Cystatin C
What it measures, and what changes the number.
Cystatin C is a small protein produced at a steady rate by nearly all cells in the body and filtered out of the blood by the kidneys. Its blood level is used as another marker of kidney filtration.
It is typically read with creatinine and eGFR. Because cystatin C depends much less on muscle mass than creatinine does, it can help when creatinine may be misleading, and some eGFR equations combine both markers.
Fasting is not usually required. Thyroid activity, corticosteroid medicines and inflammation can influence the level. Results depend on the assay, and international standardization of calibrators means older and newer results, or results from different labs, may not be directly comparable.
Often read alongside: Creatinine, eGFR (CKD-EPI), Creatinine clearance, Urea