Skip to content
Hakeemly
ع

Fractional Excretion of Sodium (FENa)

Fractional excretion of sodium from paired plasma and urine sodium and creatinine.

For healthcare professionalsKidneyv1.0.1

For healthcare professionals / clinical use. Not a self-diagnosis tool.

Your measurements

mmol/L
mmol/L
mg/dL

Calculated on your device. Nothing is sent to a server.

Your result will appear here

Enter your measurements, then press Calculate.

Methodology

FENa (Espinel 1976)

FENa = (UNa × PCr) ÷ (PNa × UCr) × 100

Interpretation is an aid in the context of oliguric AKI and must be combined with clinical assessment. Espinel 1976 reported under 1% in prerenal azotaemia and over 3% in acute tubular necrosis; the 2% boundary is a later convention.

Who it is for
Adults with acute kidney injury and oliguria, not on diuretics.
Version
1.0.1

How this calculator works

Where the formula comes from

  • Espinel (1976) measured FENa in the oliguric phase of acute renal failure: prerenal azotaemia gave values below 1% and acute tubular necrosis above 3%.[1]

The inputs

  • FENa = (urine sodium × plasma creatinine) ÷ (plasma sodium × urine creatinine) × 100.[1]

Versions and alternatives

  • A 2024 meta-analysis found FEUrea above 35% had limited accuracy (sensitivity 66%, specificity 75%) and was not a better alternative to FENa, even in patients on diuretics.[2]

Where accuracy falls (published evidence)

  • A 2022 meta-analysis (19 studies, 1,287 patients) found pooled sensitivity 90% and specificity 82% at a 1% threshold for intrinsic versus prerenal AKI.[3]
  • Specificity fell to 54% in patients on diuretics; in oliguric patients without CKD or diuretics sensitivity was 95% and specificity 91%.[3]

Sources

  1. Espinel CH. The FENa test. Use in the differential diagnosis of acute renal failure. JAMA. 1976;236(6):579–581. doi:10.1001/jama.1976.03270060029022
  2. 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. doi:10.1016/j.amjms.2024.04.021
  3. Abdelhafez MO, Nayfeh T, Atieh A, 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. doi:10.2215/CJN.14561121

Compiled from the published sources above. Every statement was agreed independently by three AI research tools (Claude, ChatGPT, DeepSeek) and its source verified in PubMed or Crossref by the Hakeemly editorial team. Not reviewed by a doctor unless stated. Agreed on

Worked example: how to calculate FENa

Inputs

Plasma sodium
140 mmol/L
Urine sodium
20 mmol/L
Plasma creatinine
2 mg/dL
Urine creatinine
100 mg/dL

Result

FENa
0.29 %

Under 1%: pattern associated with prerenal states

Calculation:

20×2 / (140×100) × 100 = 0.286

An illustration with sample values, worked out with the same method the tool above uses.

Limitations

  • Less reliable in people with chronic kidney disease or taking diuretics; most useful in selected patients with oliguria (2022 meta-analysis). Interpret with the clinical picture.
  • FEUrea was not a better alternative, even with diuretics (2024 meta-analysis).

References

  1. Espinel CH. The FENa test. Use in the differential diagnosis of acute renal failure. JAMA. 1976;236(6):579–581. doi:10.1001/jama.1976.03270060029022
  2. Abdelhafez MO, Nayfeh T, Atieh A, 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. doi:10.2215/CJN.14561121
  3. 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. doi:10.1016/j.amjms.2024.04.021

You may also find useful