Calculate paired urine and serum FEUrea (FEUN) after independent exact normalization of BUN mg/dL or whole urea mmol/L and urine/serum creatinine in mg/dL or µmol/L, with no AKI-cause or treatment output.
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This page calculates FEUrea from paired urine and serum specimens using urine and serum BUN or whole urea plus urine and serum creatinine. Each urea-related result is normalized independently to BUN mg/dL and each creatinine result to mg/dL. It reports one percentage and four normalized audit values. FEUN and FEUrea refer here to the same fractional-excretion- of-urea-nitrogen arithmetic. It does not calculate FENa, AKI diagnosis, stage, or cause.[1, 11, 12, 13]
Carvounis 2002 supported the historical prerenal context at or below 35%, including its diuretic-treated study group. Later Pépin and Darmon studies found substantially weaker performance. The 35% convention was never a certain etiologic boundary for every person with AKI, and this page never assigns prerenal, intrinsic, or ATN labels.[1, 2, 3]
Current syntheses remain limited: the 2024 review found limited utility without universal superiority over FENa, while the 2026 review reported moderate accuracy and a possible adjunct role with high heterogeneity and low certainty. Loop diuretics, critical illness, and specimen timing can change interpretation. This page does not diagnose, stage, determine cause, or generate treatment advice.[4, 5, 6, 7, 8, 9]
creatinine mg/dL = creatinine µmol/L ÷ 88.4
Urine and serum creatinine are independently normalized before the equation. [13]
Display: FEUrea and four normalized concentrations to two decimals
Intermediate arithmetic is not rounded; true zero remains 0.00 and a tiny positive value is shown as <0.01. Software input limits are not clinical reference intervals or diagnostic thresholds.
| Quantity | What it represents | Calculated here |
|---|---|---|
| Urine BUN or whole urea | Urea-related concentration in paired urine | Input / normalized |
| Serum BUN or whole urea | Urea-related concentration in paired serum or plasma | Input / normalized |
| Urine creatinine | Urine concentration denominator adjustment | Input / normalized |
| Serum creatinine | Paired filtration-related concentration | Input / normalized |
| FEUrea / FEUN | Calculated fractional-excretion percentage | Yes |
| FENa | Fractional excretion of sodium | No |
| Historical 35% classification | Study-specific interpretive convention | No |
| AKI diagnosis or stage | Serum-creatinine and urine-output clinical framework | No |
| Prerenal, intrinsic, or ATN cause | Etiologic assessment | No |
| Fluid, diuretic, dialysis, or treatment decision | Individual management | No |
FEUrea is a percentage, not a urine urea concentration; FEUrea and FEUN name the same arithmetic concept on this page. The same percentage can arise from different absolute analyte values. It cannot replace the raw analytes, trends, urine output, urinalysis, microscopy, imaging, or clinical assessment, and the page generates no 35% classification, diagnosis, cause, or treatment output.
It does not read baseline or serial creatinine, urine-output history, diuretic type, dose or timing, CKD, sepsis, urinalysis, microscopy, volume status, hemodynamics, obstruction imaging, diagnosis, or treatment information.
