Alternative names
FE sodium; FENa

Definition

Fractional excretion of sodium (FENa) is the fraction of sodium actually excreted by your body relative to the amount filtered 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 serum chemistry tests are necessary to perform this calculation.

How the test is performed

Samples of blood and urine are taken simultaneously and analyzed for sodium and creatinine levels.

URINE COLLECTION

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.

For infants, 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.

BLOOD COLLECTION

Blood is drawn from a vein, usually from the inside of the elbow or the back of the hand. The puncture site is cleaned with antiseptic, and an elastic band or blood pressure cuff is placed around the upper arm to cause the veins to swell 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 band 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.

In infants or young children, 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.

For infants and children:
The preparation you can provide for this test depends on your child’s age, previous experiences, and level of trust. For general information regarding how you can prepare your child, see the following:

 
• 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 who are severely ill with acute kidney failure.

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. The test helps answer the question - is this drop in urine because of reduced blood flow to the kidney (like in dehydration, bleeding, low blood pressure) or is it due to kidney damage itself?

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 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 test is only of value in specific circumstances, such as sudden reduced urine output.  
• Use of diuretic medicines before performing the tests may give misleading results.