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Fractional excretion of sodium

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

FENa is not a test. Instead it is a calculation based on the concentrations of sodium and creatinine in the blood and urine. Urine and blood chemistry tests are needed to perform this calculation.

How the Test is Performed

Blood and urine samples are collected at the same time and sent to a lab. There, they are examined for salt (sodium) and creatinine levels.

How to Prepare for the Test

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

If needed, you may be told to temporarily stop medicines that interfere with test results. For example, some diuretic medicines (water pills) can affect test results.

How the Test will Feel

When the needle is inserted to draw blood, some people feel moderate pain. Others feel only a prick or stinging. Afterward, there may be some throbbing or slight bruising. This soon goes away.

Why the Test is Performed

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

What Abnormal Results Mean

A meaningful interpretation of the test can be made only when your urine volume has dropped to less than 500 milliliters per day (mL/day).

FENa of lower than 1% indicates decreased blood flow to the kidney. This can occur with kidney damage due to dehydration or heart failure.

FENa higher than 1% suggests damage to the kidney itself.

Risks

Veins and arteries vary in size from one patient to another and from one side of the body to the other. Obtaining a blood sample from some people may be more difficult than from others.

Other risks associated with having blood drawn are slight but may 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

There are no risks with the urine sample.

Alternative Names

FE sodium; FENa

References

Emmet M, Fenves AZ, Schwartz JC. Approach to the patient with kidney disease. In: Taal MW, Chertow GM, Marsden PA, et al., eds.Brenner and Rector's The Kidney.

Update Date 11/7/2013

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