Cystatin C
Cystatin C is a small protein (13 kDa) that inhibits certain enzymes (cysteine proteases). All your nucleated cells make it at a steady rate. Your kidneys' filters (glomeruli) freely filter it, and the next part (the proximal tubule) reabsorbs and breaks down almost all of it. That makes it a sensitive marker of how well you filter blood. Unlike creatinine, its blood level barely depends on your muscle mass, sex, or protein intake. (It does rise with higher body fat, thyroid problems, and steroids, which you should factor in.) The 2021 race-free CKD-EPI equations, using creatinine plus cystatin C, give more accurate eGFR estimates where muscle mass throws creatinine off. (The cystatin-C-only equation still in use is the 2012 form, which was already race-free.) A high cystatin C strongly predicts heart events, cognitive decline, and death from all causes. And some analyses suggest it tracks true kidney aging more faithfully than creatinine-based eGFR.
Last reviewed:
This definition is educational and is not medical advice, a diagnosis, or treatment. Talk to a doctor about any health decisions. Read our full medical disclaimer