Cystatin C

Cystatin C is a small protein produced at a steady rate by virtually every cell in your body. It is filtered out of the blood by your kidneys and then broken down, making it an exceptionally precise marker of how well your kidneys are performing their filtration job. This test is particularly valuable because it overcomes some of the limitations of the more commonly used creatinine-based kidney function tests. Creatinine levels are significantly influenced by muscle mass, which means that athletes, bodybuilders, and anyone with a muscular build can receive misleadingly high creatinine readings, and consequently a falsely low estimated glomerular filtration rate, even when their kidneys are perfectly healthy. Cystatin C is not affected by muscle mass, diet, or sex, so it provides a more reliable picture of kidney function across a wide range of body types.

Your kidneys filter your entire blood volume roughly forty times every day, acting like an extraordinarily efficient purification system. As blood passes through the tiny filtering units called nephrons, small molecules like cystatin C are removed from the bloodstream. In healthy kidneys, this process is highly efficient and cystatin C levels in the blood remain low and stable. If the kidneys' filtering capacity starts to decline, cystatin C is not cleared as effectively and its level in the blood begins to rise. Because the body produces cystatin C at a remarkably constant rate regardless of age, muscle mass, or inflammation, any change in your blood level is a strong signal that the kidneys themselves are responsible.

Very few factors outside of kidney function significantly affect cystatin C, which is part of what makes it such a reliable marker. Thyroid disorders can influence production rates, an overactive thyroid may slightly raise it and an underactive thyroid may slightly lower it, but these effects are modest. High-dose corticosteroid medication has also been reported to have a small influence. Unlike creatinine, a high-protein meal or an intense workout the day before the test will not skew your result. If your cystatin C level comes back higher than expected, it is important to discuss this with your GP. They can compare it with your creatinine and estimated glomerular filtration rate to build a complete picture of your kidney health, and determine whether any lifestyle adjustments, monitoring, or further investigation would be appropriate.