What labs are needed to calculate Fractional Excretion of Sodium (FENa)?

Labs Needed for Calculating Fractional Excretion of Sodium (FENa)

To calculate Fractional Excretion of Sodium (FENa), you need to collect both urine and serum samples for sodium and creatinine measurements.

Required Laboratory Tests

1. Serum Measurements:

   • Serum sodium (Na⁺)
   • Serum creatinine

2. Urine Measurements:

   • Urine sodium (Na⁺)
   • Urine creatinine

Calculation Formula

FENa is calculated using the following formula:

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

Where:

• Urine Na = Urine sodium concentration
• Serum Cr = Serum creatinine concentration
• Serum Na = Serum sodium concentration
• Urine Cr = Urine creatinine concentration

Clinical Application and Interpretation

FENa is primarily used to differentiate between prerenal causes of acute kidney injury (AKI) and intrinsic renal causes such as acute tubular necrosis (ATN):

• FENa < 1%: Typically suggests prerenal causes including volume depletion or decreased effective arterial blood volume 1
• FENa > 1%: Suggests intrinsic renal causes such as ATN 1

Important Considerations

• Timing of collection: Both urine and serum samples should be collected at approximately the same time for accurate calculation
• Recent diuretic use: Can falsely elevate FENa even in prerenal states
• Chronic kidney disease: Can affect interpretation - concordance between FENa and actual kidney injury is poor in patients with preexisting CKD 2
• Cirrhosis: In patients with cirrhosis, an FENa of <1% has high sensitivity (100%) but poor specificity (only 14%) for diagnosing prerenal causes of AKI 1

Limitations and Pitfalls

1. Diuretic effect: Recent diuretic administration can increase sodium excretion and elevate FENa despite prerenal etiology
2. Diagnostic accuracy: Recent studies show that relying solely on FENa to exclude acute tubular injury should be abandoned 2
3. Concurrent conditions: Sepsis, contrast nephropathy, and certain medications can affect sodium handling
4. False interpretations: About 38% of patients with muddy brown granular casts (indicating ATN) still have FENa <1% 2

Alternative Assessments

• Fractional Excretion of Urea (FEUrea): May better discriminate between hepatorenal syndrome and other causes of AKI in cirrhosis patients, with FEUrea <28.16% having 75% sensitivity and 83% specificity for hepatorenal syndrome 1
• Urinary sodium:creatinine ratio: For routine practice, this is easier to measure and correlates with FENa (corresponding range 17-52 mmol/mmol) 1

Remember that FENa should be interpreted in the clinical context and not used as the sole determinant for diagnosis of kidney injury etiology.