Cystatin C is a low-molecular-weight cysteine protease inhibitor produced at a constant rate by all nucleated cells and freely filtered by the glomerulus, making it a sensitive endogenous marker of glomerular filtration rate (GFR). Unlike serum creatinine, cystatin C levels are largely independent of muscle mass, age, sex, and diet, offering a more reliable estimate of kidney function across diverse populations. It is used both as a standalone GFR estimator and in combination with creatinine-based equations (CKD-EPI creatinine-cystatin C) to improve accuracy. Cystatin C is constitutively expressed by virtually all nucleated cells at a stable rate governed primarily by transcriptional regulation, resulting in a steady-state serum concentration that reflects glomerular filtration efficiency. After free filtration at the glomerulus, it is almost completely reabsorbed and catabolized by proximal tubular cells, with negligible tubular secretion, making serum levels a reliable surrogate for GFR. When GFR declines, cystatin C accumulates in the blood proportionally, often rising earlier than creatinine in mild-to-moderate kidney function impairment.
Cystatin C is a low-molecular-weight protein produced at a constant rate by all nucleated cells and filtered freely by the glomerulus. Unlike creatinine, its serum concentration is not significantly influenced by muscle mass, diet, or sex, making it a more accurate marker of glomerular filtration rate (GFR) in populations where creatinine-based estimates are unreliable.
Elevated cystatin C indicates reduced GFR and impaired renal filtration. It detects early-stage kidney dysfunction that creatinine-based eGFR may miss, particularly in individuals with higher-than-average or lower-than-average muscle mass. Combined creatinine-cystatin C equations (CKD-EPI 2021) provide the most accurate GFR estimation.
Creatinine is a byproduct of creatine phosphate metabolism in skeletal muscle, so individuals with high lean mass routinely have elevated serum creatinine that does not reflect kidney impairment. Cystatin C avoids this confound entirely. For muscular individuals, cystatin C-based GFR estimation is more physiologically representative of actual kidney function.
Turnaround Time
3 days (up to 15 days)
Fasting Required
No
Method
Turbidimetric (directly traceable to ERM-DA471/IFCC, the international standard reference material)
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