Your Kidneys and How They Work
How Will My Doctor Detect Kidney Disease? continued...
The doctor may refer to the measure of
creatinine in your blood as your serum creatinine. Do not confuse your serum
creatinine number with your creatinine clearance number.
A creatinine clearance test shows how fast
your kidneys remove creatinine from the blood. Clearance is measured in
milliliters per minute (ml/min).
To measure your creatinine clearance, you
will need to collect urine for 24 hours. Your doctor or nurse will give you a
container to collect urine and special instructions for timing the 24-hour
When you take the collected urine to your
doctor or laboratory, you will also give a blood sample at that time. Your
doctor will measure your creatinine clearance by comparing the amount of
creatinine in your urine to the amount of creatinine in your blood.
For men, a normal creatinine clearance rate
is 97 to 137 ml/min. For women, the normal rate is 88 to 128 ml/min. If your
number is below this normal range, your kidneys are not working at full
Blood Urea Nitrogen (BUN)
Blood carries protein for use by cells
throughout the body. After the cells use the protein, the remaining waste
product is returned to the blood as urea, a compound containing nitrogen.
Healthy kidneys take urea out of the blood and send it to the bladder in the
urine. If your kidneys are not working well, the urea will stay in the
Normal blood contains 7 to 20 milligrams of
urea per deciliter of blood. If your BUN is more than 20 mg/dl, your kidneys
may not be working at full strength. Other possible causes of an elevated BUN
include dehydration and heart failure.
Healthy kidneys take wastes out of the
blood but leave in protein. Impaired kidneys may fail to separate the protein
from the wastes. Proteinuria means protein in the urine, and it is a sign of
poor kidney function. If your urine makes foam in the toilet, it may contain
high levels of protein. Your doctor may test for protein using a dipstick in a
small sample of your urine taken in the doctor's office. The color of the
dipstick indicates the presence or absence of proteinuria. For a more precise
measurement, you may need to collect urine for 24 hours.
If blood and urine
tests indicate reduced kidney function, your doctor may recommend additional
tests to help identify the cause of the problem. Renal imaging (taking pictures
of the kidneys) methods include ultrasound, computed tomography (CAT scan), and
magnetic resonance imaging (MRI). These tools are most helpful in finding
unusual growths or blockages to the flow of urine.
Renal biopsy. Your doctor may want
to see a tiny piece of your kidney tissue under a microscope. To obtain this
tissue sample, the doctor will perform a renal biopsy -- a hospital procedure
in which the doctor inserts a needle through your skin into the back of the
kidney. The needle retrieves a strand of tissue about 1/2 to 3/4 of an inch
long. You will lie prone (on your stomach) on a table and will receive local
anesthetic to numb the skin. The sample tissue will help the doctor identify
problems at the cellular level.