Feurea Calculator - Carvounis 2002 AKI Bands

A feurea calculator is a free clinical tool that turns four routine AKI workup labs - plasma creatinine, plasma urea, urine creatinine, and urine urea - into the fractional excretion of urea percentage. The result is matched against the 35% and 50% Carvounis 2002 bands to suggest whether an acute kidney injury is more likely prerenal or intrinsic. It is the diuretic-safe companion to the FENa percentage in the AKI workup.

• • AKI pattern triage: Separate prerenal, indeterminate, and intrinsic AKI patterns from a single percentage when urine output drops or serum creatinine rises.
• • Diuretic-safe AKI workup: Confirm the prerenal band in patients on loop or thiazide diuretics, where FENa can look falsely intrinsic.
• • Teaching and review: Show trainees how the FEUrea formula links plasma and urine labs to the Carvounis 2002 AKI pattern bands.
• • Chart check: Re-check a reported FEUrea against the four input values before discussing the case with the on-call team.

Acute kidney injury shows up in three broad patterns. Prerenal AKI is a drop in kidney perfusion (volume loss, heart failure, sepsis) with healthy kidney tissue. Intrinsic AKI is direct tubular, interstitial, or glomerular damage, with acute tubular necrosis as the classic example. Postrenal AKI is obstruction and is normally confirmed with renal ultrasound rather than a fractional excretion percentage.

The feurea calculator fits the prerenal-vs-intrinsic question. A low FEUrea means the kidney is hanging on to filtered urea, which fits prerenal physiology. A high FEUrea means the damaged tubule is letting urea escape, which fits intrinsic AKI.

When the AKI workup is being done on a patient on loop or thiazide diuretics, the FENa Calculator is the most natural complement because it shares the same plasma/urine lab idea but tracks sodium instead of urea.

The Carvounis 2002 formula is used to convert four laboratory values into a single percentage. The percentage is matched against the prerenal, indeterminate, and intrinsic AKI bands.

• Serum Creatinine (SCr): Plasma creatinine from the same draw as the urine sample, in mg/dL. A high value with a low urine creatinine usually means concentrated urine and fits prerenal physiology.
• Serum Urea (SU): Plasma urea (or BUN) from the same draw, in mg/dL. Acts as the denominator against the filtered load.
• Urine Creatinine (UCr): Urine creatinine from a spot or timed collection, in mg/dL. A dilute spot urine can exaggerate the FEUrea percentage.
• Urine Urea (UU): Urine urea from the same sample, in mg/dL. A low urine urea with a high plasma urea means the kidney is reabsorbing urea, the prerenal signal.

The formula is a clearance ratio. Multiplying urine urea by serum creatinine captures how much urea the kidney lets through; dividing by serum urea times urine creatinine normalizes against urine concentration; and the factor of 100 turns the ratio into a percentage.

Interpretation is a band assignment against the Carvounis 2002 thresholds. Below 35% is the prerenal band, 35% to 50% is an indeterminate window, and above 50% fits intrinsic AKI such as acute tubular necrosis.

According to Carvounis et al, Kidney International 2002, the 35% and 50% Carvounis AKI pattern bands

Reading the feurea percentage next to the estimated GFR is often the next step, so the GFR Calculator is a useful companion for the same clinical window.

Four ideas shape how the FEUrea percentage should be read.

A correct FEUrea result still needs a clean paired sample, a working medication history, and a working knowledge of whether the picture is acute or chronic. The tool makes the math transparent but cannot replace a clinical exam, urinalysis, urine microscopy, kidney imaging, or a discussion with the on-call nephrology team.

FEUrea is one of several fractional excretion tests used during an AKI workup. FENa tracks sodium, FEUrea tracks urea, and FePhos is occasionally used in paracetamol toxicity. Each test has its own band thresholds.

The same paired-sample idea shows up in proteinuria work, where the Protein Cr Ratio Calculator expresses urine protein as a ratio to urine creatinine and fits the same lab-driven AKI workup.

