Kidney function marker independent of muscle mass — more accurate than creatinine in the muscular, elderly, or ill.
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.
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.
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.
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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.