Cystatin C

Cystatin C is a protein produced by most cells in the body and filtered by the kidneys at a constant rate. Unlike creatinine, its levels are less affected by muscle mass or diet, making it potentially a more sensitive marker of early kidney dysfunction, especially when creatinine-based eGFR is borderline or muscle mass is atypical. Elevated levels indicate reduced glomerular filtration rate. Use eGFR calculated from Cystatin C (eGFRcys) or combined Creatinine-Cystatin C (eGFRcr-cys) for the best assessment of kidney function.

Estimated Glomerular Filtration Rate (eGFR, cystatin-C-based) is a measure of kidney function that uses cystatin C—a protein produced at a relatively constant rate by all nucleated cells—along with age and sex. Unlike creatinine, cystatin C is not influenced by muscle mass, making it especially useful in people with unusually high or low muscle mass, the elderly, or those with borderline creatinine-based eGFR values. It is calculated using the CKD-EPI 2012 cystatin C equation. eGFR is reported on the same scale (mL/min/1.73m²), and CKD stages use the same cutoffs (e.g., Stage 3 <60). Values ≥90 are generally ideal, while sustained values below 60 indicate reduced kidney function and warrant medical follow-up.