Cystatin C is a protein produced by all nucleated cells and is considered a more reliable marker of kidney function than creatinine. Elevated levels of cystatin C in the blood may indicate reduced kidney function or early stages of kidney disease, as it is less influenced by factors like muscle mass or diet. It is commonly used to estimate glomerular filtration rate (GFR), helping detect kidney problems earlier, especially in cases where traditional markers like creatinine may not be accurate.