Back Blood-tests Explained


GFR


At a Glance

Why Get Tested?

To assess kidney function and diagnose, stage, and monitor chronic kidney disease (CKD)

When to Get Tested?

With a blood creatinine test as part of a routine health exam or when your healthcare practitioner thinks that you may have kidney damage or that you may be at risk for developing kidney disease

Sample Required?

A blood sample drawn from a vein in your arm; the eGFR is a calculated estimate of the actual glomerular filtration rate and is based on your blood creatinine level along with other variables such as your age, sex, and race, depending on the equation used.

Test Preparation Needed?

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.

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.

Drawing of a kidney and the urinary tract

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.

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.

The Test



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