A FeNa calculator helps you quickly estimate how much of the filtered sodium load is excreted in urine. In the right context—especially oliguric AKI—FeNa can support the distinction between prerenal physiology and intrinsic tubular injury. It’s a calculation from paired blood and urine chemistries, not a standalone lab test.
What FeNa measures?
Fractional excretion of sodium (FeNa) is the percentage of filtered sodium that is ultimately excreted. In states of under-perfusion (e.g., volume depletion, reduced effective arterial blood volume), intact nephrons conserve sodium—so FeNa tends to be low. In intrinsic tubular injury, sodium reabsorption is impaired—so FeNa tends to be higher. This is why FeNa is often used at the bedside during FENa AKI evaluations to help separate FENa prerenal from intrarenal causes.
FeNa is most informative when urine output is low and measurements are taken together (paired urine and blood sampling). It should be read alongside the history, exam, urinalysis, and the patient’s trajectory—not in isolation.
The FeNa formula (so you can calculate FeNa by hand)
FeNa (%) = 100 × (Urine Na × Plasma Cr) ÷ (Plasma Na × Urine Cr)
That expression falls directly out of mass-balance: sodium excreted versus filtered, with creatinine acting as a filtration marker, which cancels the flow terms.
Units that :
- Sodium may be mEq/L or mmol/L (numerically identical for Na⁺).
- Creatinine can be mg/dL or μmol/L. Conversion: 1 mg/dL = 88.4 μmol/L.
If you’re doing a manual fena calculation, keep sodium units consistent and convert creatinine when necessary. (Our fena calculator handles this for you.)
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,” supportive of a prerenal process when the clinical story fits.
Interpreting FeNa (with caution)
Classic bedside cut-offs (heuristics):
- FeNa < 1% → supports prerenal physiology (e.g., hypovolemia, reduced effective arterial volume).
- FeNa > 2–3% → supports intrinsic tubular injury (e.g., ATN).
These rules of thumb come from longstanding observational work and remain common in teaching. Treat them as guides, not absolutes.
Why FeNa can mislead (pitfalls you should know):
- Diuretics increase urinary sodium and can falsely raise FeNa in otherwise prerenal states.
- CKD, glomerulonephritis, obstruction, sepsis, contrast, and non-oliguric AKI can all blur discrimination.
- Modern reviews emphasize that FeNa—and FeUrea—are adjuncts, with performance varying by population and study design.
Guideline context (don’t chase numbers with diuretics):
KDIGO recommends against using diuretics to prevent AKI and suggests against using them to treat AKI, except for managing volume overload—a reminder to use FeNa to inform physiology, not to justify routine diuresis.
FeUrea vs FeNa: when diuretics muddy the water
If the patient recently received a loop diuretic, consider fractional excretion of urea (FeUrea) in addition to FeNa. Urea handling is less affected by loops; a FeUrea < 35% often supports prerenal physiology, though results across studies are mixed and context-dependent. Recent Kidney360 commentaries summarize both the supportive data and the caveats—use these indices as support, not as sole arbiters.
Practical bedside workflow for FENa AKI
- Confirm AKI (creatinine rise and/or reduced urine output) and address immediacies (hyperkalemia, acidosis, fluid status).
- History & exam: GI losses, hemorrhage, heart/liver disease, nephrotoxins (NSAIDs, contrast), timing of any diuretics.
- Urinalysis and sediment: muddy brown granular casts support tubular injury; bland sediment leans prerenal (not definitive).
- Order paired labs: spot urine Na & Cr with plasma Na & Cr drawn around the same time; calculate FeNa (and FeUrea if diuretics are a factor).
- Treat the cause & avoid harm: optimize perfusion, avoid nephrotoxins, and reserve diuretics for volume overload per KDIGO.
Scenario snapshots (to make the numbers real)
1) Vomiting with oliguria
Young adult, orthostasis, dry mucosae. FeNa 0.3%; after careful fluids, urine output rises and creatinine falls—fits prerenal physiology.
2) Septic shock after loop diuretic
FeNa 2.1%, FeUrea 28%. Diuretics can inflate FeNa, while FeUrea <35% leans prerenal. Sediment shows granular casts and creatinine rises despite resuscitation → likely mixed prerenal + tubular injury.
3) AKI on CKD
Baseline CKD 4 on ACEi + loop. FeNa 1.4% is non-diagnostic; integrate clinical picture, sediment, and imaging for obstruction.
Tips to keep your fena calc honest
- Pair samples in time. Draw urine and blood around the same time so FeNa reflects one physiologic state.
- Mind your units. For sodium, mmol/L = mEq/L; for creatinine, 1 mg/dL = 88.4 μmol/L—convert if needed, or calculate FeNa with a unit-aware tool.
- Check the sediment. Urine microscopy (e.g., granular casts) can be more specific for ATN than any single ratio.
- Follow the trajectory. Response to fluid optimization over 24–48 hours often clarifies physiology better than a single FeNa snapshot.
- Don’t over-generalize. In non-oliguric AKI, advanced CKD, GN, obstruction, or sepsis, FeNa’s accuracy falls—treat results as one piece of the puzzle.
Frequently asked questions (FAQ) — FeNa calculator
What does a FeNa calculator actually do?
It computes the fraction of filtered sodium excreted in urine using paired urine/blood Na and creatinine values, returning FeNa (%) to support your AKI differential in context.
How do I calculate FeNa by hand?
Use FeNa (%) = 100 × (Urine Na × Plasma Cr) ÷ (Plasma Na × Urine Cr), keeping sodium units consistent and converting creatinine if needed (1 mg/dL = 88.4 μmol/L). Or just calculate FeNa with a unit-aware fena calculator.
What FeNa value suggests prerenal AKI?
Classically, FeNa < 1% supports prerenal physiology; >2–3% suggests intrinsic tubular injury. Treat these as heuristics; diuretics and comorbid states can break the rule.
Can I trust FeNa if the patient is on diuretics?
Be cautious—loop diuretics raise urinary sodium and can falsely elevate FeNa. Consider FeUrea (often <35% supports prerenal), but recognize that both indices have limitations and should be paired with the clinical picture.
Do guidelines recommend diuretics for AKI?
No. KDIGO recommends against diuretics to prevent AKI and suggests against them to treat AKI (except for volume overload). Manage perfusion, avoid nephrotoxins, and use fractional indices as supportive data.
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