FE sodium

Definition

Fractional excretion of sodium (FENa) is the amount of salt (sodium) that leaves your body through urine versus the amount filtered and reabsorbed by the kidney.

FENa is not a test, but rather a calculation based on the concentrations of sodium and creatinine in your blood and urine. Urine and blood chemistry tests are necessary to perform this calculation.

Alternative Names

FE sodium; FENa

Why the FE sodium is Performed

The test is usually done for patients who are severely ill with acute kidney failure. The test helps determine if the drop in urine is due to reduced blood flow to the kidney or to kidney damage itself.

How the FE sodium is Performed

Blood and urine samples are sent to a lab, where they are examined for their salt (sodium) and creatinine levels.

For information on how a blood sample is taken from a vein see venipuncture.

For information on giving a urine sample see clean catch urine sample.

How to Prepare for the FE sodium

Eat a normal diet with a normal amount of salt, unless otherwise instructed by your doctor.

Your health care provider will instruct you, if necessary, to discontinue drugs that may interfere with test results. Certain diuretic medicines may affect test results.

How the FE sodium Will Feel

When the needle is inserted to draw blood, some people feel moderate pain, while others feel only a prick or stinging sensation. Afterward, there may be some throbbing.

Risks

The urine sample has no risk. The risks of drawing blood include:

• Excessive bleeding
• Fainting or feeling light-headed
• Hematoma (blood accumulating under the skin)
• Infection (a slight risk any time the skin is broken)
• Multiple punctures to locate veins

Considerations

The test is only of value in specific circumstances such as sudden reduced urine output.

What Abnormal Results Mean

A meaningful interpretation of the test can be made only when your urine volume has dropped to less than 500 ml/day.

A FENa of less than 1% indicates decreased blood flow to the kidney while a FENa greater than 1% (and usually greater than 3%) suggests kidney damage.

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Typical mistypes for FE sodium
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