FENa Calculator – Best Ultimate Tool – BilalMD

A FeNa calculator helps you rapidly screen whether acute kidney injury (AKI) appears to be prerenal (under-perfusion) or intrinsic (e.g., acute tubular injury). You enter spot urine sodium, urine creatinine, plasma sodium, and plasma creatinine, and the tool returns FeNa (%)—then you interpret it in a clinical context.

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What FeNa actually measures

FeNa = the fraction of filtered sodium that the kidney excretes in urine. In prerenal states, the nephron avidly reabsorbs sodium to conserve volume, so FeNa tends to be low; in intrinsic tubular injury, sodium reabsorption is impaired, so FeNa tends to be higher. That’s why it can help you triage AKI at the bedside, especially in oliguric patients.

FeNa is a calculation, not a lab test. You compute it from paired urine and blood chemistries (Na⁺ and creatinine) drawn at roughly the same time.

How to calculate FeNa (step by step)

Formula

FeNa (%) = 100 × (Urine Na × Plasma Cr) / (Plasma Na × Urine Cr)

Units that “just work”

• Sodium can be mEq/L or mmol/L (numerically the same for Na⁺).
• Creatinine can be mg/dL or µmol/L (convert: 1 mg/dL = 88.4 µmol/L).

Worked example

• Plasma Na = 140 mEq/L; Plasma Cr = 2.0 mg/dL
• Urine Na = 15 mEq/L; Urine Cr = 100 mg/dL
• FeNa = 100 × (15 × 2.0) / (140 × 100) = 0.21% → low, which supports a prerenal process when matched to the story and exam.

FENa Interpretation Guide

FENa Value	Clinical Interpretation
< 1%	Suggests prerenal cause (volume depletion, hypoperfusion)
> 2%	Suggests intrinsic renal cause (e.g., ATN)
1–2%	Indeterminate zone – consider clinical context

Interpreting FeNa (without over-interpreting it)

Classic bedside cutoffs (heuristics):

• FeNa < 1% → supports prerenal AKI (e.g., volume depletion, effective arterial underfilling).
• FeNa > 2% (some use >3%) → supports intrinsic tubular injury such as ATN.
  These are guidelines, not absolutes; always consider them in conjunction with the clinical picture.

Why FeNa can mislead (pitfalls):

• Diuretics increase urine sodium and can falsely raise FeNa even in prerenal states.
• CKD, glomerulonephritis, obstruction, sepsis, contrast injury, and early disease—all can blunt the correlation between FeNa and the true mechanism.
• Many early FeNa studies excluded these groups, so real-world accuracy varies.

Bottom line: FeNa is supportive evidence, not a diagnosis. Combine it with history, exam (volume status), urinalysis (casts, sediment), ultrasound when needed, and labs (BUN/Cr ratio, bicarbonate, potassium), plus the overall course.

FeUrea vs FeNa: when diuretics muddy the waters

If the patient recently received loop diuretics, clinicians often look at fractional excretion of urea (FeUrea) instead, because urea handling is less affected by loop action. A FeUrea < 35% can support prerenal physiology, whereas higher values suggest intrinsic tubular dysfunction—again, only in the right context. Evidence is mixed: some experts find FeUrea/FeNa useful tools, others emphasize limitations and selection biases. Use them as adjuncts, not arbiters.

Quick decision pathway for FENa AKI questions

1. Is this true AKI? Confirm rising creatinine and/or reduced urine output; consider baseline and timeline. (Remember: eGFR equations are unreliable in non-steady-state AKI.)
2. Get the basics: History (GI losses, hemorrhage, heart failure, cirrhosis), meds (ACEi/ARBs, NSAIDs, diuretics, contrast), vitals, JVP, edema, mucous membranes.
3. Urinalysis/sediment: Muddy brown casts push you toward ATN; bland sediment favors prerenal.
4. Run your tests together: Spot urine Na and Cr with paired plasma Na and Cr → use your fena calculator.
5. Interpret with caution:
   • FENa prerenal: <1% and the story fits (hypovolemia, CHF, cirrhosis) → resuscitate/optimize perfusion.
   • FENa intrinsic: >2–3% with compatible sediment → evaluate for ATN or other intrarenal processes.
   • On diuretics/CKD or mixed pictures → consider FeUrea, urine osmolality, and repeat assessments.

When not to lean on FeNa

• Non-oliguric AKI: the traditional performance of FeNa is best in oliguria; outside that, its discrimination drops.
• Recent diuretics: FeNa may be falsely high; try FeUrea, but remain skeptical.
• Advanced CKD or GN: baseline tubular handling is different; FeNa thresholds don’t port reliably.
• Obstruction: postrenal causes can give variable FeNa; image when suspected.
• Sepsis or multi-hit AKI: physiologies are mixed; one number won’t capture it.

