Cystatin C
Cystatin C is a small protein made by all nucleated cells at a fairly constant rate. It is freely filtered by the kidneys and not reabsorbed, making blood cystatin C a good estimate of glomerular filtration rate (GFR). Unlike creatinine, cystatin C is minimally affected by muscle mass, age, sex, or diet — useful in elderly, malnourished, bodybuilders, and children where creatinine-based eGFR is unreliable.
Reference Ranges
Male
0.6 – 1
mg/L
Female
0.6 – 1
mg/L
Child
0.6 – 1.1
mg/L
Slightly higher below age 1 and above age 50
Reference ranges may vary by laboratory. Always compare with the range printed on your lab report.
What Low Levels Mean
A cystatin C below 0.6 mg/L is rare and usually not clinically meaningful. Very low values can be seen with hyperthyroidism (increased cystatin C clearance) and high-dose corticosteroid therapy, which suppresses cystatin C production.
What High Levels Mean
Elevated cystatin C suggests reduced glomerular filtration — early kidney dysfunction. Cystatin C rises earlier than creatinine in mild CKD and is useful when creatinine is deceptively normal (low muscle mass). A cystatin C-based eGFR, or a combined creatinine-cystatin equation, gives the most accurate estimate. Non-renal causes of elevation include hyperthyroidism (rare direction), steroid use, and active inflammation.
Frequently Asked Questions
When should cystatin C be used instead of creatinine?
When creatinine-based eGFR is likely inaccurate: very low muscle mass (elderly, cirrhosis, amputation, malnutrition), very high muscle mass (bodybuilders), children, pregnancy, and when precise GFR matters for drug dosing (chemotherapy, antivirals in HIV, contrast studies). KDIGO guidelines recommend confirming early-stage CKD (eGFR 45–60) with cystatin C before labeling the patient.
Is cystatin C affected by diet or exercise?
Minimally. Unlike creatinine, cystatin C is not affected by meat intake or recent exercise — no fasting is needed. This makes it more convenient for outpatient use. However, thyroid dysfunction and high-dose steroids can alter cystatin C production independent of kidney function.
Why does my cystatin C-based eGFR differ from creatinine-based?
Creatinine and cystatin C can diverge when muscle mass is unusual. In a person with low muscle, creatinine-eGFR overestimates kidney function while cystatin-eGFR is more accurate. In a muscular person the reverse. Combined creatinine-cystatin equations give the best estimate and are increasingly used in clinical decision-making.
This information is for educational purposes only and should not replace professional medical advice. Always consult your doctor for interpretation of your test results.
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