Fractional Excretion of Sodium (FENa).

Though I have listed this under tests it is better called a calculation based on the results of tests measuring the concentration of sodium and creatinine in the blood and urine.

The calculation gives the fraction of sodium that your body actually excretes in relation to the amount that the kidney filters.

Why it is done.
This calculation can help in deciding whether impaired kidney function is due to dehydration or actual kidney damage.

Patients going for this test must refrain from taking diuretics for a period (your doctor will tell you how long) before the test.

How does one get the results?
Blood and urine samples are taken and analysed to find their serum sodium and creatinine levels. The results from these are entered into a formula and the fractional excretion of sodium mathematically calculated and given as a percentage.

What is the formula?
%. value FENa =
((urine Na X serum Cr) / (urine Cr X serum Na)) X 100

Na stands for sodium
Cr stands for creatinine

Normal values -
These range from 1-3%.

If the value is below 1% dehydration can be the cause of impaired kidney function.

To confirm dehydration, the below factors, which also give values below 1% must be ruled out :-

1. Acute Glomerulonephritis - a kidney disease in which the kidneys' glomeruli (filters) become inflamed and scarred, and slowly lose their ability to remove wastes and excess water from the blood to make urine

2. Hepatorenal syndrome - acute kidney failure occurring without other cause in a person with severe liver disease.

3. Prerenal azotemia - abnormally high level of nitrogen-type wastes in the
bloodstream caused by conditions that reduce blood flow to the kidneys such as congestive heart failure.

4. Their urinary tract may be partially obstructed.

However if the value is above 3% then acute tubular necrosis and severe obstruction of the urinary drainage of both kidneys is suggested and dehydration can be ruled out.

Back to top