As a nephrologist, I order blood and urine tests to help me check if your kidneys are properly removing waste and excess fluid from your body. Urine tests can also detect whether the kidneys are leaking abnormal amounts of protein, a sign of kidney damage.
Here is a quick guide to the tests used to measure kidney function:
Blood tests
1. Serum creatinine. Creatinine is a waste product that comes from the normal wear and tear of muscles in the body, and levels can vary depending on age, race and body size. Elevations in creatinine typically indicate that the kidneys are not working properly.
2. Estimated glomerular filtration rate (eGFR). This is calculated from the serum creatinine level using age and gender, using a mathematical formula called MDRD or the CKD-EPI equation). Normal eGFR can vary according to age, decreasing as you get older. The normal value for eGFR is 90 or above, while an eGFR below 60 is a sign that the kidneys are not working well enough. Once the eGFR decreases below 15, one is at high risk for needing treatment for kidney failure, such as dialysis or a kidney transplant.
3. Blood urea nitrogen (BUN). Urea nitrogen comes from the breakdown of protein in the foods you eat. A normal BUN level is between 7 and 20. Although other factors may cause fluctuations in BUN levels, we typically associate an increase with a decrease in kidney function.
NORMAL BUN level is between 7 and 20.
Urine tests
Some urine tests require only a couple tablespoons of urine. Other tests require collection of all urine produced for a full 24 hours. A 24-hour urine test shows how much urine your kidneys produce, and it can also give a more accurate measurement of how well your kidneys are working and how much protein leaks from the kidneys into the urine in one day.
1. Urinalysis. This includes microscopic examination of a urine sample as well as a dipstick test. The dipstick is a chemically treated strip, which is dipped into a urine sample. The strip changes color in the presence of abnormalities such as excess amounts of protein, blood, pus, bacteria and sugar. A urinalysis can help to detect a variety of kidney and urinary tract disorders, including chronic kidney disease, diabetes, bladder infections, and kidney stones.
2. Urine protein. This may be done as part of a urinalysis or by a separate dipstick test. You usually should not have protein in your urine. An excess amount of protein in the urine is called proteinuria, and can indicate kidney damage. A positive dipstick test (1+ or greater) should be confirmed using a quantitative measurement called a random urine protein-to-creatinine ratio.
Imaging tests
1. Ultrasound. This test uses sound waves to get a picture of the kidney. It may be used to look for abnormalities in size or position of the kidneys or for obstructions such as stones or tumors.
2. CT scan. This imaging technique uses X-rays to picture the kidneys. It may also be used to look for structural abnormalities and the presence of obstructions. This test may require the use of intravenous contrast dye which can be of concern for those with kidney disease.
Other tests
1. Glomerular filtration rate (GFR) scan. A GFR scan is a test that shows the function and blood flow of your kidneys using a radioactive tracer. The amount of radiation used in this test is small and well within limits that are not harmful to you. The test is usually done in the Nuclear Medicine Department of your hospital and takes about four hours.
2. Kidney biopsy. A kidney biopsy is performed by using a thin needle with a sharp cutting edge to slice small pieces of kidney tissue for examination under a microscope. A biopsy may be done occasionally for one of the following reasons:
•To identify a specific disease process and determine whether it will respond to treatment
•To evaluate the amount of damage that has occurred in the kidney
•To find out why a kidney transplant may not be doing well
•It should be noted that not all of these tests are done at the same time, and should only be done when appropriate, as advised by your physician.