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The Test Sample
What is being tested?
Glomerular filtration rate (GFR) is a measure of the function of the kidneys. This test measures the level of creatinine in the blood and uses the result in a formula to calculate a number that reflects how well the kidneys are functioning, called the estimated GFR or eGFR.
Glomeruli are tiny filters in the kidneys that allow waste products to be removed from the blood, while preventing the loss of important constituents, including proteins and blood cells. Every day, healthy kidneys filter about 200 quarts of blood and produce about 2 quarts of urine. The GFR refers to the amount of blood that is filtered by the glomeruli per minute. As a person's kidney function declines due to damage or disease, the filtration rate decreases and waste products begin to accumulate in the blood.
Chronic kidney disease (CKD) is associated with a decrease in kidney function that is often progressive. CKD can be seen with a variety of conditions, including diabetes and high blood pressure. Early detection of kidney dysfunction can help to minimize the damage. This is important as symptoms of kidney disease may not be noticeable until as much as 30-40% of kidney function is lost.
Measuring glomerular filtration rate directly is considered the most accurate way to detect changes in kidney status, but measuring the GFR directly is complicated, requires experienced personnel, and is typically performed only in research settings and transplant centers. Because of this, the estimated GFR is usually used.
The eGFR is a calculation based on a serum creatinine test. Creatinine is a muscle waste product that is filtered from the blood by the kidneys and released into the urine at a relatively steady rate. When kidney function decreases, less creatinine is eliminated and concentrations increase in the blood. With the creatinine test, a reasonable estimate of the actual GFR can be determined.
Different equations may be used to calculate eGFR. The following two are most common and require a person's blood creatinine result, age, and assigned values based upon sex and race.
- Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) creatinine equation (2009)—recommended by the National Kidney Foundation for calculating eGFR in adults
- Modification of Diet in Renal Disease Study (MDRD) equation—some laboratories continue to use this equation
The results reported using one equation versus the other will not be identical but should give a healthcare practitioner similar information.
A different set of CKD-EPI calculators was published in 2012. These equations use the result of a cystatin C test. (For more on these, see Common Questions #7.) There is also a modified equation for people 18 and under that takes the blood urea nitrogen (BUN) level into consideration along with the factors listed above.
How is the sample collected for testing?
A blood sample is taken by needle from a vein in the arm. Depending on the formula used, a person's age, sex, race, height, and weight may also be needed.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
This test uses a blood creatinine level to calculate a result. You may be instructed to fast overnight or refrain from eating cooked meat; some studies have shown that eating cooked meat prior to testing can temporarily increase the level of creatinine.