FENa Calculator

FENa Calculator for Nephrology. A low value indicates sodium retention at sampling. It does not prove reversible hypovolemia; a higher value does not independently diagnose tubular necrosis.

For shared guidance on choosing a tool and documenting results, read the clinical calculator workflow.

How this calculator works

FENa =100 x (urine sodium x plasma creatinine)/(plasma sodium x urine creatinine). Sodium units must match each other and creatinine units must match each other.

When to use this calculator

As a contextual urine index during AKI assessment using appropriately paired specimens. It is most interpretable in selected situations resembling the original oliguric populations.

Inputs used

  • Urine sodium
  • Serum sodium
  • Urine creatinine
  • Serum creatinine

Clinical interpretation

A low value indicates sodium retention at sampling. It does not prove reversible hypovolemia; a higher value does not independently diagnose tubular necrosis.

Worked example

Urine sodium 12, plasma sodium 140, urine creatinine 90 and plasma creatinine 3, with matching units, give 100 x 36/12600=0.286%.

Limitations and safety notes

Diuretics, CKD, timing and several kidney-injury mechanisms can alter the result. FEUrea is not a diuretic-proof replacement. Neither index directly measures fluid responsiveness.

Frequently asked questions

Does FENa below 1%require a fluid challenge?

No. A fluid decision needs volume and perfusion assessment and consideration of harm.

Can I mix urine creatinine mg/dL with plasma micromol/L?

No. Convert them to the same units before calculating.

References

  • Espinel CH. The FENa test. JAMA. 1976. DOI: 10.1001/jama.1976.03270060029022. https://jamanetwork.com/journals/jama/article-abstract/347299
  • Cox ZL et al. Effect of Loop Diuretics on the Fractional Excretion of Urea in Decompensated Heart Failure. 2020. PMID: 32007554. https://pubmed.ncbi.nlm.nih.gov/32007554/

Editorial review and citation methodology

Maintained by the Quick Medical Calculator Editorial Team. Content record date: September 7, 2026. A content date is not evidence of independent clinical review. Individual clinical sign-off is not recorded on this page.

  • Prefer original validation studies for scoring systems and prediction tools.
  • Use current specialty society guidance, transplant allocation policy, public health guidance, or regulator resources when they govern clinical use.
  • Include limitations and safety notes when a calculator is population-specific, context-dependent, or unsuitable as a standalone decision tool.

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