Cystatin C
A protein filtered by the kidneys that estimates kidney function independent of muscle mass.
| Typical reference range | 0.52–1.03 mg/L |
Ranges vary by laboratory, method, age and sex: always use the range printed on your own report. Why labs differ.
What it measures
Cystatin C is a small protein produced at a constant rate by all nucleated cells and filtered freely by the kidneys. Unlike creatinine, its production is largely independent of muscle mass and diet.
Why it matters
Cystatin C provides a more accurate kidney function estimate in people with unusual muscle mass (athletes, bodybuilders, creatine users, frail or elderly people). Higher levels also independently predict cardiovascular events and mortality.
What a high result can mean
High values suggest reduced kidney filtration. Levels can also increase with high-dose corticosteroids, hyperthyroidism, inflammation, obesity and smoking.
What a low result can mean
Low values are generally not a concern; hypothyroidism can lower cystatin C.
What affects the result
No fasting required. Thyroid dysfunction and steroids affect it. Combining cystatin C with creatinine (eGFRcr-cys) gives the most accurate estimate. Reference ranges vary by assay and lab.
Units and conversion
Usually reported in mg/L.
- From mg/dL: multiply by 10 to get mg/L
- From nmol/L: multiply by 0.013 to get mg/L
Also called on lab reports
Cystatin-C, Cystatin C, Serum
Related tests: Kidney Function
This page is educational and does not replace advice from a healthcare professional.