The Better Kidney Filter Test
Cystatin C is a small protein produced by all your cells at a steady rate. Unlike creatinine, it's not affected by muscle mass, diet, or exercise—making it a more accurate and unbiased measure of kidney function, especially for people who are very muscular, elderly, or have unusual body compositions.
What is Cystatin C?
Cystatin C is a cysteine protease inhibitor produced at a constant rate by all nucleated cells. It's freely filtered by kidneys and not significantly reabsorbed, making it an ideal filtration marker. Cystatin C-based eGFR is increasingly used alongside creatinine-based eGFR.
↑ What High Cystatin C Means
Your kidneys aren't filtering as well as they should. Because cystatin C isn't affected by muscle mass, it can catch early kidney disease that creatinine might miss.
Common symptoms:
Same as low eGFR: often asymptomatic early, fatigue, swelling, foamy urine in advanced stages
↓ What Low Cystatin C Means
Not typically concerning.
Common symptoms:
Not typically significant
Why It Matters
When normal:
More accurate than creatinine in muscular or elderly individuals
Not affected by muscle mass, diet, or gender
Detects early kidney decline before creatinine rises
Better predictor of cardiovascular events
Risks if abnormal:
Elevated cystatin C: kidney dysfunction, increased cardiovascular risk
May detect kidney disease 1-2 years before creatinine does
What Can Cause Abnormal Levels?
Early Kidney Disease
55% likelyCystatin C rises earlier than creatinine when kidney function starts declining.
Diabetes or Hypertension
45% likelyThe same conditions that damage kidneys will elevate cystatin C.
Thyroid Dysfunction
Hyperthyroidism can increase and hypothyroidism can decrease cystatin C production, independent of kidney function.
High-Dose Corticosteroids
Can increase cystatin C production.
What You Can Do
Same kidney-protective lifestyle as for eGFR
Impact: Blood pressure control, hydration, limit NSAIDs \u00B7 Timeline: Ongoing
Control blood sugar if diabetic
Impact: Prevents kidney damage progression \u00B7 Timeline: Ongoing
If lifestyle changes aren't enough:
Regular monitoring with both creatinine and cystatin C-based eGFR
Impact: Most accurate kidney function tracking \u00B7 Timeline: Every 3-6 months
Recommended retest: 3-6 months
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