Cystatin C
Measures kidney filtration without being skewed by muscle mass - giving a clearer read than creatinine for many people.
TypeBlood test
HowStandard blood draw.
How oftenWhen kidney function is borderline or you’re 60+.
BeforeNo special preparation.
WhoPeople with borderline eGFR, very high or low muscle mass, or 60+.
Why your usual kidney number can mislead
Routine kidney results use creatinine, which comes from muscle. Very muscular people can look worse than they are; frail people can look better. Cystatin C isn’t affected by muscle.
Creatinine eGFR
“Is my kidney number accurate?”
Good for most - but skewed by muscle mass.
Standard
Cystatin C
“Can I get a clearer answer?”
Less affected by muscle - often combined with creatinine for the best estimate.
More accurate
Why it matters
“Does it change anything?”
Kidney function guides medication doses and heart risk.
Guides decisions
Is it right for you?
Worth considering if…
- Your eGFR is borderline.
- You’re very muscular, frail, or 60+.
- You take medicines that depend on kidney function.
A better option if…
- Your kidney results are clearly normal - it rarely adds much.
How it’s measured
Cystatin C
Blood draw.
Combined eGFR
Uses both creatinine and cystatin C.
The full picture
What is it?
A protein filtered by the kidneys.
What type of test is it?
A blood biomarker.
What does it measure?
Cystatin C concentration, used to estimate filtration (eGFR).
Why might someone consider it?
For a more accurate kidney function estimate.
Who is it most relevant for?
People with borderline results or unusual muscle mass.
How is it performed?
A blood draw.
How often might it be considered?
When needed.
What can the result help you understand?
How well your kidneys are filtering.
What are its limitations?
Affected by thyroid disease and steroids.
What should it be viewed with?
Creatinine, urine albumin, blood pressure and HbA1c.
Where to get it
Through your provider
The simplest route - often covered when there’s a clear reason.
Lab services
e.g. Quest Diagnostics, Labcorp.
Direct-to-consumer panels
e.g. Function Health, InsideTracker.
Names are examples only, not endorsements. Your provider can often order the same test - and help interpret it.
Connected to
Questions to ask your provider
- Would cystatin C give a clearer picture of my kidneys?
Ziva Health
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Being listed doesn’t mean everyone needs this test. Educational only — not medical advice. Results should be interpreted with a qualified clinician. ← Back to Know What to Test