What does a Fractional Excretion of Sodium (FENA) indicate in patients with Impaired renal function?

Fractional Excretion of Sodium (FENA) in Patients with Impaired Renal Function

In patients with impaired renal function, the Fractional Excretion of Sodium (FENA) is less reliable than Fractional Excretion of Urea (FEUrea) for differentiating between pre-renal and intrinsic causes of acute kidney injury.

Understanding FENA in Renal Dysfunction

FENA measures the percentage of filtered sodium that is excreted in the urine. It is calculated using the formula:

FENA = [(Urine sodium × Serum creatinine) / (Serum sodium × Urine creatinine)] × 100

Traditional Interpretation

• FENA <1%: Traditionally suggests pre-renal causes (volume depletion, decreased perfusion)
• FENA >2%: Traditionally suggests intrinsic renal failure (ATN)

Limitations of FENA in Impaired Renal Function

Poor Specificity

• While FENA <1% has high sensitivity (100%) for pre-renal causes, it has very poor specificity (only 14%) 1
• This means many patients with intrinsic kidney disease can still present with low FENA values

Confounding Factors

• Recent diuretic use significantly alters sodium handling, making FENA unreliable 1, 2
• Water reabsorption affects FENA calculations 1
• Certain conditions can present with low FENA despite intrinsic renal failure:
  • Sepsis 3
  • Early stages of acute tubular necrosis 4
  • Contrast nephropathy
  • Rhabdomyolysis

Timing Considerations

• FENA values change over the course of kidney injury 4
• Early testing (within 1-2 days of injury) may show low FENA even in sustained acute renal failure 4

Superior Alternative: Fractional Excretion of Urea (FEUrea)

FEUrea is more reliable in patients with impaired renal function for several reasons:

• Not affected by diuretic use: FEUrea remains reliable even when patients are receiving diuretics 2, 5
• Higher specificity: FEUrea <28.16% has a sensitivity of 75% and specificity of 83% for identifying pre-renal causes 1, 2
• More consistent: FEUrea values remain similar in pre-renal states regardless of diuretic therapy 5

FEUrea is calculated as: FEUrea = [(Urine urea × Serum creatinine) / (Serum urea × Urine creatinine)] × 100

Interpretation of FEUrea

• FEUrea <35%: Strongly suggests pre-renal causes 2, 5
• FEUrea >50%: Suggests intrinsic renal failure (ATN) 5

Clinical Algorithm for Using FENA in Impaired Renal Function

1. First, determine if patient is on diuretics:

   • If on diuretics → Use FEUrea instead of FENA
   • If not on diuretics → Both FENA and FEUrea can be used (FEUrea still preferred)

2. Interpret results with caution:

   • Low FENA (<1%) + Low FEUrea (<35%) → Likely pre-renal cause
   • Low FENA + High FEUrea (>35%) → Consider intrinsic renal failure despite low FENA
   • High FENA (>1%) + High FEUrea → Strong evidence for intrinsic renal failure

3. Consider timing of test:

   • Early in course of AKI (1-2 days) → FENA may be falsely low
   • Later in course → FENA more likely to reflect true etiology

4. Look for response to volume challenge:

   • In true pre-renal states, serum creatinine should decrease to within 0.3 mg/dL of baseline after appropriate volume replacement 1

Common Pitfalls to Avoid

• Overreliance on FENA alone: Always interpret in clinical context
• Ignoring diuretic use: Recent diuretic administration invalidates FENA but not FEUrea
• Missing timing considerations: Early testing can show low FENA even in intrinsic failure
• Overlooking other clinical parameters: Urinalysis, urine sediment, and other biomarkers should be considered alongside FENA

Special Considerations in Specific Conditions

Cirrhosis and Hepatorenal Syndrome

• Patients with hepatorenal syndrome typically have urine sodium <10 mEq/L 1
• FEUrea is more reliable than FENA in these patients 1

Heart Failure

• Hemodynamic derangements in advanced heart failure affect kidney function through reduced perfusion and venous congestion 1
• FENA may be low due to neurohormonal activation despite significant kidney dysfunction 1

Sepsis

• Sepsis can present with low FENA despite intrinsic kidney injury 3
• FEUrea is more reliable in septic patients

In conclusion, while FENA has historically been used to differentiate pre-renal from intrinsic causes of kidney injury, its reliability in patients with impaired renal function is limited. FEUrea offers a more dependable alternative, especially in patients receiving diuretics or with complex conditions affecting sodium handling.