Clinical pearls to keep your fena calculation honest

• Time your samples. Draw urine and blood around the same time to reflect the same physiologic state.
• Mind units. Your calculator handles them, but remember: Na mmol/L = mEq/L; creatinine 1 mg/dL = 88.4 µmol/L.
• Don’t forget the sediment. The presence of granular (muddy brown) casts is more specific for ATN than any fractional excretion number.
• Follow the patient. The trend in urine output and creatinine after fluid optimization often tells you more than a single FeNa snapshot.
• Guidelines on diuretics in AKI. Major guidelines recommend against using diuretics to prevent or treat AKI (except to manage volume overload)—use your numbers to reason, not to justify routine diuretics.

Example scenarios (so your fena calc isn’t abstract)

1) Vomiting and hypotension
A young adult with 3 days of vomiting, tachycardia, dry mucosae, and oliguria. FeNa comes back 0.3%. This strongly supports prerenal volume depletion. With cautious fluids, urine output rises and creatinine falls over 24–48 hours—diagnosis and treatment line up.

2) Septic shock on diuretics
An older patient, diuretic exposure, hypotension, and vasopressors. FeNa is 2.1%, FeUrea 28%. Mixed data: diuretics can inflate FeNa, whereas FeUrea <35% can suggest prerenal physiology—but urine sediment shows granular casts and creatinine keeps rising. Consider a mixed prerenal + ATN picture. Manage perfusion, avoid nephrotoxins, and monitor closely.

3) Chronic kidney disease with AKI on CKD
Person with CKD 4, peripheral edema, on ACEi and loop diuretic. FeNa is 1.4%. Thresholds are less reliable in CKD; rely on history, exam, ultrasound, and labs. Fractional indices are adjuncts only here.

👉 Read the complete FeNa Calculator guide now.

Frequently asked questions — Fractional excretion of sodium calculator

What is FeNa and what does a FeNa calculator do?

FeNa is the fraction of filtered sodium excreted in urine. A FeNa calculator plugs in urine and plasma Na and creatinine to return FeNa (%), helping you classify AKI physiology (prerenal vs intrinsic) when used with the clinical picture.

How do I calculate FeNa by hand?

Use FeNa (%) = 100 × (Urine Na × Plasma Cr) / (Plasma Na × Urine Cr). Keep sodium units the same and convert creatinine if needed (1 mg/dL = 88.4 µmol/L). Or just calculate FeNa with the widget above.

What FeNa value is “normal”?

There isn’t a “normal” in the abstract; FeNa is contextual. In many prerenal states, it’s <1%, while >2–3% suggests intrinsic tubular injury—but exceptions are common, so interpret alongside the story, exam, and urinalysis.

Can I trust FeNa if the patient is on diuretics?

Be careful—loop diuretics raise urinary sodium and can falsely elevate FeNa. Consider adding FeUrea (cutoff around <35% to support prerenal), but remember that studies show mixed performance; neither index is definitive.

Is FeNa useful in non-oliguric AKI?

Less so. Its historical utility is strongest in oliguric patients; accuracy falls in broader AKI populations and in conditions like CKD, GN, and obstruction.

What else should I check besides FeNa?

Volume status, recent fluid balance, urine sediment (casts), medication review, renal ultrasound when indicated, and the trajectory of creatinine/urine output after resuscitation.

Do guidelines recommend diuretics for AKI?

No. Major guidance advises against routine diuretics to prevent or treat AKI (use only for volume overload). FeNa or FeUrea should not be used to justify diuretic use.

Where do the FeNa cutoffs come from?

From classic observational work that associated <1% with prerenal physiology and higher values with ATN, but many modern critiques highlight the limitations and exclusions in that literature—hence the emphasis on context.

References

• StatPearls — Renal Function Tests: Definition, FeNa formula, and clinical use. NCBI
• StatPearls — Prerenal Kidney Failure: Classic <1% prerenal and >2% ATN heuristic (context-dependent). NCBI
• MedlinePlus: Clarifies FeNa is a calculation from paired blood/urine tests. MedlinePlus
• Kidney360 (open access): Pro/con commentaries on FeNa/FeUrea accuracy and real-world pitfalls; caution in diuretics/CKD. PMC
• Kidney360 & related literature: Urine sediment (muddy brown casts) as a specific ATN clue. Lippincott Journals
• Creatinine unit conversion: 1 mg/dL = 88.4 µmol/L (clinical chemistry references). eClinpath
• Guidelines: KDIGO/AAFP—don’t use diuretics to prevent/treat AKI (except for volume overload). KDIGO