FEUN emphasizes the nitrogen portion of urea; FEUrea is the common clinical shorthand used here for the same fractional-excretion-of-urea-nitrogen arithmetic. The runtime converts a whole-urea concentration to a BUN basis. BUN and whole urea are different reporting quantities, so changing only the unit label is unsafe. Independent urine and serum normalization is especially important when the two specimens use different reporting systems.[1, 11, 12]
Urine urea/BUN is converted first, serum urea/BUN is converted separately, and the two creatinine values are each converted independently before the cross-product is formed. Even when a shared conversion factor could cancel in an ideal same-basis ratio, this implementation still normalizes every input so mixed units remain explicit and the audit values expose unit errors. No intermediate value is rounded.[11, 12, 13]
The four analytes should represent the same clinical time point as closely as practical. Fluids, diuretics, hemodynamics, catabolism, and changing kidney function can alter the inputs. Combining different times can produce a percentage that described no single assessment. Clearing confirmation and stale output after a value or unit changes is a software safety measure.[2, 3, 9]
FEUrea is indirect arithmetic for the fraction of a filtered urea-nitrogen load appearing in urine, using creatinine ratios to approximate the filtration relationship. It is not a direct tubular-injury measurement, GFR, eGFR, creatinine clearance, or daily urea excretion. A spot result describes one sampling time point.[1, 9]
The early study evaluated 102 episodes of acute renal failure: 50 prerenal episodes without diuretics, 27 prerenal episodes with diuretics, and 25 cases of acute tubular necrosis. In the diuretic-treated prerenal group, 89% had FEUN below 35%. Those observations established the historical 35% context, but this was one selected cohort rather than universal modern validation.[1]
Low FEUrea was historically associated with a prerenal context. Later diagnostic-accuracy syntheses sometimes align direction so a higher value predicts intrinsic injury. Study direction is therefore not uniform. This page does not display low/high, positive/negative, prerenal, intrinsic, ATN, persistent, or transient labels and does not turn 35% into a color boundary.[5, 6]
In 99 patients with AKI, FEUrea sensitivity and specificity were 48% and 75% without diuretics, and 79% and 33% with diuretics. The authors concluded that limited specificity prevented FEUrea from serving as a reliable automatic alternative. This page selects no cutoff from that comparison.[2]
Darmon 2011 included 203 ICU patients. FEUrea distinguished transient from persistent AKI with an AUC of 0.59; at 35%, sensitivity was 63% and specificity 54%, with similar results in the diuretic subgroup. Critical illness, sepsis, and complex kidney physiology cannot be assigned by one percentage.[3]
Cox 2020 followed 297 patients with decompensated heart failure. After IV loop diuretic, FEUrea increased by a mean 5.6 percentage points and 27% crossed the 35% category; change correlated with diuretic response. This directly challenges the assumption that FEUrea is unaffected by loop diuretics, but the heart-failure study did not validate every AKI cause.[4]
The 2024 meta-analysis included 11 studies and 1,108 hospitalized patients. Across eight studies and 915 patients using FEUrea above 35% to distinguish intrinsic from prerenal AKI, sensitivity was 66% and specificity 75%. In four studies with 244 diuretic-treated patients, FEUrea sensitivity and specificity were 52% and 82%, compared with 92% and 44% for FENa. The synthesis found limited utility and did not establish universal superiority over FENa, even with diuretics; these are study estimates, not patient-level probabilities.[5]
The 2026 analysis included 12 studies and 1,240 patients. Overall sensitivity was 0.74, specificity 0.78, and SROC AUC 0.83, with I² of 90% for sensitivity and 93% for specificity. The diuretic subgroup had sensitivity 0.70, specificity 0.87, and AUC 0.86, while subgroup heterogeneity remained substantial. GRADE certainty was low. The authors described moderate accuracy and a possible adjunct role—not stand-alone diagnosis, universal replacement, or certain etiology.[6]
FEUrea and FENa use different analytes, so one result cannot be converted into the other. The linked PRO, CON, and commentary articles document an expert debate rather than consensus. The 2024 and 2026 reviews used different study sets, diagnostic-direction handling, and statistical models; their pooled results must not be mechanically averaged. This page claims neither test is universally superior and never recommends calculating or abandoning FENa from a submitted FEUrea result.[5, 6, 7, 8, 9]
Urine BUN 300 mg/dL, serum BUN 60 mg/dL, urine creatinine 100 mg/dL, and serum creatinine 2 mg/dL produce raw FEUrea 10%, displayed as 10.00%, with normalized audit values 300.00, 60.00, 100.00, and 2.00 mg/dL.
Implementation, unit, and display audit only—not AKI diagnosis, staging, cause classification, or fluid, diuretic, or treatment instruction.
