Cystatin C is a protein produced at a steady rate by nearly all cells and filtered by the kidneys, which makes its blood concentration a measure of kidney filtration. It is generally more sensitive than creatinine for detecting early changes, particularly in people with high or low muscle mass, where creatinine is systematically misleading.
- Below 0.95 mg/L sits within the standard reference range, with tighter bands as app interpolations.
- Unlike creatinine, it isn’t confounded by muscle mass, which matters for anyone who lifts.
- Excluded from Longevity Coach IQ projections, because it responds to conditions rather than to habits.
- Blood pressure and glucose control are the real upstream levers.
What cystatin C measures
Cystatin C is a small protein produced at a roughly constant rate by virtually all nucleated cells in the body. It is filtered out of the blood by the kidneys and then broken down, rather than being reabsorbed.
That combination is what makes it useful. Production is steady and largely independent of what you do, so the concentration left in the blood reflects how well the kidneys are filtering. When filtration declines, the level rises.
Kidney function matters for longevity beyond the kidneys themselves. Reduced filtration is independently associated with cardiovascular events and all-cause mortality, and it often progresses silently for years before conventional testing flags it.
Why cystatin C beats creatinine
Creatinine is the standard kidney marker and has a structural flaw: it is a breakdown product of creatine in muscle, so how much of it you produce depends on how much muscle you have.
There is a second argument that matters more for a longevity marker. Cystatin C-based estimates have been more strongly associated with mortality and cardiovascular outcomes than creatinine-based estimates, which moves the case from "creatinine misleads in specific populations" to "cystatin C is the better predictor of the outcomes this site is about".
| Creatinine | Cystatin C | |
|---|---|---|
| Source | Muscle breakdown | Nearly all cells |
| Affected by muscle mass | Yes, substantially | No |
| Affected by creatine supplements | Yes | No |
| Detects early decline | Later | Earlier |
Only the standard reference range is externally established, and it varies by assay and laboratory. The three tiers below it are Longevity Coach IQ interpolations rather than guideline thresholds.
The practical consequences are specific. A muscular person who lifts and supplements creatine can show elevated creatinine and be told their kidney function is impaired when it isn’t. An older person with low muscle mass can show reassuringly normal creatinine while filtration is genuinely reduced. Both errors are common, and both disappear with cystatin C.
How cystatin C is turned into an eGFR
A raw cystatin C concentration is not the clinically meaningful figure. It is converted into an estimated glomerular filtration rate, expressed in mL/min/1.73m², which is what describes how well the kidneys are filtering.
eGFR can be estimated from creatinine alone, from cystatin C alone, or from both together. The combined equation is more accurate than either on its own, which is the finding of the Inker 2012 paper cited below.
That is why the two tests are complementary rather than alternatives. Neither replaces the other, and the equation using both outperforms either.
Because eGFR is derived from the concentration, anything that shifts the concentration shifts the estimate. Age is the largest of those factors.
What a cystatin C result means
| Tier | Level (mg/L) | Basis |
|---|---|---|
| Standard range | ~0.6-1.2 | Laboratory reference |
| Top 10% | Below 0.95 | App interpolation |
| Top 5% | Below 0.80 | App interpolation |
| Top 1% | Below 0.70 | App interpolation |
Only the reference range is externally established. The tiers below it are interpolations within that range so the marker sits on the same scale as the rest, and they are labeled here so you know which carries guideline weight.
Levels rise with age as filtration naturally declines. A result that would be unremarkable at 70 would warrant attention at 35.
Cystatin C interpreter
Enter your result to see it against the reference range. Levels rise naturally with age, so read the number in that context.
Why cystatin C is excluded from projections
Like Lp(a), this marker is deliberately left out of the consistency projections, and the app states that openly on My Plan:
“Not projected: Lp(a), Cystatin C, TSH, and Free T3 are excluded here. Lp(a) is ~90% genetically determined, and there isn’t credible evidence the others respond meaningfully to habits and training within a realistic timeframe. They’re still tracked normally in Your Lab.”
The reason differs from Lp(a)'s. Lp(a) is fixed genetically. Cystatin C can change, but it changes in response to underlying conditions rather than to habits, and not on a timescale a projection could honestly represent. Drawing a line showing improvement from consistent training would misrepresent how kidney function actually works.
