Sarcopenia Index Calculator
Enter creatinine and cystatin C from the same blood draw. Both must come from one panel, since the whole point of the ratio is that the two markers respond differently to muscle mass.
Educational tool, not a diagnosis. The idea is simple: creatinine comes from muscle, cystatin C does not, so their ratio carries a muscle signal neither marker shows alone. In 458,702 UK Biobank participants it discriminated sarcopenia with an area under the curve of roughly 0.72 — acceptable, not excellent. Two honest limits: it tracks grip strength better than it tracks muscle mass, and its performance degrades when kidney function is impaired. It has also never been validated specifically in people taking GLP-1 medications, which is the group most likely to be watching muscle loss. Treat it as a cheap trend line, not a test.
Why these two markers together
Creatinine is a waste product of muscle metabolism, so how much you produce depends on how much muscle you carry. Cystatin C is produced at a near-constant rate by every nucleated cell and is not affected by muscle mass. Both are filtered by the kidneys and both are used to estimate kidney function — which means their ratio strips out the kidney signal and leaves something closer to a muscle signal.
That same asymmetry is why creatinine alone can mislead. Losing muscle produces less creatinine, so a creatinine-based eGFR drifts upward and kidney function looks like it is improving when the real story is lost muscle. The trap is covered in detail in GLP-1 muscle loss and what your labs really show.
What to do with the number
Treat it as a baseline, not a verdict. The value of a cheap marker like this is the trend across repeat panels, and it is most informative when paired with something you can actually measure at home — grip strength, or your working weights in the gym. The wider case for why this matters is in muscle mass and longevity, and it belongs alongside the rest of a longevity panel.