Cystatin C is a small protein produced at a constant rate by all nucleated cells in the body. Unlike creatinine, cystatin C is unaffected by muscle mass, exercise, diet, or sex — making it a more reliable marker of kidney filtration in people where creatinine can be misleading, such as elderly individuals, athletes, people with low muscle mass, and those with obesity.
Cystatin C is freely filtered at the glomerulus and completely reabsorbed and broken down in the kidney tubules — none enters the urine. As kidney filtration declines, cystatin C accumulates in the blood. Cystatin C-based eGFR equations are increasingly recommended as adjuncts or alternatives to creatinine-based eGFR, particularly for detecting early-stage kidney disease where creatinine may still appear normal.
FAQs
Is cystatin C better than creatinine for kidney testing?
Cystatin C is more accurate in elderly individuals, those with low muscle mass, athletes, and people with obesity. Combined creatinine and cystatin C equations provide the most accurate GFR estimate overall.
Can cystatin C be used instead of creatinine for eGFR?
Yes. Cystatin C-based eGFR is a validated alternative to creatinine-based eGFR. The combined CKD-EPI Cr-CysC equation is considered the most accurate and is increasingly used in clinical practice.
Does cystatin C appear in urine?
No. Unlike creatinine, cystatin C is completely reabsorbed and catabolised in kidney tubules and does not appear in normal urine. Urine cystatin C can be used as a marker of tubular injury.
Why is cystatin C not routinely tested like creatinine?
Cystatin C testing is more expensive and not as universally available. As costs decrease and guidelines increasingly recommend it for CKD staging and cardiovascular risk assessment, its use is growing.
