Cystatin C is a small protein produced at a steady rate by nearly all nucleated cells and cleared almost entirely by the kidneys. Its concentration in blood reflects how well the glomeruli filter, so it serves as a marker of kidney filtration function. Unlike creatinine, it depends little on muscle mass, which makes it useful when creatinine is hard to interpret.
No readings for this marker yet.
Raised values most often point at reduced glomerular filtration, whether from acute kidney injury or chronic kidney disease; they may also appear in thyroid overactivity and with corticosteroid use. Lowered values are less common and carry little independent meaning, though underactive thyroid can shift the result downward. Cystatin C tends to move earlier than creatinine when filtration begins to decline. A single out-of-range result is a reason to repeat the test and discuss it with a doctor, not a conclusion in itself.
Thyroid status, systemic corticosteroids, smoking, obesity and ongoing inflammation can shift the value independently of kidney function, and assays differ between laboratories, so repeat testing is best done in the same lab. It is read alongside creatinine, urea and eGFR, since eGFR is often calculated from cystatin C alone or from cystatin C together with creatinine. Uric acid speaks more to purine metabolism and gout, and is only loosely related here.
Reference ranges are general adult values and depend on lab, assay, age and sex. For orientation, not diagnosis — discuss results with your doctor.