What do fractional excretion of sodium (FeNa) values indicate about the type and management of acute kidney injury (AKI)?

Fractional Excretion of Sodium in Acute Kidney Injury Diagnosis

Fractional excretion of sodium (FENa) is a valuable diagnostic tool that helps differentiate between prerenal and intrinsic causes of acute kidney injury (AKI), with values <1% typically indicating prerenal failure and values >2% suggesting intrinsic renal damage such as acute tubular necrosis (ATN).

Understanding FENa

FENa measures the percentage of filtered sodium that is excreted in the urine, calculated using the formula:

FENa = [(Urine Na × Serum Cr) / (Serum Na × Urine Cr)] × 100

Interpretation of FENa Values

• FENa <1%: Strongly suggests prerenal AKI (volume depletion, heart failure, cirrhosis)
• FENa >2%: Typically indicates intrinsic renal failure, particularly ATN
• FENa 1-2%: Indeterminate range requiring clinical correlation

Limitations of FENa

FENa has important limitations that affect its diagnostic accuracy:

1. Diuretic use: Diuretics increase sodium excretion, artificially elevating FENa despite prerenal etiology 1
2. Sodium-avid states: Patients with cirrhosis, heart failure, or nephrotic syndrome may have falsely low FENa (<1%) despite having intrinsic renal failure 2
3. Chronic kidney disease: Baseline tubular dysfunction affects sodium handling
4. Early or recovering ATN: May have lower FENa values than expected 3

Alternative Biomarker: Fractional Excretion of Urea (FEUrea)

When FENa is unreliable (especially with diuretic use), FEUrea provides better diagnostic accuracy:

• FEUrea <35%: Indicates prerenal AKI (sensitivity 89% in patients on diuretics) 1
• FEUrea >50%: Suggests intrinsic renal failure/ATN
• Formula: FEUrea = [(Urine urea × Serum Cr) / (Serum urea × Urine Cr)] × 100 4

A 2002 study showed that while only 48% of diuretic-treated prerenal AKI patients had FENa <1%, 89% had FEUrea <35%, making it more reliable in this context 1.

Special Considerations in Cirrhosis

In patients with cirrhosis and AKI, FENa interpretation requires adjustment:

• A lower FENa cutoff of 0.57% (rather than 1%) provides better discrimination between ATN and other forms of AKI (sensitivity 89%, specificity 71%) 5
• FENa cannot reliably differentiate between hepatorenal syndrome (HRS) and prerenal AKI in cirrhosis 5
• The International Club of Ascites recommends volume expansion with albumin (1g/kg for two days) for diagnostic purposes in suspected HRS 6

Pediatric Considerations

In children with AKI:

• FEUrea <35% has higher sensitivity and specificity than FENa <1% for differentiating prerenal from intrinsic renal failure 7
• Optimal cutoff values in pediatric patients are approximately 30% for FEUrea and 1.6% for FENa 7

Clinical Application Algorithm

1. Measure both FENa and FEUrea in all AKI patients
2. If patient is NOT on diuretics:
   • FENa <1% → Likely prerenal AKI → Volume expansion trial
   • FENa >2% → Likely intrinsic renal failure → Treat underlying cause
3. If patient IS on diuretics:
   • Rely on FEUrea (<35% suggests prerenal)
   • Consider holding diuretics if possible
4. In cirrhosis patients:
   • Use lower FENa cutoff (0.57%)
   • Consider albumin challenge (1g/kg/day for 2 days)
5. Monitor response to therapy:
   • Rising FENa after volume expansion suggests improving ATN
   • Persistently low FENa despite adequate volume suggests ongoing prerenal state or HRS

Pitfalls to Avoid

• Don't rely solely on FENa in patients receiving diuretics
• Don't assume low FENa always indicates prerenal AKI in sodium-avid states
• Don't forget that FENa may change during the course of AKI (particularly during recovery)
• Don't interpret FENa in isolation from clinical context and other laboratory parameters

Remember that while FENa and FEUrea are valuable diagnostic tools, they should be interpreted within the clinical context, considering the patient's underlying conditions, medication use, and overall clinical presentation.