Cystatin C
For informational purposes only — not medical advice. Always consult a qualified healthcare provider before making changes to your health regimen. Full disclaimer →
- Cystatin C is more accurate than creatinine for kidney function assessment in women, older adults, people with low muscle mass, and anyone whose body composition makes creatinine a biased estimate. It is the preferred GFR marker for these populations.
- Normal creatinine does not rule out kidney function decline. Studies consistently show that 5–15% of individuals classified as having normal kidney function by creatinine-based eGFR have impaired function when assessed by cystatin C — and these individuals have cardiovascular and mortality risk that matches confirmed kidney disease.
- Cystatin C predicts cardiovascular death independent of GFR. Beyond its value as a kidney marker, elevated cystatin C is an independent predictor of cardiovascular events and mortality. This suggests it may capture systemic biological aging and vascular burden beyond kidney-specific information.
- CKD-EPI cystatin C equation is more accurate than CKD-EPI creatinine for GFR estimation across populations. When both biomarkers are available, the combined cystatin C plus creatinine equation is the most accurate GFR estimate currently available without a formal clearance measurement.
- Cystatin C is not affected by diet. A high-protein meal or intense exercise transiently elevates creatinine; cystatin C is unaffected by either. This makes it more reliable for single-sample GFR assessment.
Why Cystatin C Is the Kidney Marker Longevity Medicine Uses
Creatinine has been the standard marker for kidney function for decades. It is cheap, widely available, and familiar to physicians. It is also systematically biased in ways that make it unreliable precisely in the populations where kidney function monitoring matters most — older adults, women, and people with altered muscle mass.
The fundamental problem is that creatinine is a waste product of muscle metabolism. Its serum concentration is determined by two variables: kidney clearance (what we want to measure) and creatinine production (which varies with muscle mass and is therefore confounded by sex, age, and body composition). A 35-year-old competitive cyclist and a 70-year-old woman with sarcopenia may have identical creatinine levels — but their kidney function is entirely different.
Cystatin C eliminates this confound. Produced at a constant rate by all nucleated cells and cleared almost entirely by glomerular filtration, its serum level is determined almost exclusively by GFR. Multiple studies comparing cystatin C-based and creatinine-based GFR estimates have confirmed that cystatin C is substantially more accurate — particularly in older adults, women, and people with low muscle mass, where creatinine systematically overestimates kidney function.
Cystatin C and Cardiovascular Mortality
What makes cystatin C particularly valuable from a longevity perspective is its relationship to cardiovascular and all-cause mortality that extends beyond its kidney-function role. The Cardiovascular Health Study — a prospective cohort of 4,663 adults over age 65 followed for over a decade — found that cystatin C predicted mortality, heart failure, and cardiovascular events more powerfully than creatinine-based eGFR, even after adjustment for conventional cardiovascular risk factors and kidney function. 1
This excess predictive value suggests cystatin C may capture biological aging and vascular burden beyond what kidney filtration explains. Elevated cystatin C has been independently associated with endothelial dysfunction, atherosclerosis burden, and systemic inflammation — suggesting it may function as a multi-system biomarker of physiological aging rather than purely a kidney marker.
| Level | Range | Interpretation | Notes |
|---|---|---|---|
| Optimal | < 0.80 mg/L | Excellent kidney function | Lowest mortality risk quintile |
| Good | 0.80–0.90 mg/L | Normal, within longevity target | Normal kidney function, low-normal risk |
| Borderline | 0.90–1.15 mg/L | Within normal range but elevated risk | Normal by standard criteria; CV risk elevated vs. optimal |
| Mildly impaired | 1.15–1.50 mg/L | Above normal — kidney function decline | Warrants investigation and follow-up testing |
| Impaired | > 1.50 mg/L | Significant kidney function impairment | CKD likely — physician evaluation required |
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Analyze My Biomarkers →The CKD Misclassification Problem
Chronic kidney disease (CKD) is defined by an eGFR below 60 mL/min/1.73m² on two separate measurements at least 3 months apart. CKD is estimated to affect 15% of US adults — but this estimate is based primarily on creatinine-based eGFR, which systematically overestimates kidney function in women and older adults.
Studies that have compared creatinine-based and cystatin C-based GFR classifications in the same populations have found that approximately 5–15% of individuals classified as CKD-free by creatinine are reclassified as having CKD when cystatin C is used. These reclassified individuals have cardiovascular and mortality risk that is statistically indistinguishable from people with confirmed CKD — meaning the creatinine-based normal classification is providing false reassurance.
This misclassification is not random: it disproportionately affects older adults (where muscle loss makes creatinine unreliable) and women (who have lower muscle mass than men at every age). These are also the groups where early kidney disease detection and intervention would be most beneficial. The practical implication is clear: cystatin C should be part of any comprehensive longevity assessment, particularly in women over 40 and men over 55.
