Fractional excretion of sodium

What is Fractional Excretion of Sodium?

Fractional excretion of sodium (FENa) is a diagnostic test to help assess the cause of acute renal failure. It is used in conjunction with other laboratory tests. The FENa value helps identify whether pre-renal, intra-renal or post-renal causes are responsible for acute renal failure.

Preparation for Fractional Excretion of Sodium

There is no real preparation needed prior to a FENa test. The only requirement is to ensure that the patient is maintaining adequate hydration. This is important because dehydration will falsely elevate the FENa value, which could lead to an incorrect diagnosis.

Procedure for Fractional Excretion of Sodium

During a FENa, the patient is first hydrated with fluids and the baseline values of urine and serum electrolytes are established. Then, the patient is infused with a diuretic, typically furosemide (Lasix). After two to four hours, the urine and serum electrolytes are re-measured to determine whether there has been a shift in the electrolyte concentrations. From there, the FENa value can be calculated.

Types of Fractional Excretion of Sodium

The FENa value is typically expressed as a fraction. It is calculated as (urine sodium/serum sodium) divided by (urine creatinine/serum creatinine). The normal FENa value is less than 1%, with values greater than 1% indicating pre-renal causes of acute renal failure.

Risks of Fractional Excretion of Sodium

The FENa is generally a safe procedure with no major risks. However, as with any medical procedure, there is a small risk of infection and adverse reactions to the diuretic.

Why is Fractional Excretion of Sodium used?

The FENa test is used to differentiate between pre-renal, intra-renal and post-renal causes of renal failure in order to guide further testing and treatment decisions. It can also be used to monitor the effectiveness of treatment.

When is Fractional Excretion of Sodium used?

The FENa test is typically used when a patient presents with symptoms of acute renal failure, and a diagnosis is needed to guide treatment. It can also help differentiate between renal and non-renal causes of acute renal failure.