What it measures
Cystatin C is a small protein made at a steady rate by all the body's cells and cleared by the kidneys. Unlike creatinine, it is not affected by muscle mass, so it gives a second measure of how well the kidneys filter.
Why people test it
- Used to confirm a borderline kidney result from creatinine and eGFR.
- Useful where muscle mass is unusually high or low, as in very muscular or frail people.
- Combined with creatinine to give an eGFR that leans less on muscle mass.
- Used by people taking creatine or carrying a lot of muscle, whose creatinine can read high.
Typical range
Ranges vary between labs, with the method used, age and sex, and your report shows the lab's own range. One NHS trust lab gives adult ranges that fall between 0.31 and 0.99 mg/L, depending on age and sex. Source: Oxford University Hospitals NHS Foundation Trust: biochemistry reference ranges.
Finger-prick or vein?
A finger-prick sample is usually fine for cystatin C. A vein draw works too and is used when it is part of a larger kidney panel.
Creatine raises creatinine, which can make kidney results look worse than they are. Cystatin C gives a truer read.