What actually lowers cystatin C
There is no direct lever here, but there are real indirect ones, and they are the two leading causes of chronic kidney disease.
- Blood pressure. High pressure damages the small vessels the kidneys filter through. Controlling it is the single most effective way to protect filtration over decades.
- Glycaemic control. Diabetic kidney disease is the other leading cause. The same work that improves HbA1c protects the kidneys.
- Hydration and caution with NSAIDs. Neither is dramatic, but chronic heavy NSAID use is a genuine and avoidable contributor.
The framing worth keeping: this marker tells you whether the upstream work is succeeding. It is a readout, not a target.
Who cystatin C matters most for
Cystatin C sits in Systemic Health, in the Complete tracking mode, and it is scored while being excluded from projections.
For most people its practical value is as a check on whether blood pressure and metabolic health are being managed well enough over time. For anyone lifting seriously or supplementing creatine, it has a second use: it gives an accurate read on kidney function where creatinine would raise a false alarm.
Sources
Key references for the claims on this page. Where a figure is attributed to a specific study or body, it is named here.
- Reference ranges for cystatin C vary by assay and by laboratory rather than sitting at a single established figure, so the range printed on your own result is the one to read against. The Top 10, Top 5 and Top 1 percent sub-splits are Longevity Coach IQ interpolations rather than guideline thresholds.
- Inker LA, Schmid CH, Tighiouart H, et al. Estimating glomerular filtration rate from serum creatinine and cystatin C. New England Journal of Medicine, 2012;367(1):20–29. The combined equation and the case for cystatin C where creatinine is unreliable.
- Shlipak MG, Matsushita K, Arnlov J, et al. Cystatin C versus creatinine in determining risk based on kidney function. New England Journal of Medicine, 2013;369(10):932–943. Cystatin C-based estimates more strongly associated with mortality and cardiovascular outcomes.
- Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. KDIGO 2012 clinical practice guideline for the evaluation and management of chronic kidney disease. Kidney International Supplements, 2013;3(1):1–150. Recommends cystatin C for confirmatory testing where creatinine-based estimates are uncertain.
Frequently asked
What is a normal cystatin C level?
Standard laboratory ranges run roughly 0.6 to 1.2 mg/L, though ranges differ between labs and assays. Values rise naturally with age, so the same number means different things at 30 and at 75.
Why is it better than creatinine?
Creatinine is a muscle breakdown product, so its level depends on how much muscle you carry. A heavily muscled person can show elevated creatinine with entirely normal kidneys, and a frail person can show normal creatinine while filtration is genuinely impaired. Cystatin C is produced by nearly all cells at a steady rate, which removes that confounding.
Should I ask for cystatin C instead of creatinine?
Not instead. The two are complementary, and the combined equation using both is more accurate than either alone. Cystatin C is most valuable where creatinine is likely to mislead: high muscle mass, low muscle mass, amputation, or an ambiguous creatinine-based result.
Can I improve my cystatin C with diet or exercise?
Not directly, which is why it is excluded from projections. It reflects kidney filtration, and that responds to underlying conditions such as blood pressure and diabetes rather than to training or diet in any short timeframe. Manage the upstream causes and the marker follows.
Does creatine supplementation affect it?
Creatine supplementation raises creatinine, sometimes enough to look like reduced kidney function when nothing is wrong. It doesn’t affect cystatin C, which is one of the more practical reasons for anyone supplementing creatine to know this marker.
What raises cystatin C?
Reduced kidney filtration is the main driver. Thyroid dysfunction, corticosteroid use, obesity and smoking are also associated with higher levels, which is part of why an isolated elevation warrants investigation rather than assumption.
Is an elevated result serious?
It is a signal to investigate, not a diagnosis. Kidney function is assessed from several inputs over time rather than one reading, and a single elevated value can reflect dehydration, an acute illness or laboratory variation.
How often should I test it?
Annually is reasonable for most people, in line with the general retesting guidance, or as directed if you have hypertension, diabetes or existing kidney concerns. It changes slowly, so more frequent testing rarely adds information.