Sources
- Shlipak MG, et al. "Cystatin C and the Risk of Death and Cardiovascular Events among Elderly Persons." New England Journal of Medicine, 2005. PubMed →
| Range Type | Value (mg/L) | Notes |
|---|---|---|
| Standard Clinical Range | 0.62–1.15 mg/L | Designed to identify disease risk — not longevity optimisation. |
| Longevity-Optimal Target | < 0.9 mg/L |
Associated with reduced all-cause mortality and extended healthspan.
Within the standard normal range, cystatin C shows a dose-response relationship with cardiovascular and all-cause mortality risk. People with cystatin C below 0.9 mg/L have substantially lower mortality risk than those in the 0.9–1.15 mg/L range, even though both fall within the normal reference interval. The Cardiovascular Health Study and multiple other cohorts have validated this within-normal-range risk gradient.
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Why is cystatin C more accurate than creatinine for measuring kidney function?
Creatinine is a breakdown product of creatine phosphate in muscle tissue, meaning its production rate is directly proportional to muscle mass. A highly muscular person produces more creatinine and will have higher serum creatinine than a frail, sarcopenic person — even if their kidney function is identical. This creates systematic biases: muscle-rich individuals (athletes, young men) have their kidney function underestimated by creatinine (their high creatinine suggests worse kidneys than they have); frail individuals (elderly women, people with chronic illness) have their kidney function overestimated (their low creatinine suggests better kidneys than they have). Cystatin C, produced at a constant rate by all cells regardless of muscle mass, eliminates this bias. It is particularly more accurate in women, older adults, individuals with chronic illness-related muscle wasting, and anyone at the extremes of body composition.
What is eGFR and how is it calculated from cystatin C?
eGFR (estimated glomerular filtration rate) is calculated from cystatin C using the CKD-EPI Cystatin C 2012 equation, which incorporates cystatin C concentration and age. The result — expressed in mL/min/1.73m² — estimates how much blood the kidneys filter per minute per standardized body surface area. An eGFR above 90 is considered normal; 60–89 is mildly reduced; 45–59 is moderately reduced (CKD Stage 3a); 30–44 is moderate-severely reduced (CKD Stage 3b); 15–29 is severely reduced (CKD Stage 4); below 15 is kidney failure requiring dialysis or transplant. When both creatinine and cystatin C are available, the combined CKD-EPI creatinine-cystatin C equation provides the most accurate eGFR estimate currently available without formal clearance measurement.
Can cystatin C be elevated for reasons other than kidney disease?
Yes. Cystatin C is elevated in hyperthyroidism (thyroid hormones increase cystatin C production), suppressed in hypothyroidism. High-dose corticosteroids can increase cystatin C levels independent of kidney function. Some inflammatory states elevate cystatin C through mechanisms separate from filtration impairment. Malignancy may produce elevated cystatin C. Conversely, cystatin C may be suppressed in hypothyroidism and possibly in protein-calorie malnutrition. These confounders are generally less problematic than the muscle mass confounding of creatinine, but they should be considered when cystatin C is discordant with other kidney markers. The combination of cystatin C and creatinine provides more robust GFR estimation than either alone, as their confounders are largely non-overlapping.
Should I get cystatin C if my creatinine and eGFR are normal?
Yes, if you are: over age 50 (where sarcopenia increasingly makes creatinine unreliable), female (women have consistently lower muscle mass than men, making creatinine a biased overestimate of kidney function), have lost significant muscle mass for any reason, are an athlete or have unusually high muscle mass (where creatinine may underestimate kidney function), have a family history of kidney disease, have hypertension or diabetes (conditions that damage kidneys silently for years before creatinine rises), or are undergoing a comprehensive longevity evaluation. Normal creatinine with elevated cystatin C identifies a meaningful clinical situation: kidney function is actually impaired, the creatinine is giving you a false sense of security, and your cardiovascular and mortality risk are elevated. This is the 'CKD misclassification' problem, and it disproportionately affects women and older adults.
What is the relationship between kidney function and longevity?
Kidney function is one of the most consistent predictors of longevity across large population studies. eGFR and cystatin C predict all-cause mortality in continuous, dose-response fashion — not only in people with clinical kidney disease, but across the full spectrum of kidney function within the normal range. The mechanisms are multiple: the kidney produces erythropoietin (EPO), activates vitamin D, regulates blood pressure, and eliminates metabolic waste products including homocysteine and uric acid. Impaired kidney function allows accumulation of uremic toxins, drives oxidative stress, worsens cardiovascular risk, and contributes to anemia of chronic disease. Because kidney function declines gradually with age — approximately 1 mL/min/1.73m² per year after age 40 — tracking it longitudinally via cystatin C allows early intervention before decline becomes clinically significant.