Cystatin C
Also written as: CysC
A small protein used to estimate kidney filtering, less swayed by muscle mass than creatinine.
What it measures
The concentration of cystatin C, a small protein that nearly all cells release into the blood at a steady rate. Because the kidneys filter it out continuously, its level reflects how well the kidneys are filtering. Labs often convert it into an estimated filtration rate, or eGFR.
Why a doctor orders it
Ordered to estimate kidney function, often as a cross-check on the more common creatinine-based estimate. It is especially useful when muscle mass could make creatinine misleading, such as in very muscular people, older adults, or people who have lost a limb.
What is generally associated with a higher result
Broadly associated with reduced kidney filtration. Levels can also be nudged upward by an overactive thyroid, corticosteroid medications, smoking, inflammation, and larger body size, independently of the kidneys.
What is generally associated with a lower result
Broadly associated with brisk kidney filtration and with an underactive thyroid. Low values are generally not pursued.
Everyday things that move this number
Not every shift in a value reflects disease. These are ordinary factors known to affect this particular measurement:
- Corticosteroid (steroid) medications
- Thyroid status and thyroid medication
- Smoking
- Body size and composition
- Recent illness or inflammation
Questions to take to your appointment
- Was this ordered because creatinine might be misleading for my body type?
- How does the eGFR from cystatin C compare with my creatinine-based eGFR?
- Could my thyroid or any of my medications be influencing this result?
Build a printable questionnaire with your own results →
Where to read more
MedlinePlus, from the US National Library of Medicine, publishes patient-facing pages on individual lab tests and is a reliable next stop: medlineplus.gov/lab-tests .
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