Fractional excretion of sodium

Fractional Excretion of Sodium (FENa) is a diagnostic calculation used in the field of medicine, particularly in nephrology and internal medicine, to help evaluate the status of renal function and to differentiate between various types of acute kidney injury (AKI). The FENa measures the percentage of the sodium filtered by the kidney which is excreted in the urine. It is a valuable tool in determining the cause of kidney failure and assessing the volume status of a patient.

Calculation

The formula for calculating the Fractional Excretion of Sodium is:

\[FENa = \left( \frac{{\text{Urine Sodium} \times \text{Plasma Creatinine}}}{{\text{Plasma Sodium} \times \text{Urine Creatinine}}} \right) \times 100\]

where:

• Urine Sodium is the concentration of sodium in the urine (measured in mmol/L),
• Plasma Creatinine is the concentration of creatinine in the blood plasma (measured in mg/dL),
• Plasma Sodium is the concentration of sodium in the blood plasma (measured in mmol/L),
• Urine Creatinine is the concentration of creatinine in the urine (measured in mg/dL).

Clinical Significance

The FENa is particularly useful in distinguishing prerenal azotemia from intrinsic acute kidney injury. In prerenal azotemia, the FENa is typically less than 1%, indicating that the kidneys are responding appropriately to a decrease in blood flow by conserving sodium. In contrast, a FENa greater than 2% suggests intrinsic renal damage, where the kidneys are unable to properly reabsorb sodium, indicative of conditions such as acute tubular necrosis.

Limitations

The FENa calculation has limitations and should not be used in isolation for diagnosing kidney issues. It is less reliable in patients with chronic kidney disease, those on diuretics, or in cases of glomerulonephritis. Clinical judgment and other diagnostic tests are necessary to accurately diagnose and manage renal conditions.

Related Calculations

Other related calculations include the Fractional Excretion of Urea (FEUrea) and the Renal Failure Index (RFI), which can be used in conjunction with FENa to provide a more comprehensive assessment of renal function, especially in patients on diuretics where FENa may be less accurate.

See Also

• Acute Kidney Injury
• Nephrology
• Renal Function
• Diuretics