Inputs

1. Enter urine urea concentration and select the unit (urea mg/dL, BUN mg/dL, or mmol/L).
2. Enter plasma urea in the same unit system.
3. Enter urine creatinine and plasma creatinine with appropriate units (mg/dL or µmol/L).
4. Indicate whether the patient is on diuretics (this guides interpretation emphasis).
5. Review FEUrea result, AKI classification, and the FEUrea vs FENa comparison table.

Example Calculation

Tips & Best Practices

• FEUrea is most valuable when the patient is on diuretics — otherwise, FENa is generally the first-choice test.
• Collect urine and blood samples simultaneously for accurate fractional excretion calculations.
• BUN (blood urea nitrogen) and urea are different measurements — make sure to select the correct unit type.
• In sepsis-related AKI, both FENa and FEUrea may be misleading due to the mixed hemodynamic-inflammatory pathophysiology.
• Urine microscopy (muddy brown casts) remains the most specific finding for ATN and should always accompany urinary indices.
• Serial measurements may be more informative than a single value — trending FEUrea over 24–48 hours can reveal the direction of injury.

Why FEUrea Can Help

FEUrea is most often reviewed when a patient has already received diuretics and the usual sodium-based indices become harder to interpret. The calculation does not solve AKI by itself, but it can add another structured data point when volume status, urine sediment, and medication exposure are all being reviewed together.

Keep the Limitations Visible

CKD, sepsis, gastrointestinal bleeding, low urea production, urine-flow changes, and mixed etiologies can all make FEUrea less decisive. The overlap zone is common, which is why the page keeps the indeterminate range visible instead of forcing a binary answer.

Read It Beside the Rest of the Workup

Urine microscopy, the clinical course, ultrasound when obstruction is possible, and repeat chemistry often do more to sort out AKI than one urinary index alone. FEUrea is most useful when it helps organize that broader picture rather than replacing it.

Frequently Asked Questions

• Why is FEUrea often used when diuretics are on board?

  Loop and thiazide diuretics directly change urinary sodium handling, which can push FENa upward even when the patient is still sodium avid. FEUrea is often less distorted by that effect, so it can be a helpful additional clue after diuretics have been given. It is still not perfect, and the number should be read alongside urine sediment, volume status, and the rest of the AKI workup.

• What are the limitations of FEUrea?

  FEUrea has several limitations: 1) Less extensively validated than FENa (fewer studies), 2) Overlap zone (35–50%) can be wide, 3) High-protein diet or GI bleeding can increase urea production independently, 4) CKD alters baseline urea handling, 5) Severe liver disease reduces urea production, 6) It doesn't account for the same intrinsic-renal exceptions as FENa (contrast nephropathy, rhabdomyolysis). Like FENa, it should be one part of the AKI workup, not the sole determinant.

• Can I use both FENa and FEUrea together?

  Yes — when both are available, concordant results strengthen the diagnosis. If FENa and FEUrea both point to prerenal (<1%, <35%), prerenal is very likely. If FENa is elevated (>1%) but FEUrea is low (<35%) in a patient on diuretics, prerenal is likely (diuretics explain the elevated FENa). If both are elevated (FENa >2%, FEUrea >50%), intrinsic AKI is strongly suggested. Discordant results in a patient NOT on diuretics require additional workup.

• What is the difference between BUN and urea?

  BUN (Blood Urea Nitrogen) measures only the nitrogen portion of the urea molecule, while "urea" measures the entire molecule. The conversion factor is: Urea = BUN × 2.14 (molecular weight ratio: 60/28). In the US, labs typically report BUN (mg/dL), while international labs often report urea (mmol/L or mg/dL). This distinction matters for calculations — using BUN values in a formula expecting urea (or vice versa) will give incorrect results. Our calculator handles the conversion automatically.

• When should I consider renal biopsy in AKI?

  Renal biopsy is considered when: 1) AKI doesn't fit a clear pattern (not obviously prerenal, ATN, or obstructive), 2) Features suggest glomerulonephritis (hematuria, proteinuria, red cell casts, low complement), 3) AKI persists despite appropriate management (unexplained persistent AKI beyond 3–4 weeks), 4) Systemic disease workup is positive (ANCA, anti-GBM, ANA), 5) Drug-induced interstitial nephritis is suspected but steroids are being considered. Biopsy provides definitive histological diagnosis and guides specific therapy.

• How do novel AKI biomarkers compare to FEUrea?

  Novel biomarkers (NGAL, KIM-1, IL-18, TIMP-2×IGFBP7) detect tubular injury hours before creatinine rises, whereas FEUrea (like FENa) requires creatinine elevation to calculate. The biomarkers identify "subclinical AKI" — tubular damage before it manifests as decreased GFR. TIMP-2×IGFBP7 (NephroCheck) is FDA-cleared for AKI risk assessment in ICU patients. However, these biomarkers complement rather than replace traditional indices: they detect injury early, while FENa/FEUrea help classify the mechanism after AKI is established.