Tests to Measure Kidney Function, Damage and Detect Abnormalities
Table of Contents
Healthy kidneys remove wastes and excess fluid from the blood. Blood and urine tests show how well the kidneys are doing their job and how quickly body wastes are being removed. Urine tests can also detect whether the kidneys are leaking abnormal amounts of protein, a sign of kidney damage. Here's a quick guide to the tests used to measure kidney function.
Serum creatinine: Creatinine is a waste product that comes from the normal wear and tear on muscles of the body. Creatinine levels in the blood can vary depending on age and body size. A creatinine level of greater than 1.2 for women and greater than 1.4 for men may be an early sign that the kidneys are not working properly. As kidney disease progresses, the level of creatinine in the blood rises.
Glomerular filtration rate (GFR): This test is a measure of how well the kidneys are removing wastes and excess fluid from the blood. It is calculated from the serum creatinine level using age and gender. Normal GFR can vary according to age (as you get older it can decrease). The normal value for GFR is 90 or above. A GFR below 60 is a sign that the kidneys are not working properly. Once the GFR decreases below 15, one is at high risk for needing treatment for kidney failure, such as dialysis or a kidney transplant.
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. As kidney function decreases, the BUN level rises.
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.
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.
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
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, can give an more accurate measurement of how well your kidney are working and how much protein leaks from the kidney into the urine in one day.
Urinalysis: 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.
Urine protein: This may be done as part of a urinalysis or by a separate dipstick test. An excess amount of protein in the urine is called proteinuria (pro-TEEN-yu-ree-uh). A positive dipstick test (1+ or greater) should be confirmed using a more specific dipstick test such as an albumin specific dipstick or a quantitative measurement such as an albumin-to-creatinine ratio.
Microalbuminuria: This is a more sensitive dipstick test which can detect a tiny amount of protein called albumin in the urine. People who have an increased risk of developing kidney disease, such as those with diabetes or high blood pressure, should have this test or an albumin-to-creatinine ratio if their standard dipstick test for proteinuria is negative.
Albumin-to-creatinine ratio (ACR): This is a urine test to see how well your kidneys are working. First, the amount of albumin in your urine is measured. Albumin is the most common type of protein found in your urine. Higher amounts of albumin in the urine can be a sign that your kidneys are not working as well as they should. ACR is calculated by dividing the amount of urine albumin by the amount of urine creatinine. An ACR below 30 is considered normal. An ACR between 30-300 means you have moderately increased albuminuria. An ACR above 300 means you have severely increased albuminuria.
Creatinine clearance: Creatinine is a waste product that comes from the normal wear and tear on muscles of the body. Creatinine clearance test compares the creatinine in a 24-hour sample of urine to the creatinine level in your blood to show how much waste products the kidneys are filtering out each minute.
Kidney Numbers and the CKD Heat Map
Click here to download a printable resource that describes the kidney numbers that everyone should know and how your doctor uses them to diagnose chronic kidney disease (CKD). You will also learn how the kidney numbers fit onto the CKD Heat Map, which helps to guide your treatment plan.