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Fractional excretion of sodium

Alternative names

FE sodium; FENa


Fractional excretion of sodium is not a test, but rather a calculation based on the concentrations of Sodium (Na) and Creatinine (Cr) in the blood and the urine. Urine chemistry (based on urinalysis) and serum chemistry tests are necessary to perform this calculation.

The information from this calculation is helpful in defining the extent of sodium conservation by the kidney, and it may also help identify certain diseases of the kidney.

How the test is performed

Samples of blood and urine are taken simultaneously and analyzed for the serum sodium and creatinine levels. The fractional excretion of sodium can then be mathematically calculated according to the formula:

FENa = ([urine Na]x[serum Cr]/[urine Cr]x[serum Na])x100%, where FENa represents fractional excretion of sodium and "[]" represents concentration values.

Child or adult:
Collect a "clean-catch" (midstream) urine sample. To obtain a clean-catch sample, men or boys should wipe clean the head of the penis . Women or girls need to wash the area between the lips of the vagina with soapy water and rinse well. As you start to urinate, allow a small amount to fall into the toilet bowl (this clears the urethra of contaminants). Then, in a clean container, catch about 1 to 2 ounces of urine and remove the container from the urine stream. Give the container to the health care provider or assistant.

Thoroughly wash the area around the urethra. Open a urine collection bag (a plastic bag with an adhesive paper on one end), and place it on your infant. For males, the entire penis can be placed in the bag and the adhesive attached to the skin. For females, the bag is placed over the labia. Place a diaper over the infant (bag and all). Check your baby frequently and remove the bag after the infant has urinated into it. For active infants, this procedure may take a couple of attempts -- lively infants can displace the bag, causing an inability to obtain the specimen. The urine is drained into a container for transport back to the health care provider.

Adult or child:
Blood is drawn from a vein ( venipuncture ), usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and a tourniquet (an elastic band) or blood pressure cuff is placed around the upper arm to apply pressure and restrict blood flow through the vein. This causes veins below the tourniquet to distend (fill with blood). A needle is inserted into the vein, and the blood is collected in an air-tight vial or a syringe. During the procedure, the tourniquet is removed to restore circulation. Once the blood has been collected, the needle is removed, and the puncture site is covered to stop any bleeding .

Infant or young child:
The area is cleansed with antiseptic and punctured with a sharp needle or a lancet. The blood may be collected in a pipette (small glass tube), on a slide, onto a test strip, or into a small container. Cotton or a bandage may be applied to the puncture site if there is any continued bleeding.

How to prepare for the test

Consume a normal diet with a normal amount of salt.

Infants and children:
The physical and psychological preparation you can provide for this or any test or procedure depends on your child's age, interests, previous experiences, and level of trust. For specific information regarding how you can prepare your child, see the following topics as they correspond to your child's age:
  • Infant test or procedure preparation (birth to 1 year)
  • Toddler test or procedure preparation (1 to 3 years)
  • Preschooler test or procedure preparation (3 to 6 years)
  • Schoolage test or procedure preparation (6 to 12 years)
  • Adolescent test or procedure preparation (12 to 18 years)
If the collection is being taken from an infant, a couple of extra collection bags may be necessary.

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 severely ill with acute renal failure .

Normal Values

Values vary depending on absence or presence of disease states and the state of hydration of the individual being tested. Typically, normal values range from 1 to 3%.

What abnormal results mean

An abnormal result is a value that is very low or very high. If the value is very low (less than 1%), this suggests bleeding , other causes of low blood volume, or insufficient blood flow to the kidneys (pre-renal failure; prerenal azotemia ). If the value is high (more than 3%), this suggests acute tubular necrosis .

Additional conditions under which the test may be performed:
  • Acute renal failure

What the risks are

The urine sample has no risk. Risks of venipuncture 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

Special considerations

The normal value depends on consumption of fluid and salt. Consequently, the test is only of value in specific circumstances, such as sudden decreased urine output .

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.

Update Date: 1/21/2002

Andrew Koren, M.D., Department of Nephrology, NYU-Mount Sinai Medical Center, New York, NY. Review provided by VeriMed Healthcare Network.

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Last updated: Tue, 06 Jan 2009 00:20:03 GMT