What is a characteristic of prerenal azotemia?

Fractional Excretion of Sodium <1% is a Characteristic of Prerenal Azotemia

The correct answer is (b) fractional excretion of sodium value <1%, which is a characteristic finding in prerenal azotemia 1.

Pathophysiology of Prerenal Azotemia

Prerenal azotemia represents a functional decline in kidney function due to decreased renal perfusion, without intrinsic damage to the renal parenchyma. In this condition:

• The kidneys respond to reduced perfusion by increasing sodium and water reabsorption to maintain intravascular volume 1
• This adaptive response results in a low fractional excretion of sodium (FENa <1%) as the kidneys avidly retain sodium 1
• The condition is potentially reversible if the underlying cause of hypoperfusion is corrected 1

Diagnostic Markers in Prerenal Azotemia

Fractional Excretion of Sodium (FENa)

• FENa <1% is highly sensitive (100%) for prerenal causes of acute kidney injury, though specificity may be lower (14%) 1
• FENa reflects sodium handling more accurately than urinary sodium concentration alone 1
• Low FENa indicates volume depletion and prerenal causes of kidney injury 1

Urine Sodium Concentration

• Urine sodium in prerenal azotemia is typically <10 mEq/L (not <40 mEq/L as suggested in option c) 1
• This value may be altered in patients who have recently received diuretics 1

Blood Urea Nitrogen/Serum Creatinine Ratio

• While BUN/Cr ratio >10 is often associated with prerenal states, it is less specific than FENa 2
• BUN/Cr ratio can be affected by other factors including protein intake, catabolic states, and gastrointestinal bleeding 3

Urine Sediment

• Prerenal azotemia typically presents with bland (normal) urine sediment, not abnormal sediment as suggested in option (a) 1
• Patients with prerenal azotemia have bland urine microscopic examination 1

Clinical Implications

• FENa <1% helps differentiate prerenal azotemia from acute tubular necrosis (ATN), where FENa is typically >1% 1, 2
• This distinction is crucial as management differs significantly - prerenal azotemia typically responds to volume repletion 1

Important Caveats

• FENa may be less reliable in patients receiving diuretics, which can artificially increase sodium excretion despite prerenal status 2
• In patients on diuretics, fractional excretion of urea (FEUrea) may be more reliable, with values <35% suggesting prerenal causes 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 1

Understanding these laboratory markers is essential for correctly identifying prerenal azotemia and implementing appropriate management strategies to prevent progression to intrinsic renal damage.