In one sentence
A small protein that every cell in the body makes at a steady rate and the kidneys clear, so its level in the blood tracks how well they filter without depending on how much muscle you carry or whether you take creatine.
Why it matters
It fixes the creatinine problem. In the same 170 healthy adults in whom creatinine rose with lean mass and training, cystatin C was unrelated to either (Baxmann 2008), and in a trial of creatine at about 10 g a day for three months, cystatin C fell as the men trained, which is the direction of better filtering, while creatinine held still (Gualano 2008). It also sharpened the kidney figure's meaning: in a meta-analysis of 11 general-population cohorts with 90,750 people, an estimate built on cystatin C, alone or with creatinine, strengthened the association between the filtration rate and death and end-stage kidney disease, and moving a person's estimate up or down by adding cystatin C moved their observed outcomes with it (Shlipak 2013). The 2021 CKD-EPI equations that use both markers were the most accurate of the set and showed the smallest differences between groups (Inker 2021).
What moves it
Less than creatinine, and different things: thyroid state (an overactive thyroid raises it, an underactive one lowers it), corticosteroid medicines, smoking, and, modestly, body fat and inflammation. Not muscle, not meat, not creatine.
How to measure it well
On the same draw as the renal panel, so the two estimates can be compared. In New Zealand it is usually a self-pay addition; its availability and price through the community laboratories is a question on the panel's review list. Same laboratory each time, because assays differ.
How Vitality Tracker uses it
Stored as reported with the laboratory's range and flag, never overridden; typical laboratory reference 0.6 to 1.0 mg/L, and VT adds no working line of its own beneath it. From it VT computes the cystatin-based eGFR (its own entry, with the creatinine and the combined equations) and, when the Supplement Tracker holds creatine or the five-segment scale reading shows little muscle, uses that estimate as the kidney panel's headline figure and keeps the creatinine estimate beside it as context. PhenoAge still takes creatinine and is marked when creatine is logged. Re-test with the renal panel.
How far to trust the number
Laboratory: an immunoassay, now standardized against an international reference material, with small analytical error; the thyroid and steroid effects above are the main reasons it can mislead.
Sources
- Shlipak MG, et al. Cystatin C versus creatinine in determining risk based on kidney function. N Engl J Med 2013;369:932–43. doi:10.1056/NEJMoa1214234
- Inker LA, et al. N Engl J Med 2021. doi:10.1056/NEJMoa2102953
- Baxmann AC, et al. Clin J Am Soc Nephrol 2008. doi:10.2215/CJN.02870707
- Gualano B, et al. Eur J Appl Physiol 2008. doi:10.1007/s00421-007-0669-3
- Vitality Tracker, Recommended baseline lab panel v0.1.