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.

Creatinine Clearance

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 collection.

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 strength.

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 blood.

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.

Proteinuria

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.

Additional Tests

Renal imaging. 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.