Cystatin C

If your kidneys have been damaged by kidney disease, they can't filter your blood as fast as they should. A GFR test checks for kidney disease by measuring how much blood your kidneys filter each minute.

GFR can be measured directly, but it is a complicated test to do. So health care providers usually estimate GFR based on the amount of certain waste substances in your blood. An estimated GFR is called an eGFR.

eGFR may also be calculated using the level of cystatin C in your blood. This is a protein that many cells in your body make. Cystatin C levels are not affected by muscle size, age, or diet, so some researchers think cystatin C provides a more accurate estimate of GFR than creatinine. In certain cases, creatinine and cystatin C levels are both used to calculate eGFR in adults.

Cystatin C levels rise as kidney function falls, so a higher level usually means slower filtering by the kidneys, while a lower level usually means better filtering.

Because cystatin C is not strongly affected by your muscle mass, age, sex, or race, it can be more accurate than creatinine for estimating kidney function in people who are very muscular, very thin, much older, or losing muscle from illness or aging.

Cystatin C also reacts more quickly than creatinine when kidney function changes suddenly, which can be helpful for people in the hospital or going through a rapid illness.

An eGFR below 60 mL/min/1.73 m² that lasts for more than 3 months is one of the signs of chronic kidney disease, and lower numbers point to more serious kidney problems. Babies under 3 months old have higher cystatin C than older children, because their kidneys are still maturing.

If you're diagnosed with kidney disease, your provider will explain what you can do to protect your kidneys from more damage and help prevent kidney failure.