Fractional sodium excretion

Fractional Sodium Excretion
Synonyms	FENa
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Fractional Sodium Excretion (FENa) is a clinical calculation used to assess a patient's renal function, particularly in the context of acute kidney injury (AKI). It is a measure of the percentage of the sodium filtered by the kidneys that is excreted in the urine.

Physiology

The kidneys play a crucial role in maintaining homeostasis by regulating the balance of electrolytes, including sodium. Sodium is filtered at the glomerulus and then reabsorbed along the nephron. The amount of sodium excreted in the urine is a reflection of the kidney's ability to reabsorb sodium, which is influenced by various physiological and pathological conditions.

Calculation

The formula for calculating the fractional sodium excretion is:

FENa (%) = \( \frac{\text{Urine Na} \times \text{Plasma Cr}}{\text{Plasma Na} \times \text{Urine Cr}} \times 100 \)

Where:

• Urine Na = Concentration of sodium in the urine (mEq/L)
• Plasma Na = Concentration of sodium in the plasma (mEq/L)
• Urine Cr = Concentration of creatinine in the urine (mg/dL)
• Plasma Cr = Concentration of creatinine in the plasma (mg/dL)

Clinical Significance

FENa is primarily used to differentiate between prerenal and intrinsic renal causes of acute kidney injury.

Prerenal Azotemia

In prerenal azotemia, the kidneys are functioning normally, but there is a decrease in renal perfusion, often due to hypovolemia, heart failure, or cirrhosis. In this condition, the kidneys conserve sodium to maintain blood volume, resulting in a low FENa (typically <1%).

Intrinsic Renal Failure

In intrinsic renal failure, there is damage to the renal parenchyma, such as in acute tubular necrosis (ATN). The damaged tubules are unable to reabsorb sodium effectively, leading to a higher FENa (typically >2%).

Limitations

While FENa is a useful diagnostic tool, it has limitations. It may not be reliable in patients on diuretics, as these medications alter sodium handling by the kidneys. In such cases, the fractional excretion of urea (FEUrea) may be more informative.

Interpretation

• FENa < 1%: Suggests prerenal azotemia
• FENa 1-2%: Indeterminate, may require further investigation
• FENa > 2%: Suggests intrinsic renal failure

See Also

• Acute kidney injury
• Renal physiology
• Electrolyte imbalance

External Links

• [Link to nephrology resources]