Neither number is your kidney function. Both are estimates, built from a substance the kidney happens to filter out of blood — and the two substances carry very different, very avoidable sources of error.
| Cystatin C | creatinine (eGFR) | |
|---|---|---|
| What it is | A protein produced by many cells in the body | A byproduct of muscle metabolism |
| Affected by muscle mass | Less affected than creatinine | Higher or lower muscle mass can influence the estimate |
| Affected by a high-meat meal | Less affected than creatinine | A meal can transiently raise creatinine |
| Affected by creatine supplementation | Not affected in the same way as creatinine | Can raise creatinine without a change in filtration |
| Other influences | Thyroid dysfunction, corticosteroids, obesity, inflammation and smoking | Muscle mass, diet, exercise and hydration |
| Used by this calculator | A cystatin C-only eGFR is a scoring input | A creatinine-based eGFR is also scored at lower weight, unless creatine use is flagged |
| KDIGO 2024 | A combined creatinine–cystatin C estimate can improve accuracy when available | Clinical use depends on the situation and availability |
GFR can be measured using specialized methods, but clinical care usually uses an estimate derived from blood markers. Creatinine and cystatin C are two such markers; each estimate can be affected by factors other than kidney filtration.
Creatinine is a byproduct of muscle metabolism, produced continuously in rough proportion to muscle mass. Cystatin C is a small protein produced by essentially all nucleated cells in the body at a comparatively steady rate, largely unrelated to muscle. Both rise in blood as filtration falls — but what they were doing before the kidney got involved is where the two diverge.
Because creatinine production is influenced by muscle mass as well as kidney function, a creatinine-based eGFR can be wrong in either direction. Higher muscle mass or creatine supplementation can raise creatinine and make the estimate appear lower even without a change in filtration. An unexpected result still deserves clinical evaluation.
The more consequential direction runs the other way. Someone with low muscle mass — common in frailty and older age — produces less creatinine to begin with, so a creatinine-based eGFR can report a reassuring number even once real kidney function has meaningfully declined. This is precisely the population in which an early, silent decline is most likely to be missed by the single most widely used kidney marker.
Cystatin C is less affected by muscle mass, but thyroid dysfunction, corticosteroids, obesity, inflammation and smoking can influence it independently of filtration. Neither marker is perfect; combining them can improve the estimate, though it does not eliminate uncertainty.
The population-level stakes for eGFR generally are large regardless of which input is used: the CKD Prognosis Consortium (Matsushita et al., The Lancet, 2010), pooling data on over a million people across more than 14 cohorts, found mortality risk was essentially flat between eGFR 75 and 105, then rose steadily below 60 — all-cause mortality was 57% higher at an eGFR of 45 and more than tripled at 15.
The case for the cystatin C version specifically runs through functional independence rather than mortality alone: the Framingham Offspring Study (Liu et al., Journal of Gerontology: Medical Sciences, 2014) found cystatin-C-defined chronic kidney disease in adults over 60 predicted both incident mobility disability — inability to walk half a mile or climb stairs — and measurably faster gait-speed decline over 6.6 years, independent of age, BMI and inflammation.
The calculator can score a cystatin C-only eGFR and a creatinine-based eGFR. It gives the creatinine-based input less weight and excludes it when creatine supplementation is flagged. Do not enter a combined creatinine–cystatin C estimate in the cystatin C-only field.
KDIGO 2024 supports using the combined creatinine–cystatin C equation when cystatin C is available, and in situations where a more accurate estimate would affect clinical decisions. Discuss which estimate is appropriate with your clinician; a combined estimate is not interchangeable with this calculator's cystatin C-only input.
Discuss the disagreement with your clinician. Muscle mass and other factors can influence either marker; a combined estimate may help, but an unexpected result still needs evaluation.
Creatine can raise serum creatinine without a change in filtration, making creatinine-based eGFR appear lower. Cystatin C is not affected in the same way, but it has other limitations.
Cystatin C is less affected by muscle mass. The calculator also scores creatinine-based eGFR at a lower weight unless creatine supplementation is flagged; neither estimate is a diagnosis.
Related: egfr · organ · longevity blood panel
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Discuss the disagreement with your clinician. Muscle mass and other factors can influence either marker; a combined estimate may help, but an unexpected result still needs evaluation.
Creatine can raise serum creatinine without a change in filtration, making creatinine-based eGFR appear lower. Cystatin C is not affected in the same way, but it has other limitations.
Cystatin C is less affected by muscle mass. The calculator also scores creatinine-based eGFR at a lower weight unless creatine supplementation is flagged; neither estimate is a diagnosis.
Chronological age counts birthdays. Biological age estimates how old your body appears to be functioning, based on measurable biomarkers. Two people born the same year can differ by a decade or more in vascular stiffness, insulin signalling or cardiorespiratory fitness. Biological age makes that gap explicit — and unlike your birthday, it can move in either direction.
Each biomarker you enter is scored against longevity-optimised reference ranges and grouped into one of eight dials. Every dial produces its own biological age, and those are combined using fixed weights into a single composite, then expressed as a Longevity Score from 0 to 100. The weighting reflects how strongly each system predicts healthspan: structural and vascular carry the most, hormonal the least.
No. The short form contains a subset of scoring inputs plus context-only fields, and you can leave any of them blank. It is not a recommendation to obtain tests. Missing values are excluded from the calculation — never treated as zero or as 'normal'. A dial with no scored data is dropped entirely and the remaining weights renormalise.
Some inputs can be measured without bloodwork. Blood pressure needs a home cuff and waist-to-height needs a tape measure. Wearable estimates vary; the floor-based sitting-rising test carries a fall risk and should only be attempted if safe.
Both terms describe what this tool does, and which one fits depends on what you are asking. "Anti-aging" is the term most people search, and loosely means anything aimed at slowing or reversing age-related decline — this tool fits that description if you score your dials and act on what you find. "Healthspan calculator" is the more precise term: healthspan is the number of years lived without serious chronic disease or disability, and that cannot be measured directly, so this tool scores eight systems that each function as an early-warning proxy for it. Either way, the method is the same: real biomarkers, published thresholds, no vanity-metric framing.
Several popular tools ask only about habits — exercise days, sleep hours, smoking, diet quality — and return a single precise-looking age. We looked at the research behind that approach and chose not to build it. Published studies do show real, individually-measured links between specific habits and epigenetic age acceleration, but those effect sizes come from different study populations, different DNA-methylation clocks and different measurement units. Adding them together into one number is not something the underlying research actually supports — it produces a precise-looking result built on an imprecise method. Our dials use direct physiological measurements instead: a real blood pressure reading, a real VO₂ max, a real ApoB value. Several dials score from home-measurable data alone with no quiz-guessing involved — see what you can measure without a blood test, above.
Standard reference ranges describe what is common in a general population, which now includes a great deal of metabolic dysfunction. They are tuned to flag disease, not to identify what is optimal for a long healthspan. A fasting insulin of 20 µIU/mL sits inside most labs' normal range and is strongly associated with insulin resistance. This tool uses longevity-optimised thresholds instead, so a flag here means room for optimisation — not a diagnosis.