Cys-c (Cystatin C)

Cystatin C is a small protein produced steadily by cells throughout the body and cleared from the blood by the kidneys. Because its level in the blood depends closely on how well the kidneys are filtering, measuring cystatin C provides an accurate assessment of kidney function. It is increasingly used as an alternative or complement to the more traditional creatinine test.

What it can help assess

• Kidney function: cystatin C is a sensitive marker of how well your kidneys are filtering waste from the blood
• Detecting early kidney changes: it can pick up a reduction in kidney function earlier than creatinine in some cases
• Assessing function more reliably in certain people: unlike creatinine, cystatin C is largely unaffected by muscle mass, age, sex, and diet, which makes it particularly useful in people where creatinine can be misleading, such as those with very high or very low muscle mass
• Estimating filtration rate: it can be used to calculate an estimated glomerular filtration rate (eGFR), a key measure of kidney health

An important note on interpretation

Cystatin C reflects kidney function and is usually interpreted alongside other kidney markers and your overall clinical picture. As with any single test, an abnormal result indicates that further assessment may be needed rather than giving a diagnosis on its own. Any abnormal result should be discussed with your GP, who can interpret it in context and advise on any further testing.

Why it matters

Kidney problems often develop quietly, with few symptoms until function is significantly reduced. Cystatin C offers a precise and reliable way to assess kidney function, helping to detect changes early and monitor kidney health over time, particularly where a more accurate measure than creatinine is needed.