Cystatin C: A Sharper View of Kidney Function

Cystatin C is a small protein made by nearly all cells in the body. It circulates in the blood and is filtered by the kidneys, which is why it is widely used as a marker of kidney function. Unlike creatinine, cystatin C levels are less affected by muscle mass, diet, and sex. Beyond the kidneys, research shows that cystatin C can also be influenced by other processes in the body, such as cardiovascular disease, diabetes, inflammation, and thyroid function. This makes it not just a kidney marker, but a broader indicator of overall health.

Cystatin C provides the most accurate estimation of kidney function more accurately, especially when combined with creatinine. Your cystatin C level also carries information about your general health, with higher levels being linked to a greater risk of conditions such as cardiovascular disease and diabetes. Studies in large populations have shown that people with elevated cystatin C tend to have a higher likelihood of health complications and an increased overall risk of mortality, even when kidney function is normal. In this way, cystatin C is valuable both as a kidney test and as a marker of broader health risk.

Rather than a single strict cutoff, cystatin C is best thought of as a “continuous marker,” meaning that risk tends to rise gradually as levels increase. Using data from large populations, we have chosen to define an optimal cystatin C level as <0.8 mg/L, indicating the lowest risk of poor health outcomes (Köttgen et al., 2008). Most people will have a level of 0.8-1.12 mg/L, which may indicate that there is room for improvement in some health areas, especially heart and metabolic health. A level of >1.12 mg/L indicates an increased risk of cardiovascular disease, diabetes, and inflammation, and it is recommended that you take efforts to lower your risk factors for these conditions.