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.

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.

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.

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.

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.