Cystatin C
Kidney function marker independent of muscle mass - more accurate than creatinine in the muscular, elderly, or ill.
How Cystatin C ranges relate
The standard lab range vs the optimization-focused target. Illustrative trajectory shows what a 12-week improvement pattern looks like — not real user data.
What Cystatin C Measures
Cystatin C is a low-molecular-weight protein produced by all nucleated cells at a steady rate, freely filtered by the glomerulus, and not secreted. Unlike creatinine (which varies with muscle mass), cystatin C gives a cleaner read on glomerular filtration rate - particularly in populations where creatinine misleads (very muscular individuals, the elderly, patients with sarcopenia).
Cystatin-based eGFR equations (like CKD-EPI cystatin) are increasingly used in research and complex clinical cases.
What Affects This Biomarker
Cystatin C is influenced by: glomerular filtration rate (the signal of interest), thyroid disease (hyperthyroidism raises), corticosteroid use (raises), and some inflammatory states. Unaffected by muscle mass, diet, or recent exercise - its main advantages.
In the Context of Peptide Protocols
Optional add-on when creatinine-based eGFR is ambiguous (e.g., very muscular lifters often show artificially low eGFR on creatinine). If available, run alongside creatinine every 6–12 months in peptide users.
Upload any lab PDF and MyProtocolStack maps your values to Cystatin C and 40+ other biomarkers. StackAI interprets the trend in context of your protocol.
Start tracking →Informational only - not medical advice. Reference ranges vary by lab and individual context. Work with a licensed provider to interpret your specific results.