Cystatin C may be used as an alternative to creatinine and creatinine clearance to screen for and monitor kidney dysfunction in those with known or suspected kidney diseases. It may be especially useful in detecting early decreases in kidney function (as measured by GFR) and in those for which creatinine measurement is a problem: for instance in those who have liver cirrhosis, are very obese, malnourished or have a very small muscle mass (creatinine is produced in the muscles). It may also have a role along with other tests, such as urine microalbumin to monitor kidney function over time in those with diseases such as diabetes.

When available Cystatin C may be requested when you have a known or suspected disease that affects or potentially affects kidney function and GFR. Your doctor requests Cystatin C to check for early kidney dysfunction and/or to monitor known impairment over time. It may be requested by itself or be done along with other tests such as urine microalbumin.

Early kidney impairment may not cause elevated creatinine levels or symptoms. If your doctor is not satisfied with your creatinine clearance test or feels that your GFR would be better estimated by another test, Cystatin C may prove to be a useful alternative.

An elevated serum Cystatin C indicates a low GFR and kidney dysfunction. Since Cystatin C is produced throughout the body at a constant rate and removed by glomerular filtration (and subsequently broken down in another part of the kidney), it should remain at a steady state in the blood if the kidneys are working efficiently and the GFR is normal.

There are indications that Cystatin C levels may also be elevated in cancer and rheumatic diseases, even when kidney function is normal, and levels can be affected by some drugs: corticosteroids can increase levels, while cyclosporine can decrease them. They are not, however, generally affected by most drugs, by infections, diet, inflammation or by tumour burden (amount of cancer someone has). They also do not appear to be affected by gender, age (except very young and old), or race.

Cystatin C is not currently routinely available.