Clinical Overview
Fractional excretion of sodium is a bedside decision-support calculation used to standardize a defined part of clinical assessment. It reports the fraction of filtered sodium excreted and can support physiologic assessment in selected patients with acute kidney injury. The result should be interpreted with the patient’s presentation, baseline function, serial observations, laboratory method, medication exposure, and the protocol used by the treating institution. It is an aid to communication and risk stratification rather than an independent diagnosis or treatment order.
Enter contemporaneous values and confirm every unit before calculating. When physiology is changing, repeat assessment and prioritize the clinical trajectory over a single number. Document the input values alongside the result so another clinician can reproduce the calculation and recognize assumptions. Thresholds describe populations and do not remove the need to investigate discordant symptoms, examination findings, imaging, or biomarkers.
When to Use
- Adjunctive evaluation of oliguria or acute kidney injury
- Paired urine and plasma sodium and creatinine samples
Limitations & Safety
Diuretics and many kidney disorders alter tubular sodium handling; classic cutoffs perform imperfectly outside oliguric patients without CKD. Validation cohorts, case mix, prevalence, and treatment era affect observed event rates. Pregnancy, extremes of age or body composition, critical illness, and major comorbidity may reduce transportability unless specifically represented in the original model. Do not extrapolate beyond the stated population or substitute this estimate for a validated local pathway.
Before acting, check for missing data, measurement error, competing diagnoses, contraindications, and time-sensitive emergencies. A low-risk label never overrides clinician concern; a high-risk label does not prove the target condition. Discuss consequential decisions with the appropriate senior or specialty team and use current regional guidance.
- Diuretics, CKD, sepsis, contrast injury, pigment nephropathy, and obstruction can confound interpretation
- Never classify AKI etiology from FeNa alone
Formula / Scoring Criteria
FeNa (%) = 100 × (Urine Na × Plasma creatinine) / (Plasma Na × Urine creatinine)
Interpretation
- <1% — Sodium-avid pattern: May support prerenal physiology. Assess volume, perfusion, and competing causes.
- 1–2% — Indeterminate: Substantial overlap. Use sediment, history, imaging, and trends.
- >2% — Higher sodium excretion: May support intrinsic tubular injury. Evaluate intrinsic, obstructive, drug, and other causes.
Evidence & References
- Espinel CH. The FENa test. Use in differential diagnosis of acute renal failure.
- Seethapathy H, Fenves AZ. Fractional Excretion of Sodium and Urea in AKI.
Clinical FAQs
Can FeNa diagnose prerenal AKI?
No. It is an adjunct and must be combined with clinical findings and urine microscopy.
What happens after diuretics?
Diuretics raise urinary sodium and can make FeNa misleading; alternative indices also have limitations.
Must creatinine units match?
Yes. Urine and plasma creatinine must use the same concentration units so they cancel correctly.