Last Updated: July 26, 2023
Medically reviewed by NKF Patient Education Team
Table of Contents
Purpose
Usually, healthcare providers check your eGFR (estimated glomerular filtration rate) with a blood test for creatinine, which is a waste product made by your body's muscles. Healthy kidneys keep the level of creatinine just right. The level of creatinine in your blood along with your age and gender are used to calculate your eGFR.
Sometimes, healthcare providers use a blood test for cystatin C to calculate eGFR. A cystatin C blood test can be useful if:
- A past test for kidney function did not give clear results about your kidneys, so your healthcare provider wants to check again with a cystatin C test to be sure.
- You are older or have lots of muscle (such as a bodybuilder), where creatinine levels can vary. In these cases, a cystatin C test may give a more accurate result than a creatinine test.
Sometimes the cystatin C test may not be accurate, such as in people with untreated thyroid conditions and those who are on steroids.
During the test
A healthcare provider will take a blood sample from a vein in your arm, using a small needle. After the needle is inserted, a small amount of blood will be collected into a test tube or vial. You may feel a little sting when the needle goes in or out. This usually takes less than five minutes.
