Fractional Excretion of Sodium in Prerenal Azotemia

Pathophysiology and Diagnostic Markers

• The kidneys respond to reduced perfusion by increasing sodium and water reabsorption to maintain intravascular volume, resulting in a low fractional excretion of sodium (FENa <1%) 1
• This adaptive response is a characteristic finding in prerenal azotemia, where the kidneys avidly retain sodium 2
• Prerenal azotemia is potentially reversible if the underlying cause of hypoperfusion is corrected 1
• FENa <1% is highly sensitive (100%) for prerenal causes of acute kidney injury, though specificity may be lower (14%) 2
• FENa reflects sodium handling more accurately than urinary sodium concentration alone 2
• Low FENa indicates volume depletion and prerenal causes of kidney injury 3
• Urine sodium in prerenal azotemia is typically <10 mEq/L 2, 3
• Urine sediment in prerenal azotemia typically presents with bland (normal) urine sediment 3
• Patients with prerenal azotemia have bland urine microscopic examination 2
• FEUrea <28.16% has been reported to have a sensitivity of 75% and specificity of 83% in separating hepatorenal syndrome from non-hepatorenal syndrome causes 2

Clinical Implications

• FENa <1% helps differentiate prerenal azotemia from acute tubular necrosis (ATN), where FENa is typically >1% 2
• Prerenal azotemia typically responds to volume repletion 1

Renal Indices in Prerenal Azotemia

Diagnostic Approach to Prerenal Azotemia

• The American Gastroenterological Association suggests that diuretic use can falsely elevate Fractional Excretion of Sodium (FENa) even in prerenal states, making it less reliable 4
• The American Gastroenterological Association recommends that in such cases, fractional excretion of urea (FEUrea) <28% may better discriminate prerenal azotemia from acute tubular necrosis, with 75% sensitivity and 83% specificity 4