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how do physicians test your kidneys function

24-Hour Urine Sample
A 24-hour urine sample is a creatinine clearance test. It gives your doctor an idea of how much creatinine your body expels over a single day.

On the day that you start the test, urinate into the toilet as you normally would when you wake up.

For the rest of the day and night, urinate into a special container provided by your doctor. Keep the container capped and refrigerated during the collection process. Make sure to label the container clearly and to tell other family members why it’s in the refrigerator.

On the morning of the second day, urinate into the container when you get up. This completes the 24-hour collection process.

Cap and label the container, and follow your doctor’s instructions about where to drop it off. You may need to return it either to your doctor’s office or a laboratory.

Blood Samples
BUN and serum creatinine tests require blood samples taken in a lab or doctor’s office.

The technician drawing the blood will tie an elastic band around your upper arm. This makes the veins stand out. The technician will clean the area over the vein. They will then slip a hollow needle through your skin and into the vein. The blood will flow back into a test tube that will be sent for analysis.

You may feel a sharp pinch or prick when the needle enters your arm. The technician will place gauze and a bandage over the puncture site after the test. The area around the puncture may develop a bruise over the next few days. However, you shouldn’t feel severe or long-term pain.