Cystatin C
Kidney markersWhat the test generally measures, and what to ask about it.
Longetic keeps every lab and record you’ve had organized and easy to find, so your own Cystatin C values sit together on one chart and you walk into your next appointment prepared.
Cystatin C is a protein your kidneys filter out of blood at a steady rate, used to estimate kidney function without creatinine's muscle-mass bias. Doctors increasingly pair it with creatinine in race-free eGFR equations. Longetic keeps your cystatin C results organized with your other labs and helps you prepare specific questions for your doctor.
What is Cystatin C?
Cystatin C is a protein your kidneys filter out of blood at a steady rate, used to estimate kidney function without creatinine's muscle-mass bias.
Cystatin C is a small protein that almost every cell with a nucleus produces at a fairly constant rate. Healthy kidneys filter it out of the blood continuously, so in someone with normal kidney function, blood levels stay low and steady. When filtration slows, cystatin C backs up in the blood, which labs read as a sign of reduced kidney function. The reason it exists as a separate test from creatinine comes down to where each molecule comes from. Creatinine is a byproduct of muscle breakdown, so a bodybuilder and a frail 80-year-old can post very different creatinine numbers with identical kidney function, and a single equation has to guess at how much of that difference is muscle rather than kidneys. Cystatin C isn't generated by muscle tissue at all, so it sidesteps that specific problem, according to Lab Tests Online UK, which is why it's the marker of choice when muscle mass makes creatinine hard to read. That doesn't make cystatin C immune to everything, though. It runs higher with inflammation, thyroid disease, higher body-mass index, and corticosteroid (steroid) use, independent of kidney function, per Lab Tests Online UK and peer-reviewed research on obesity and cystatin C levels. It trades one set of confounders for a different, usually smaller, set.
Typical reference context
| Group | Typical reference range | Source |
|---|---|---|
|
Adults, 18 and older
|
0.5-1.0 mg/L
|
University of Iowa Department of Pathology
|
Ranges vary by lab and by the specific assay used, especially for children and older adults, so the range printed on your own lab report is the one that applies to that result, not a number from another source.
What changed recently
In 2021, a joint task force from the National Kidney Foundation and the American Society of Nephrology recommended dropping race from GFR equations and increasing use of cystatin C combined with creatinine as a confirmatory kidney-function check. Adoption split into two speeds. A national laboratory survey found 65.8% of US labs had switched to the race-free creatinine-only equation by March 2023, up from 30.3% a year earlier, but only 7.3% had adopted the combined creatinine-cystatin C equation by that same date, per Clinical Chemistry.
The 2024 KDIGO guideline update pushed the combined equation further, recommending it specifically for people where creatinine alone is least reliable: those with class III obesity, some cancer patients on chemotherapy, and older adults, according to a 2025 review in the Cleveland Clinic Journal of Medicine. That review names the real holdup as upfront lab cost, not accuracy: validating a new assay and building the reporting infrastructure, not any doubt that the combined equation works.
Tests often checked alongside this
- Creatinine — paired directly in the combined CKD-EPI 2021 equation, since the two markers correct for different blind spots: muscle mass for creatinine, inflammation for cystatin C.
- eGFR (calculated) — cystatin C is an input the lab plugs into this equation rather than a standalone result on its own.
- BUN — sometimes ordered alongside for a broader look at filtration, since BUN responds to hydration and protein intake in ways cystatin C doesn't.
- CRP or another inflammation marker — occasionally checked together, because inflammation itself can raise cystatin C independent of kidney function.
What can affect this marker
Inflammation, thyroid conditions, higher body-mass index, corticosteroid medications, and smoking are all linked to higher cystatin C levels regardless of how well the kidneys are filtering, per Lab Tests Online UK and published research on obesity and cystatin C. Muscle mass, age, sex, race, and typical diet, by contrast, have little to no effect, which is the opposite pattern from creatinine.
What to ask your doctor
- Why cystatin C, or the combined equation, instead of a creatinine-only eGFR in my case?
- Is there anything in my history, such as a thyroid condition, inflammation, or steroid use, that could affect this specific result?
- Which reference range did the lab use, and does it match earlier tests closely enough to compare them directly?
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Frequently asked questions
References
About our sources
Official sources
Government health agencies, national institutes, and intergovernmental bodies whose guidelines carry regulatory or policy weight.
Clinical / professional
Medical societies, specialty colleges, and professional associations whose consensus statements reflect clinical practice standards.
Peer-reviewed research
Original research articles and systematic reviews published in indexed journals and verified through PubMed or DOI.