What is the Fractional Excretion of Sodium (FENa)?

The Fractional Excretion of Sodium (FENa) is a clinical measurement used to evaluate kidney function, particularly in the context of acute kidney injury (AKI). FENa is a ratio that helps differentiate between prerenal and intrinsic renal causes of kidney dysfunction. This simple test can help clinicians determine the underlying cause of AKI and guide appropriate treatment strategies.

FENa is calculated using the concentration of sodium in both urine and plasma. By comparing these levels, it gives an insight into how well the kidneys are filtering sodium and whether the kidneys are responding appropriately to the body’s fluid balance needs.

How to Calculate the Fractional Excretion of Sodium

The formula to calculate FENa is:

FENa = (Urine Sodium × Plasma Creatinine) / (Plasma Sodium × Urine Creatinine) × 100

Where: - Urine Sodium is the concentration of sodium in the urine (in mEq/L). - Plasma Creatinine is the concentration of creatinine in the blood (in mg/dL). - Plasma Sodium is the concentration of sodium in the plasma (in mEq/L). - Urine Creatinine is the concentration of creatinine in the urine (in mg/dL).

FENa is expressed as a percentage, and it indicates how much sodium is being excreted in the urine relative to the amount filtered by the kidneys. A low or high FENa can provide clues about the cause of kidney injury.

Example

Let’s consider a patient with the following lab results: - Urine Sodium = 40 mEq/L - Plasma Creatinine = 1.2 mg/dL - Plasma Sodium = 140 mEq/L - Urine Creatinine = 120 mg/dL

Using the FENa formula:

FENa = (40 × 1.2) / (140 × 120) × 100 = 48 / 16800 × 100 = 0.29%.

A FENa value of 0.29% suggests a prerenal cause of kidney injury, indicating that the kidneys are conserving sodium appropriately in response to a reduced blood flow to the kidneys.

Why Use the Fractional Excretion of Sodium?

The FENa test is useful because it helps differentiate between prerenal and intrinsic renal causes of acute kidney injury (AKI). AKI can occur due to a variety of causes, such as dehydration, heart failure, or kidney diseases, and determining the underlying cause is crucial for appropriate treatment.

When the kidneys are functioning normally, they regulate sodium excretion based on the body’s hydration and fluid balance. In prerenal causes (e.g., dehydration), the kidneys typically conserve sodium to maintain blood pressure and perfusion. In intrinsic renal injury (e.g., acute tubular necrosis), the kidneys lose this ability to conserve sodium. FENa provides an estimate of how well the kidneys are responding to changes in fluid status, aiding in the diagnosis and management of AKI.

Interpreting the Fractional Excretion of Sodium

The interpretation of FENa values can help identify the cause of kidney injury:

- FENa < 1%: A FENa less than 1% typically suggests prerenal causes of AKI, such as dehydration, heart failure, or blood loss. The kidneys are conserving sodium due to low perfusion.

- FENa between 1% and 2%: This range can be seen in both prerenal and intrinsic renal causes of AKI, and further investigation is needed to determine the exact cause.

- FENa > 2%: A FENa greater than 2% suggests intrinsic renal injury, such as acute tubular necrosis, where the kidneys are unable to appropriately conserve sodium.

It is important to remember that FENa is only one piece of the puzzle, and other clinical factors and tests should be considered when diagnosing and managing AKI.

Practical Applications of FENa

FENa is widely used in hospitals and clinics to help diagnose the cause of acute kidney injury (AKI). In patients who present with AKI, FENa can help guide clinicians toward the correct diagnosis, particularly when distinguishing between prerenal and intrinsic causes.

For instance, in a patient who is dehydrated, FENa can help confirm that the kidneys are conserving sodium appropriately. In contrast, if a patient has intrinsic kidney damage, FENa will likely be high, indicating that the kidneys are unable to conserve sodium.

FENa can also be useful in monitoring patients at risk for kidney injury, such as those undergoing major surgery or those with heart failure, as it can help detect early signs of kidney dysfunction.

Conclusion

The Fractional Excretion of Sodium (FENa) is a simple and valuable test in the management of acute kidney injury (AKI). By calculating the percentage of sodium excreted in the urine, FENa helps distinguish between prerenal and intrinsic renal causes of kidney injury. This allows clinicians to make more informed decisions regarding diagnosis and treatment.

While FENa provides important information, it is not a standalone diagnostic tool. It should be interpreted alongside other clinical findings and tests. With proper use, FENa can enhance the accuracy of diagnosis and improve patient outcomes in those with AKI.