Cystatin C test

Learn about the cystatin C test, how it shows how well your kidneys are working and when doctors use it.
Medically reviewed by
AKF's Medical Advisory Committee
Last updated
February 12, 2024

What is the cystatin C test? 

The cystatin C test is a type of blood test to see how well your kidneys are working.  

Cystatin C is a protein made by cells in your body. Here is how this works: 

  1. Your cells make cystatin C and release it into your blood at a steady pace.  
  2. Then, your kidneys filter out cystatin C to keep it at a normal level.  
  3. If the level of cystatin C in your blood is too high, it may mean your kidneys are not working well to filter cystatin C. 
  4. The cystatin C test shows how well your kidneys are filtering cystatin C out of your blood. This test is useful because it is not affected much by a person's age, gender or muscle size.

When do doctors use the cystatin C test? 

Usually, to measure how well your kidneys are working, doctors check your eGFR (estimated glomerular filtration rate) by using a blood test for creatinine (a waste product made by your body's muscles). The eGFR is an estimated number to tell doctors what stage of kidney disease you have. 

Sometimes doctors may use the cystatin C test to check your eGFR if: 

  • Past kidney function test results have come back unclear  
  • Your creatinine levels are likely to vary for other reasons, such as older age or having a lot of muscle mass, like a body builder 

What does my cystatin C test result mean? 

A high level of cystatin C in your blood means you may have a low eGFR, which can lead to chronic kidney disease (CKD). High levels of cystatin C may also mean you have a higher chance of developing heart disease or heart failure.

Talk to your doctor about what your results mean for you and your kidney health.