At a Glance
Why Get Tested?
In the UK, the National Institute for Health and Care Excellence (NICE) guidance (CG73) states: “Whenever a request for serum creatinine measurement is made, clinical laboratories should report an estimate of GFR (eGFR)”
When to Get Tested?
As a practical test to look for evidence of kidney damage. For a more definitive assessment of kidney function or to monitor changes in kidney function if you already have kidney disease, measurement of creatinine clearance is preferred.
eGFR is an estimate of actual glomerular filtration rate, calculated using your age, weight, gender, and serum creatinine (requires a blood sample from a vein in your arm); in some formulas, race is also used in the calculation. Measured Creatinine Clearance, which gives an assessment of Glomerular Filtration Rate (GFR), is calculated from serum and urine creatinine levels and requires a blood sample from a vein in your arm and normally a 24-hour collection of urine. Calculated creatinine clearance is an estimate of creatinine clearance based on the serum creatinine value only and used when it is either not practical or possible to collect a 24 hr urine
Test Preparation Needed?
The Test Sample
What is being tested?
Glomerular filtration rate (GFR) is a measure of the function of your kidneys. Glomeruli are tiny filters in your kidney that allow waste products to be removed from the blood, while preventing loss of important proteins and blood cells. The rate refers to the amount of blood that is filtered per minute. When a person's kidney function declines due to damage or disease, then 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 minimise the damage. This is important, as symptoms of kidney disease may not be noticeable until as much as 30 to 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 in a research setting. Because of this, an estimate – the eGFR – is usually used.
The eGFR is a calculation based on a serum creatinine test result. 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. Using the creatinine test result , a reasonable estimate of the actual GFR can be determined.
How is the sample collected for testing?
A blood sample is taken by needle from a vein in the arm; this is sufficient for eGFR - for calculation of creatinine clearance, your weight and age are needed as well. Measured creatinine clearance requires both a blood sample taken from a vein in your arm and a 24-hour sample of urine.
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.
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NOTE: This article is based on research that utilizes the sources cited here as well as the collective experience of the Lab Tests Online Editorial Review Board. This article is periodically reviewed by the Editorial Board and may be updated as a result of the review. Any new sources cited will be added to the list and distinguished from the original sources used.