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NephrologyFENa & FEUrea

FENa & FEUrea Calculator

Calculate FENa from urine and serum sodium and creatinine, with an optional separate FEUrea calculation from urine and serum urea or BUN plus the same creatinine record.

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QuickMedCalc is developed and maintained by an independent developer. Medical content is not independently reviewed by a physician.

Formula and medical content are based on the references listed on this page. See sources, About, and Sources and Review Process.

Use urine and serum values that represent the same clinical time point as closely as practical. QuickMedCalc cannot verify specimen identity or collection timing.

Shared creatinine record

Must be greater than zero.

A real unit switch clears the corresponding creatinine value.

Must be greater than zero.

A real unit switch clears the corresponding creatinine value.

FENa inputs

Sodium mmol/L and mEq/L are numerically equivalent; use the same matched assessment.

May be zero; use the value from the matched assessment.

Must be greater than zero; use the value from the matched assessment.

FEUrea inputs

Optional — leave blank when calculating FENa only. Reuses the shared creatinine record.

Changing the urea/BUN unit clears the entered value instead of relabeling the same number.

Optional FENa interpretability context
Interpretability context

These fields do not change the FENa formula. They provide evidence context for the calculated value; the calculator does not determine whether AKI, oliguria, or CKD is present. Unknown / not specified is the initial state.

Select Yes, No, or Unknown / not specified.

Do not infer exposure or a time window; select Unknown / not specified when uncertain.

Record known CKD context only; the calculator does not stage or diagnose CKD.

About

FENa and optional FEUrea share one urine/serum creatinine record but produce separate percentages. Explicit units normalize creatinine independently; whole-urea measurements normalize to a BUN basis. Use matched urine and serum measurements from the same clinical assessment.[1, 4, 11]

Neither output automatically determines AKI cause or treatment. Optional FENa context does not change arithmetic; oliguria, CKD, diuretics, specimen timing and study population affect interpretation. FEUrea is adjunctive information, not an automatic replacement.[3, 8, 9, 10]

Formula

FENa (%) = 100 × urine sodium × normalized serum creatinine ÷ (serum sodium × normalized urine creatinine). Sodium and creatinine should come from paired specimens representing the same clinical time point. Urine volume is not an equation input.[1, 2]
FEUrea (%) = 100 × normalized urine BUN × normalized serum creatinine ÷ (normalized serum BUN × normalized urine creatinine). Whole urea inputs normalize to a BUN basis; creatinine normalizes independently.[4, 8]
Creatinine mg/dL = creatinine µmol/L ÷ 88.4. Urine and serum creatinine are normalized independently, so mixed units are supported. Normalization happens before the formula, and 88.4 is the frozen conventional laboratory conversion rather than a claim of unlimited precision.[11]
Sodium units: sodium is monovalent, so mmol/L and mEq/L are numerically equal here. Urine sodium may be true zero; serum sodium must be greater than zero.
Display contract:internal arithmetic is not rounded early. FENa and normalized creatinine display two decimals; true zero is 0.00 and a tiny positive FENa displays <0.01. Software input boundaries are not clinical ranges or diagnostic thresholds.

Interpretation

FENa evidence context

The original selected oliguric populations and the historical 1% convention do not establish a universal etiologic classifier. The 1980 comparison also documented exceptions. Modern performance is strongest in oliguric patients without CKD or diuretics; population, sampling and clinical context matter.[1, 2, 3]

Kidney360 PRO, CON and commentary discuss both potential utility and important limitations. A numerical FENa alone does not identify prerenal physiology, intrinsic injury or ATN.[5, 6, 7]

FEUrea evidence context

FEUrea was proposed as adjunctive information when diuretics complicate FENa interpretation, but it is not an automatic replacement. Pépin 2007 did not establish a reliable automatic substitute. The 2024 review found limited utility without universal superiority, while the 2026 review reported moderate accuracy with low-certainty, highly heterogeneous evidence.[4, 8, 9]

Clinical boundaries

Use urine and serum values representing the same clinical time point as closely as practical; QuickMedCalc cannot verify specimen timing. AKI diagnosis and staging require a broader creatinine and urine-output framework. Neither percentage replaces urinalysis, microscopy, imaging or clinical context, or directs fluids, diuretics, dialysis or other treatment.[6, 7, 10]

References

  1. Espinel CH. The FENa test. Use in the differential diagnosis of acute renal failure. JAMA. 1976;236(6):579–581. PMID 947239. DOI 10.1001/jama.236.6.579.
  2. Espinel CH, Gregory AW. Differential diagnosis of acute renal failure. Clin Nephrol. 1980;13(2):73–77. PMID 7363517.
  3. Abdelhafez M, et al. Diagnostic Performance of Fractional Excretion of Sodium for the Differential Diagnosis of Acute Kidney Injury: A Systematic Review and Meta-Analysis. Clin J Am Soc Nephrol. 2022;17(6):785–797. PMID 35545442. PMCID PMC9269645. DOI 10.2215/CJN.14561121.
  4. Pépin MN, Bouchard J, Legault L, Ethier J. Diagnostic performance of fractional excretion of urea and fractional excretion of sodium in patients with acute kidney injury with or without diuretic treatment. Am J Kidney Dis. 2007;50(4):566–573. PMID 17900456. DOI 10.1053/j.ajkd.2007.07.001.
  5. Hamadah A, Gharaibeh K. Fractional Excretion of Sodium and Urea are Useful Tools in the Evaluation of AKI: PRO. Kidney360. 2023;4(6):e725–e727. PMID 37384887. PMCID PMC10371381. DOI 10.34067/KID.0002492022.
  6. Aron AW, Amatruda JG. Fractional Excretion of Sodium and Urea are Useful Tools in the Evaluation of AKI: CON. Kidney360. 2023;4(6):e728–e730. PMID 37384888. PMCID PMC10371370. DOI 10.34067/KID.0002512022.
  7. Brown RS. Fractional Excretion of Sodium and Urea are Useful Tools in the Evaluation of AKI: COMMENTARY. Kidney360. 2023;4(6):e731–e733. PMID 37384889. PMCID PMC10371367. DOI 10.34067/KID.0002502022.
  8. Abdelhafez MO, et al. Utility of fractional excretion of urea in acute kidney injury with comparison to fractional excretion of sodium: A systematic review and meta-analysis. Am J Med Sci. 2024;368(3):224–234. PMID 38768779. DOI 10.1016/j.amjms.2024.04.021.
  9. Pan HC, Jiang ZH, Chen HY, et al. Assessing the utility of fractional excretion of urea in distinguishing intrinsic and prerenal acute kidney injury in hospitalised patients: a systematic review and meta-analysis. BMJ Open. 2026;16(1):e100875. PMID 41535079. PMCID PMC12815071. DOI 10.1136/bmjopen-2025-100875.
  10. KDIGO Clinical Practice Guideline for Acute Kidney Injury. Kidney Int Suppl. 2012;2:1–138.
  11. NIST Chemistry WebBook. Creatinine, CAS 60-27-5. Molecular weight 113.1179.

FAQ

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Disclaimer

Educational and informational reference only. Not intended to replace professional medical advice, diagnosis, treatment, or independent verification.