Cystatin C, Serum or Plasma, Quantitative

What Is Cystatin C?

Cystatin C is a protein that your body produces constantly, regardless of diet or muscle mass. Unlike creatinine, its levels don’t change much with diet or exercise, making it a reliable marker for kidney function. Your doctor may order this test to check how well your kidneys are working, especially if you have symptoms like fatigue, swelling, or changes in urine output.

Why Might You Need This Test?

Doctors may recommend a cystatin C test to evaluate kidney function in people with risk factors like diabetes, high blood pressure, a family history of kidney disease, or symptoms such as frequent urination, swelling in the legs or ankles, or unexplained fatigue. It can also be used to monitor kidney disease progression or guide treatment decisions.

How Is the Test Performed?

A blood sample is taken from a vein in your arm. The sample is sent to a lab where it’s analyzed to measure cystatin C levels. No special preparation is needed, but you may be asked to fast for 8–12 hours before the test if instructed by your doctor.

What Do Results Mean?

Higher than expected cystatin C levels may indicate kidney problems, while lower levels might suggest conditions like severe muscle wasting or certain medications. Your doctor will interpret results in the context of your overall health and other tests.