Cystatin C
Sensitive kidney filter marker independent of muscle mass – superior GFR estimator
Also known as
Definition
Cystatin C is a low-molecular-weight protein (cysteine protease inhibitor) produced constantly by all nucleated body cells and eliminated exclusively through glomerular filtration. Unlike creatinine, cystatin C is independent of muscle mass, sex, and diet — making it a superior endogenous GFR marker, particularly in borderline cases and individuals with extreme body compositions (muscular athletes, cachectic patients, the elderly).
| Parameter | Value |
|---|---|
| Unit | mg/l |
| Reference Range | 0.50–1.00 mg/l (adults; mild rise with age, diabetes, and inflammation) |
Agree on your personal target range with your doctor.
↓ What a low value means
Very low cystatin C values (< 0.5 mg/l) are rare and clinically insignificant. Theoretically, hyperthyroidism-induced higher cell proliferation rates may mildly increase (not decrease) production. No relevant pathology known at low values.
↑ What a high value means
Elevated cystatin C (> 1.0 mg/l) can point to impaired glomerular filtration, for example in chronic kidney disease (CKD); whether CKD is present depends, according to KDIGO, on duration and further findings. Advantage over creatinine: detects mild GFR impairment (GFR 60–90 ml/min) in patients with low creatinine due to reduced muscle mass (elderly women, cachectic patients). Elevated cystatin C is also an independent cardiovascular risk marker.
✓ What can influence this value
Cystatin C reflects kidney filtration and is influenced by kidney diseases, high blood pressure, diabetes, fluid balance, and kidney-damaging substances such as NSAIDs or contrast agents. Independent of kidney function, overweight, smoking, inflammation, thyroid dysfunction, and glucocorticoids can change the value. The assessment is made by the treating doctor.
When to test
Cystatin C is measured when creatinine-based GFR appears suspiciously normal (low muscle mass), in older people, with mildly decreased eGFR (category G2), for cardiovascular risk assessment, and for medication dose adjustment. According to KDIGO 2024, the combined estimate from creatinine and cystatin C is usually more accurate than an estimate from either value alone; where greater accuracy matters, GFR is measured directly.
Frequently asked questions
Why is cystatin C better than creatinine for older adults? +
Creatinine is released from muscles — older adults lose muscle mass (sarcopenia) and produce less creatinine even as kidney function declines. Normal creatinine can mask a GFR of only 45 ml/min in a low-muscle 80-year-old. Cystatin C is produced independently of muscle mass and reliably detects this hidden kidney impairment.
Does inflammation affect cystatin C measurement? +
Yes. Pro-inflammatory cytokines (IFN-γ, IL-2) can raise cystatin C production, independent of kidney function. Thyroid function also influences cystatin C (hyperthyroidism raises, hypothyroidism mildly lowers it). These factors are taken into account when interpreting results — GFR from cystatin C may be mildly underestimated during active inflammation.
Do I need to fast for a cystatin C measurement? +
No. Unlike creatinine, cystatin C varies little with meals or time of day. It can be measured at any time. Only after very protein-rich meals do studies show minor rises, which are not clinically relevant.
When is cystatin C measured in addition to or instead of creatinine? +
Cystatin C is mainly measured in older people, with extreme muscle mass (bodybuilders with high-normal creatinine), muscle wasting (cachexia), cancer, after bariatric surgery (rapid weight loss), and when precise GFR estimation for drug dosing is important. The KDIGO 2024 guideline distinguishes three approaches: the combined estimate from creatinine and cystatin C (for example with the 2021 CKD-EPI equation) is generally more accurate than an estimate from either value alone; an estimate from cystatin C alone is an alternative in certain situations. Where even greater accuracy matters, GFR is measured directly with an exogenous filtration marker (measured GFR).
Cystatin C
Sources
- Shlipak MG et al. (2013) Cystatin C versus Creatinine in Determining Risk Based on Kidney Function. N Engl J Med.
- Stevens PE, Levin A; KDIGO CKD Guideline Development Work Group (2013) Evaluation and management of chronic kidney disease: synopsis of the KDIGO 2012 clinical practice guideline. Ann Intern Med.
- Inker LA et al. (2021) New Creatinine- and Cystatin C-Based Equations to Estimate GFR without Race. N Engl J Med.
Last updated: May 28, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.