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Learning About Kidney Function Tests

Kidneys, ureters, and bladder in body, with detail showing blood flowing from body to kidney, kidney filtering blood to remove waste (urine), urine sent to bladder, and filtered blood flowing back to body.

What is kidney function?

How well your kidneys work is called kidney function. Your kidneys have the important job of filtering your blood. They remove waste products and extra fluid from your body as urine.

Why is it important to check kidney function?

When your kidneys do not work as they should, waste can build up in your blood. This can make you sick. If you have long-term (chronic) kidney disease, you may not have any symptoms until your kidney function is very low.

Blood and urine tests can help show how well your kidneys are working. They can help your doctor know:

  • If the disease started all of a sudden or if you have had it a long time.
  • The cause of the kidney damage.
  • The best type of treatment to slow the damage.
  • How well treatment is working.
  • When to begin dialysis or have a kidney transplant.

What tests might be used?

Tests that check kidney function may include:

Estimated glomerular filtration rate (eGFR).
This test shows how well the blood is filtered by the kidneys. It's often calculated using a formula that includes a person's age, sex, and blood creatinine levels. The lower the eGFR number, the worse the kidney function.
Creatinine clearance test.
This test also measures how well your kidneys are filtering your blood. The test uses both blood and urine samples. If your creatinine clearance goes down, this means that your kidneys aren't filtering your blood as well.
Urine tests.
These tests measure certain substances, such as protein, in your urine. These tests may be done over 24 hours. Or you may do a one-time sample. Kidney damage can cause increased protein in the urine. This is because the kidneys can't filter protein as well as they used to.
Blood urea nitrogen (BUN) test.
This test measures the amount of nitrogen in your blood. This comes from a waste product called urea. If your kidneys can't remove urea from the blood normally, your BUN level rises.

How often should you be tested?

Experts recommend screening tests for people who are at higher risk of the disease. This includes people with diabetes or high blood pressure. If you have diabetes, your doctor may recommend checking your kidney function yearly. This is often done with blood and urine tests. When to begin testing depends on the type of diabetes you have.

Follow-up care is a key part of your treatment and safety. Be sure to make and go to all appointments, and call your doctor or nurse advice line (811 in most provinces and territories) if you are having problems. It's also a good idea to keep a list of the medicines you take. Ask your doctor when you can expect to have your test results.

Where can you learn more?

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