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Fractional excretion of sodium
Alternative namesFE 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.
How to prepare for the testConsume 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:
How the test will feelWhen 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 performedThe test is usually done for patients severely ill with acute renal failure .
Normal ValuesValues 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 meanAn 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:
What the risks areThe urine sample has no risk. Risks of venipuncture include:
Special considerationsThe 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: 1/21/2002Andrew Koren, M.D., Department of Nephrology, NYU-Mount Sinai Medical Center, New York, NY. Review provided by VeriMed Healthcare Network.
Last updated: Tue, 06 Jan 2009 00:20:03 GMT