Cystatin C

What is cystatin C?

Cystatin C is a protein that is produced by the cells in your body. When kidneys are working well, they keep the level of cystatin C in your blood just right. If the level of cystatin C in your blood is too high, it may mean that your kidneys are not working well.

A blood test for cystatin C can help your healthcare provider find your GFR (glomerular filtration rate). Your GFR number helps your healthcare provider know how well your kidneys are working.

What is GFR (glomerular filtration rate)?

Your GFR tells a lot about how well your kidneys are working. Your kidneys are important. They filter our waste and extra water from your body, help make red blood cells, and keep your bones strong. If your GFR number is low, your kidneys may not be working as well as they should. This may be a sign of kidney disase.

How is GFR checked?

Having a simple blood test for creatinine is the first step in checking your GFR. Creatinine is a waste product made by your body's muscles. Your kidneys usually keep the level of creatinine just right. The level of creatinine in your blood and your age, race, and gender are used in a math formula to estimate your GFR.

Another way to estimate GFR is with a simple blood test for cystatin C. Like creatinine, the cystatin C number is used in a math formula with your age, race, and gender to estimate your GFR. The cystatin C test is not the usual or regular way that your healthcare provider will estimate your GFR. But it can be very helpful in some instances.

When is cystatin C used to estimate GFR?

A cystatin C blood test can be helpful if:

• A previous test for kidney function was inconclusive or needs to be confirmed. In this instance, your healthcare provider may give you cystatin C test to confirm whether or not you have kidney disease.
• You are overweight, elderly or have lots of muscle (such as a body builder). Cystatin C-based estimates for GFR are believed to be less influenced by muscle mass or diet than creatinine-based estimates.

Will I need other tests besides GFR to check for kidney disease?

Yes. In addition to GFR, you should have a simple urine test called ACR. This tests checks for protein in your urine. Your body needs protein. But it should be in the blood, not the urine. Having a small amount of protein in your urine may mean that your kidneys are not filtering your blood well enough. This can be a sign of early kidney disease.

Can kidney disease be treated?

Yes. Finding and treating kidney disease early is important. With early treatment, it may be possible to keep kidney disease from getting worse.

If you would like more information, please contact us.

© 2015 National Kidney Foundation. All rights reserved. This material does not constitute medical advice. It is intended for informational purposes only. Please consult a physician for specific treatment recommendations.