Fractional Excretion of Sodium (FENa) Calculator: A Critical Renal Index:
Fractional Excretion of Sodium (FENa) quantifies the percentage of filtered sodium excreted in urine, helping differentiate causes of acute kidney injury (AKI). This essential calculation distinguishes prerenal (low renal perfusion) from intrinsic renal failure.
Fractional Excretion of Sodium (FENa):
FENa (%) =
Urine Na⁺ (mEq/L) × Plasma Cr (mg/dL)
Plasma Na⁺ (mEq/L) × Urine Cr (mg/dL)
× 100
Key Variables:
• Urine Na⁺: Spot urine sodium (mEq/L)
• Plasma Na⁺: Serum sodium (mEq/L)
• Urine Cr: Urine creatinine (mg/dL)
• Plasma Cr: Serum creatinine (mg/dL)
• Units: Must be consistent (mEq/L for Na⁺, mg/dL for Cr)
• Plasma Na⁺: Serum sodium (mEq/L)
• Urine Cr: Urine creatinine (mg/dL)
• Plasma Cr: Serum creatinine (mg/dL)
• Units: Must be consistent (mEq/L for Na⁺, mg/dL for Cr)
Clinical Interpretation:
• FENa < 1%: Prerenal azotemia
• FENa > 2%: Acute tubular necrosis (ATN)
• FENa 1-2%: Indeterminate zone
• Diuretic use: May falsely elevate FENa
• CKD: Less reliable in chronic kidney disease
• FENa > 2%: Acute tubular necrosis (ATN)
• FENa 1-2%: Indeterminate zone
• Diuretic use: May falsely elevate FENa
• CKD: Less reliable in chronic kidney disease
Calculation Example:
• Urine Na⁺: 20 mEq/L, Plasma Na⁺: 140 mEq/L
• Urine Cr: 100 mg/dL, Plasma Cr: 2.0 mg/dL
• FENa = [(20 × 2.0) / (140 × 100)] × 100
• = [40 / 14,000] × 100 = 0.29%
• Interpretation: Prerenal state
• Urine Cr: 100 mg/dL, Plasma Cr: 2.0 mg/dL
• FENa = [(20 × 2.0) / (140 × 100)] × 100
• = [40 / 14,000] × 100 = 0.29%
• Interpretation: Prerenal state
Physiological Basis:
• Measures sodium handling by kidneys
• Reflects tubular function in AKI
• Low FENa → intact tubular reabsorption
• High FENa → tubular damage
• Compares Na⁺ clearance to Cr clearance
• Reflects tubular function in AKI
• Low FENa → intact tubular reabsorption
• High FENa → tubular damage
• Compares Na⁺ clearance to Cr clearance
Special Considerations:
• Contrast nephropathy: Often shows low FENa initially
• Rhabdomyolysis: May have low FENa despite ATN
• Glomerulonephritis: Variable FENa patterns
• Early obstruction: Can mimic prerenal state
• Burns: Less reliable due to Na⁺ shifts
• Rhabdomyolysis: May have low FENa despite ATN
• Glomerulonephritis: Variable FENa patterns
• Early obstruction: Can mimic prerenal state
• Burns: Less reliable due to Na⁺ shifts
Limitations:
• Unreliable with diuretic use
• Invalid in advanced CKD (eGFR <15)
• Affected by sodium intake
• Less accurate with hyperglycemia
• Spot urine vs timed collection variations
• Urine Cr concentration impacts reliability
• Invalid in advanced CKD (eGFR <15)
• Affected by sodium intake
• Less accurate with hyperglycemia
• Spot urine vs timed collection variations
• Urine Cr concentration impacts reliability
Alternative Indices:
• FEUrea: Better for diuretic-treated patients
• Urine Osmolality: >500 mOsm/kg suggests prerenal
• BUN/Cr Ratio: >20:1 suggests prerenal
• Urine Sediment: Muddy brown casts in ATN
• Urine Na⁺ Alone: <20 mEq/L suggests prerenal
• Urine Osmolality: >500 mOsm/kg suggests prerenal
• BUN/Cr Ratio: >20:1 suggests prerenal
• Urine Sediment: Muddy brown casts in ATN
• Urine Na⁺ Alone: <20 mEq/L suggests prerenal
Clinical Applications:
