Cystatin C

Ranges vary between laboratory methods more than for most tests and rise gently with age; use the range the lab printed.

Cystatin C is a small protein made by almost every cell in the body at a steady rate and cleared from the blood by the kidneys. Unlike creatinine it does not depend on how much muscle a person has, so it gives a second, independent view of kidney filtering. The test measures the level in a blood sample.

Doctors request it when the creatinine-based eGFR may be misleading, for example in very muscular or very slim people, in older age, or when a result needs confirming before a decision is made. It is also used to calculate a cystatin-based eGFR.

• Reduced kidney filtering
• An overactive thyroid
• Steroid tablets
• Smoking and being overweight, by a small amount
• Some inflammatory conditions

• An underactive thyroid
• Very low body weight in some studies
• Pregnancy, because the kidneys filter more

• Thyroid activity and steroid medicines move it without any change in the kidneys
• Different laboratory assays give somewhat different numbers
• Smoking and body weight shift it slightly in the general population

No fasting needed. Usually requested alongside creatinine so the two estimates can be compared.

Written by a UK consultant physician. Typical UK adult reference ranges; your laboratory's own printed range takes precedence.