Because your kidneys remove waste, toxins, and extra fluid from the blood, a doctor will use a blood test to check your kidney function. The blood tests will show how well your kidneys are doing their job and how quickly the waste is being removed. Here are a few blood tests that are used:
Serum creatinine. A serum creatinine blood test measures the amount of creatinine in your blood. If your kidneys are not working like they should, your serum creatinine level goes up. Normal levels for you will depend on your sex, age, and the amount of muscle mass your body has.
Usually a creatinine level more than 1.2 for women and more than 1.4 for men may be a sign that the kidneys are not working like they should. If your serum creatinine test results are higher than normal, your doctor may want to run other tests.
Glomerular filtration rate (GFR). The GFR is a measure of how well your kidneys remove waste, toxins, and extra fluid from your blood. Your serum creatinine level, age, and sex are used to calculate your GFR number. Like other kidney tests, a normal GFR number for you will depend on your age and sex. If your GFR is low, your kidneys are likely not working as they should. As kidney disease progresses, your GFR goes down. The results of your test can mean the following:
- If you have a GFR number of 60 or more together with a normal urine albumin test, you are in the normal range. But you’ll still want to talk to your doctor about when you should be checked again.
- If you have a GFR number less than 60, it may mean you have kidney disease. You’ll want to talk to your doctor about treatment options that are best for you.
- If you have a GFR number less than 15, it may mean your kidneys are failing. If your results show kidney failure, you’ll likely need dialysis or a kidney transplant. You should know that if your GFR level is less than 20 consistently over a 6- to 12-month period, as a precaution, your doctor may consider you for a kidney transplant.
Blood urea nitrogen (BUN). This test measures the amount of urea nitrogen in your blood. Urea nitrogen is a waste product your body makes from the breakdown of protein in the foods you eat. Healthy kidneys filter urea nitrogen out of your blood and it leaves your body through your urine. This process helps keep your BUN level within a normal range. A normal level of urea nitrogen will depend on your age and other health conditions you may have, but usually ranges from 7 to 20. If your levels are higher than normal, this may be a sign that your kidneys are not working as well as they should. As kidney disease progresses, your BUN level goes up.
If your BUN level reveals signs of kidney disease, your doctor will use your BUN test results, along with other tests, to decide on a treatment plan that best fits your needs.