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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.
A measured GFR 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. Because of this, the eGFR 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 excreted into the urine at a relatively steady rate. When kidney function decreases, less creatinine is excreted and concentrations increase in the blood. With the creatinine test, a reasonable estimate of the actual GFR can be determined.
The most commonly used equation for calculating the eGFR, and the one currently recommended by the National Kidney Foundation (NKF) for general use, is called the MDRD (Modification of Diet in Renal Disease Study) equation. It requires a person's serum creatinine, age, and assigned values based upon sex and race.
According to the NKF, as of January 2013, many large commercial clinical laboratories have changed from using the MDRD equation for eGFR reporting to a slightly different one that uses the same factors, the CKD-EPI equation, published in 2009. The results reported using one equation versus the other will not be identical but should give the health practitioner similar information.
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?
No test preparation is needed.