Fractional excretion of sodium (FENa) measures the amount of sodium leaving your body relative to the amount of salt filtered through your kidneys and reabsorbed into your body. It is used to determine how well your kidneys are working.
FENa is not a test, but rather a calculation based on the concentrations of sodium and creatinine in your blood and urine. Creatinine is a product of creatine metabolism that is excreted through the urine. Your doctor will need to order blood and urine tests to make this calculation.
Your doctor may recommend this test if you are very ill and your kidneys are not functioning properly. The test will help your doctor determine if your decrease in urine production is due to:
- dehydration (a condition that occurs when your body has insufficient water)
- reduced blood flow to the kidney (can lead to kidney failure)
- kidney damage (can also lead to kidney failure)
The test is performed by your doctor in his or her office or in a hospital. The results are then sent to a laboratory to be analyzed. The test requires both a blood test and a urine sample test.
The Blood Sample
Blood will be drawn from a vein in the inside of your lower arm or the back of your hand. First, the doctor will swab the site with antiseptic and then wrap an elastic band around the top of your arm to cause the vein to swell with blood. Your doctor will then insert a fine needle into the vein and collect a blood sample into a tube attached to the needle. The band is removed and gauze is applied to the site to prevent any further bleeding.
The Urine Sample
Your doctor will collect a clean catch urine sample in an office or hospital. To do so, your doctor may insert a fine tube or catheter through your urethra and into your bladder. The urine is then drained into a sterilized container, after which the catheter is removed.
The FENa should show a normal reading of one to three percent if your kidneys are healthy.
A FENa result showing a level of less than one percent could indicate kidney malfunction caused by dehydration. Your doctor must rule out the following disorders to confirm this:
This results in an abnormally high level of urea in the bloodstream. It is caused by disorders such as kidney failure or congestive heart failure that reduce blood flow to the kidneys.
This is a disease causing the filters in the kidneys to become inflamed and develop scar tissue. Eventually they become unable to remove excess water and waste materials from the blood to produce urine.
This is a form of acute kidney failure in a person suffering from chronic liver disease.
Blood Test Risks
As with any blood test, there are minimal risks of experiencing minor bruising at the needle site. In rare cases, the vein may become swollen after blood is drawn. This condition can be treated with a warm compress several times each day.
Ongoing bleeding can be a problem if you suffer from a bleeding disorder or you are taking blood-thinning medication such as warfarin (Coumadin) or aspirin.
Urine Sample Risks
A urine sample has no associated risks.
There are no special preparations necessary for this test. Inform your doctor if you are taking any blood-thinning drugs such as aspirin warfarin (Coumadin). Your doctor may advise you to stop taking certain medications.
Your doctor may want to suspend your use of diuretics prior to the test.
The treatment that your doctor prescribes will depend on the type of kidney disorder and the extent of the damage to the organ. Your doctor may recommend that you remain in the hospital so your symptoms can be treated. Here, your health can be monitored and further complications may be avoided.
Your doctor may suggest:
- pain medication
- bed rest
- dietary changes
- chelation therapy (to reduce excessive iron build-up in your blood)
- dialysis (to purify the blood as a substitute for kidney function)