Cystatin C is a low‑molecular-weight (≈13 kDa) member of the cystatin superfamily of cysteine protease inhibitors. It is produced at a relatively constant rate by most nucleated cells and circulates in plasma; because it is freely filtered at the glomerulus and then reabsorbed and catabolized in proximal tubular cells, its steady‑state concentration in blood closely reflects clearance processes. First described in biochemical studies in the mid‑20th century and adopted clinically from the 1980s onward, cystatin C became of interest as an alternative to creatinine because it is less influenced by muscle mass, age, and sex. In addition to its utility as a filtration marker, cystatin C participates in extracellular protease regulation and can be modulated by inflammation, thyroid status, and certain medications—factors relevant to interpretation.

Higher blood cystatin C may be associated with reduced clearance of small proteins and thus with lower filtration capacity, but it can also rise with non‑renal influences such as systemic inflammation, corticosteroid exposure, or thyroid dysfunction. Lower cystatin C can reflect faster clearance, increased metabolic activity (for example with thyroid hormone changes), or reduced production, and may also result from analytic variability. Any single value is most informative when interpreted alongside creatinine, eGFR estimates, urine studies, inflammatory markers, and the broader clinical and laboratory picture provided by the full panel.

Cystatin C is included in the Foundation Zoomer to complement creatinine‑based markers and provide a muscle‑mass‑independent view of protein clearance and organ function. Measured alongside metabolic, inflammatory, and organ function markers, it helps reveal early imbalances and refines patterns that guide personalized assessment and monitoring.