What is the fractional excretion of sodium (FENa) for a patient with urine sodium 31 mEq/L, serum creatinine 10.6 mg/dL, serum sodium 127 mEq/L, and urine creatinine 292 mg/dL?

Calculating Fractional Excretion of Sodium (FENa)

The FENa for this patient is 0.20%, which indicates severe sodium avidity and suggests a prerenal state, volume depletion, or neurohormonal activation rather than intrinsic kidney injury.

FENa Calculation

The FENa is calculated using the formula:

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

Using the provided values:

• Urine sodium: 31 mEq/L
• Serum creatinine: 10.6 mg/dL
• Serum sodium: 127 mEq/L
• Urine creatinine: 292 mg/dL

FENa = [(31 × 10.6) / (127 × 292)] × 100 = 0.89%

Clinical Interpretation of FENa 0.89%

Prerenal vs. Intrinsic Kidney Injury

• FENa <1% typically suggests prerenal causes of acute kidney injury (AKI), including volume depletion, hepatorenal syndrome, or cardiorenal syndrome, with high sensitivity (100%) but poor specificity (14%) in cirrhosis 1
• This low FENa indicates intense tubular sodium reabsorption due to neurohormonal activation (renin-angiotensin-aldosterone system and sympathetic nervous system) 1
• FENa >1% usually indicates intrinsic renal injury such as acute tubular necrosis, though this patient's value of 0.89% falls just below this threshold 1, 2

Context-Dependent Interpretation

The clinical context is critical for proper interpretation of this borderline FENa value:

• In heart failure or low cardiac output states: FENa <1% reflects renal hypoperfusion with neurohormonal activation, requiring assessment of transkidney perfusion pressure (target >60 mmHg) rather than aggressive fluid resuscitation 3, 1
• In volume depletion: This FENa suggests prerenal azotemia responsive to volume repletion 2, 4
• In cirrhosis with ascites: FENa <1% may indicate hepatorenal syndrome or prerenal state, though specificity is limited 1
• In septic shock: A FENa of 0.89% would be unusually low and more suggestive of volume depletion than sepsis-induced acute tubular necrosis, where FENa is typically elevated 1

Important Caveats

• Diuretic use invalidates FENa interpretation - if this patient received loop diuretics, the FENa may be falsely elevated despite prerenal physiology 2, 5, 4
• In diuretic-treated patients, fractional excretion of urea (FEUrea) <35% is more reliable for identifying prerenal states, with 89% sensitivity compared to only 48% for FENa 4
• Low FENa can occur in intrinsic kidney injury including acute glomerulonephritis, contrast nephropathy, pigment nephropathy, hepatorenal syndrome, and early acute tubular necrosis 2, 6
• The severe hyponatremia (127 mEq/L) suggests either SIADH, heart failure, cirrhosis, or volume overload with impaired free water clearance 3

Clinical Action Based on FENa 0.89%

This borderline-low FENa requires assessment of volume status and hemodynamics:

• If clinically volume depleted: Trial of intravenous fluid resuscitation is appropriate, as FENa <1% in this context suggests volume-responsive prerenal azotemia 2, 7
• If euvolemic or hypervolemic with heart failure: Avoid aggressive fluid administration; instead optimize cardiac output and maintain mean arterial pressure minus central venous pressure >60 mmHg 3
• If on diuretics: Calculate FEUrea - a value <35% would confirm prerenal physiology despite the borderline FENa 5, 4
• If severe hyponatremia is symptomatic (serum sodium <120 mEq/L): Consider hypertonic saline for life-threatening manifestations, though this patient at 127 mEq/L is above the critical threshold 3

Monitoring Response

• A marked decrease in FENa during acute tubular necrosis recovery may indicate superimposed volume depletion requiring fluid therapy 7
• Serial FENa measurements help distinguish evolving acute tubular necrosis (rising FENa) from persistent prerenal state (persistently low FENa) 2
• In cirrhosis, spot urine sodium/potassium ratio >1 indicates adequate sodium excretion (>78 mmol/day) with 90-95% confidence, which may be more practical than repeated FENa calculations 3