Enter each value once, read the percentage, then read the band label.

1. 1 Enter serum creatinine: Use the serum creatinine in mg/dL from the same draw. If the lab reports creatinine in micromol/L, convert it first.
2. 2 Enter serum urea (or BUN): Use the plasma urea in mg/dL from the same draw. If the lab reports BUN only, convert BUN to plasma urea first.
3. 3 Enter urine creatinine: Use the urine creatinine in mg/dL from a spot or timed collection taken around the same time as the blood draw.
4. 4 Enter urine urea: Use the urine urea in mg/dL from the same sample. A value of 0 gives an FEUrea of 0%.
5. 5 Read the percentage and band: Read the percentage first, then the band label (prerenal, indeterminate, or intrinsic AKI).
6. 6 Document context: Write the band, the percentage, the four input values, and the time of the draw. If the patient is on diuretics, mention FENa so the team can compare the two tests.

A 64-year-old on the medical ward has serum creatinine 2.0 mg/dL, serum urea 60 mg/dL, urine creatinine 100 mg/dL, and urine urea 1000 mg/dL from a paired draw. The tool shows 33.33% with a prerenal pattern band. The team confirms low urine output, gives a small fluid challenge, and repeats the labs in four hours.

When the AKI picture raises an obstruction concern, the Kidney Stone Calculator organizes stone size, location, and symptoms so the feurea band and the obstruction context can be reviewed together.

The feurea calculator is meant to make a routine AKI workup faster and easier to read. It runs the Carvounis 2002 math and the band assignment in one pass.

• • Transparent math: Every input, the Carvounis 2002 formula, and the percentage are visible on the page. The result can be cross-checked against a hand calculation.
• • Built-in Carvounis band assignment: The prerenal (< 35%), indeterminate (35% to 50%), and intrinsic AKI (> 50%) bands are stated up front, so the percentage reads as a short triage note.
• • Diuretic-safe companion to FENa: FEUrea stays useful on loop and thiazide diuretics, where FENa can look falsely intrinsic. The result panel pairs the two tests so the diuretic history can be read into the band.
• • Pairs with the AKI workup: The tool is built to sit next to the FENa calculator, the GFR calculator, the protein-creatinine ratio, and the anion gap on the AKI workup screen.
• • Useful in teaching: A trainee can enter the same labs twice with different urine urea values to see how the band shifts, which makes prerenal versus intrinsic urea physiology easier to grasp.

This tool is also useful when an AKI workup is being done for a patient on chronic diuretic therapy. FENa can be misleading in that setting, and FEUrea is the standard fallback test.

For a longer-view kidney marker, the ACR Calculator converts spot urine albumin and creatinine into the albumin-to-creatinine ratio in mg/g, which is the KDIGO A1/A2/A3 category for ongoing kidney damage.

Several factors can move the FEUrea percentage away from the true underlying physiology. The percentage is only as clean as the sample and the medication history.

• • The tool is not a stand-alone diagnostic test. The percentage must be combined with urine microscopy, urine output history, blood pressure trend, medication review, and often renal imaging before a clinical decision is made.
• • The 35% and 50% Carvounis bands were validated in a hospital-acquired AKI cohort. They are not validated for chronic kidney disease, glomerulonephritis, postrenal obstruction, or pediatric AKI, and the band label should be read with the clinical context in mind.

The result panel prints a soft diuretic-safe reminder, but the clinician still has to check the medication list and the timing of the most recent diuretic dose before accepting the band label.

According to Perazella and Coca, CJASN 2012, FEUrea is the diuretic-safe alternative to FENa

According to National Kidney Foundation AKI overview, AKI is identified when the kidneys suddenly lose the ability to filter waste

When the feurea result has to be read with the electrolyte picture, the Anion Gap Calculator turns the basic metabolic panel into a serum anion gap so the urea and bicarbonate context can be reviewed together.