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Classification: Borderline high
Cystatin C 1.1 mg/L is mildly elevated — suggesting eGFR around 60-75 by cystatin C. This is the range where cystatin C catches reductions that creatinine might miss.
Cystatin C is a protein produced by all nucleated cells and freely filtered by the kidneys — unlike creatinine, it's independent of muscle mass, age, and sex. It's used to confirm borderline eGFR results and detect kidney disease when creatinine looks normal.
Blood test. Ordered when creatinine-based eGFR is uncertain (borderline 45-59 without albuminuria), or in people with extremes of muscle mass. KDIGO recommends cystatin C to confirm CKD in borderline cases.
Reference: CKD-EPI 2012 cystatin C equation (NEJM 2012); KDIGO 2024 guideline • Normal range: 0.6-1.0 mg/L (adults)