• Acute kidney injury differentiation
• Volume status assessment
• Diuretic response evaluation
• Transplant rejection monitoring
• Hepatorenal syndrome diagnosis
• Volume status assessment
• Diuretic response evaluation
• Transplant rejection monitoring
• Hepatorenal syndrome diagnosis
Critical Clinical Pearls:
• Always interpret FENa in clinical context – no single test is diagnostic
• In diuretic-treated patients, use FEUrea instead
• Low FENa in ATN can occur with contrast, rhabdo, or sepsis
• FENa >0.5% in neonates suggests intrinsic renal disease
• Check urine before diuresis for most accurate results
• Combine with urinalysis for comprehensive assessment
• Always interpret FENa in clinical context – no single test is diagnostic
• In diuretic-treated patients, use FEUrea instead
• Low FENa in ATN can occur with contrast, rhabdo, or sepsis
• FENa >0.5% in neonates suggests intrinsic renal disease
• Check urine before diuresis for most accurate results
• Combine with urinalysis for comprehensive assessment
🧪 Fractional Excretion of Sodium (FENa) Calculator
📐 Formula:
FENa (%) =
Urine Na⁺ × Plasma Cr
Plasma Na⁺ × Urine Cr
× 100
🖊️ Enter the Following Values:
FENa Formula:
Fractional Excretion of Sodium (FENa):
FENa (%) =
Urine Na⁺ (mEq/L) × Plasma Cr (mg/dL)
Plasma Na⁺ (mEq/L) × Urine Cr (mg/dL)
× 100
Units:
- Sodium (Na⁺): mmol/L (urine & plasma)
- Creatinine (Cr): mg/dL (urine & plasma)
Step-by-Step Calculation Example
Patient Values:
- Plasma Na⁺: 140 mmol/L
- Urine Na⁺: 60 mmol/L
- Plasma Cr: 2.0 mg/dL
- Urine Cr: 80 mg/dL
Calculation:
- Numerator:
Urine Na⁺ × Plasma Cr= 60 × 2.0 = 120 - Denominator:
Plasma Na⁺ × Urine Cr= 140 × 80 = 11,200 - Ratio:
120 / 11,200= 0.0107 - FENa:
0.0107 × 100= 1.07%
Interpretation of FENa Values
| FENa (%) | Clinical Implication | Common Causes |
|---|---|---|
| < 1% | Prerenal AKI | Dehydration, HF, Sepsis |
| 1–2% | Indeterminate/Gray Zone | Transition phase, Diuretic use |
| > 2% | Intrinsic Renal AKI | ATN, Glomerulonephritis |
Key Clinical Applications
- AKI Workup:
- FENa < 1% suggests responsive kidneys conserving sodium due to hypoperfusion.
- FENa > 2% indicates tubular dysfunction (e.g., acute tubular necrosis).
- Diuretic Use:
- FENa may be falsely elevated → Use FEUrea instead (cutoff < 35% suggests prerenal).
- Special Populations:
- CKD Patients: Interpret cautiously (baseline Cr alters accuracy).
- Hyponatremia: FENa < 0.2% suggests SIADH.
Limitations & Caveats
- Non-Diagnostic in:
- Chronic kidney disease (CKD)
- Contrast-induced nephropathy
- Glomerulopathies or vasculitis
- Confounders:
- Recent diuretic administration (use FEUrea)
- Hyperglycemia (pseudohyponatremia)
- Low sodium intake (may mask prerenal state)
Clinical Pearls
- Timing Matters: Measure FENa before fluid resuscitation/diuretics.
- Correlate with Exam: FENa complements (not replaces) clinical assessment.
- Repeat Testing: Track trends if AKI etiology remains unclear.
📌 Pro Tip: In prerenal states, urine osmolality > 500 mOsm/kg and urine [Na⁺] < 20 mmol/L support FENa findings.
FENa remains a rapid, cost-effective tool for initial AKI triage—but always contextualize results with the full clinical picture. Validate institutional protocols for precise diagnostic thresholds.
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