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Your Kidneys and How They Work

How Will My Doctor Detect Kidney Disease? continued...

In the lab, your blood will be tested to see how many milligrams of creatinine are in one deciliter of blood (mg/dl). Creatinine levels in the blood can vary, and each laboratory has its own normal range. In many labs, the normal creatinine range is 0.6 to 1.2 mg/dl. If your creatinine level is only slightly above this normal range, you probably will not feel sick, but the elevation is a sign that your kidneys are not working at full strength. One formula for estimating kidney function equates a creatinine level of 2.0 mg/dl to 50 percent of normal kidney function and 4.0 mg/dl to 25 percent. But, because creatinine values are so variable and can be affected by diet, you may need to have your creatinine measured regularly to see whether your kidney function is decreasing.

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.

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