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.

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.

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

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.

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.

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.

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

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