Urine urea 100 mmol/L, serum urea 20 mmol/L, urine creatinine 8840 µmol/L, and serum creatinine 176.8 µmol/L normalize to 280.112, 56.0224, 100, and 2 mg/dL, producing raw FEUrea 10% and display 10.00%.
Implementation, unit, and display audit only—not AKI diagnosis, staging, cause classification, or fluid, diuretic, or treatment instruction.
Urine BUN 280.112 mg/dL, serum urea 20 mmol/L, urine creatinine 100 mg/dL, and serum creatinine 176.8 µmol/L produce the same normalized values, raw 10%, and display 10.00%.
Implementation, unit, and display audit only—not AKI diagnosis, staging, cause classification, or fluid, diuretic, or treatment instruction.
Urine BUN 0 mg/dL with the other main-vector inputs produces raw FEUrea 0 and display 0.00%. This arithmetic alone cannot determine an AKI cause.
Implementation, unit, and display audit only—not AKI diagnosis, staging, cause classification, or fluid, diuretic, or treatment instruction.
Urine BUN 0.049 mg/dL with serum BUN 60 mg/dL, urine creatinine 100 mg/dL, and serum creatinine 2 mg/dL produces raw FEUrea about 0.00163333%, displayed as <0.01%; normalized urine BUN displays 0.05 mg/dL rather than a false zero.
Implementation, unit, and display audit only—not AKI diagnosis, staging, cause classification, or fluid, diuretic, or treatment instruction.
Reliability may be reduced by unpaired specimens, rapidly changing AKI, CKD, sepsis or critical illness, active or recent diuretics, heart failure or rapid decongestion, catabolism, high protein intake, gastrointestinal bleeding, glucocorticoids, altered hepatic urea synthesis, dialysis, unusual muscle mass, assay or unit error, or reviewing the percentage without all four analytes.[2, 3, 4, 8]
It cannot determine AKI diagnosis or stage, prerenal, intrinsic, ATN, glomerular or obstructive cause, volume status, tubular-injury severity, fluid responsiveness, microscopy, imaging, dialysis risk, referral, admission, medicine, or treatment. AKI diagnosis and staging belong to the broader serum-creatinine and urine-output framework.[9, 10]
FEUrea (%) = 100 × normalized urine BUN × normalized serum creatinine ÷ (normalized serum BUN × normalized urine creatinine). Whole-urea inputs are independently converted to a BUN basis with 2.80112, and creatinine inputs are independently converted to mg/dL with 88.4, before the unrounded calculation.
No. Carvounis 2002 supplied the early favorable context, but Pépin 2007 did not establish a reliable automatic substitute and Cox 2020 showed that loop diuretics can change FEUrea. The 2024 review found limited utility without universal superiority, while the 2026 review reported moderate accuracy but low-certainty, highly heterogeneous evidence. FEUrea or FEUN may be an adjunct in some settings; it is not an automatic replacement.
Critical illness, sepsis, CKD, loop diuretics, heart-failure decongestion, catabolism, sampling time, and study population can change the inputs or the meaning of a result. The 2024 and 2026 reviews therefore support cautious adjunctive use rather than a universal diagnosis rule.
FENa
Calculate fractional excretion of sodium from paired urine and serum sodium plus independently normalized urine and serum creatinine in mg/dL or µmol/L, returning a percentage without a universal cutoff, AKI-cause classification, or treatment output.
BUN/Cr Ratio
Calculate a same-specimen or clinically paired conventional BUN-to-creatinine ratio after exact normalization of BUN mg/dL or urea mmol/L and creatinine mg/dL or µmol/L, without diagnostic or treatment output.
Urine Output
Calculate measured urine output for one completed collection interval as average mL/hr and average mL/kg/hr using protocol-selected weight, without AKI diagnosis, stage, or treatment output.
Disclaimer
Educational and informational reference only. Not intended to replace professional medical advice, diagnosis, treatment, or independent verification.