When to order a Fractional Excretion of Sodium (FENa) in a patient with acute kidney injury (AKI) of uncertain cause?

When to Order Fractional Excretion of Sodium (FENa) in AKI of Uncertain Cause

FENa should be ordered in patients with acute kidney injury of uncertain etiology, particularly when distinguishing between prerenal and intrinsic renal causes would alter management decisions, and ideally before diuretic administration. 1, 2

Indications for FENa Testing

• Primary indications:

  • New-onset oliguric AKI without clear etiology
  • Need to differentiate between prerenal azotemia and acute tubular necrosis (ATN)
  • AKI with conflicting clinical features suggesting both volume depletion and intrinsic renal injury

• Optimal timing:

  • Before administration of diuretics when possible
  • Early in the course of AKI before multiple etiologies develop
  • When volume status assessment is clinically challenging

Interpretation of FENa Results

• FENa < 1%: Typically indicates prerenal causes (volume depletion, decreased effective arterial blood volume) 1, 3
• FENa > 1%: Suggests intrinsic renal causes such as ATN 1, 3
• FENa > 3%: Strongly indicative of ATN 3

Clinical Scenarios Where FENa is Most Useful

• Oliguric patients without CKD or diuretic use: FENa has highest diagnostic accuracy with sensitivity of 95% and specificity of 91% 2
• Volume status uncertainty: When physical examination findings are equivocal
• Monitoring recovery: A decreasing FENa in a patient with established ATN may indicate development of volume depletion that could delay recovery 4

Limitations and Caveats

• Reduced accuracy in specific populations:

  • Patients on diuretics: Sensitivity drops to 80%, specificity to 54% 2
  • Patients with CKD: Diagnostic performance is diminished 2
  • Patients with cirrhosis: FENa has poor specificity (14%) for prerenal causes despite high sensitivity 1, 5

• Alternative tests when FENa is less reliable:

  • Fractional Excretion of Urea (FEUrea): More reliable in patients on diuretics 6
    • FEUrea < 35% suggests prerenal causes even in patients on diuretics 6
    • In cirrhosis, FEUrea < 28.16% has 75% sensitivity and 83% specificity for hepatorenal syndrome 1
  • Urinary sodium:creatinine ratio: Easier to measure and correlates with FENa 1

Algorithm for FENa Testing in AKI

1. Confirm AKI diagnosis using KDIGO criteria:

   • Increase in serum creatinine by ≥0.3 mg/dL within 48h, or
   • Increase in serum creatinine to ≥1.5 times baseline within 7 days, or
   • Urine volume <0.5 mL/kg/h for 6h 7

2. Assess if etiology is uncertain after initial evaluation

3. Order FENa if:

   • Patient is not on diuretics AND
   • Patient does not have advanced CKD AND
   • Distinguishing prerenal from intrinsic causes would change management

4. Consider FEUrea instead of FENa if:

   • Patient is on diuretics OR
   • Patient has cirrhosis

5. Interpret results in clinical context and adjust management accordingly

Remember that while FENa is a valuable diagnostic tool, its interpretation must be integrated with other clinical and laboratory findings for optimal management of AKI.