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Fractional excretion of sodium

Definition

Fractional excretion of sodium (FENa) is the amount of salt (sodium) that leaves your body through urine compared to 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

How the test 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 test

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 test 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.

Why the test is performed

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

Normal Values

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.

What the risks are

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

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

Special considerations

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

References

Bazari H. Approach to the patient with renal disease. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007:chap 115.