What does it measure?
Many body cells produce this small protein, and the kidneys remove it from circulation. It can be used to calculate an estimated filtration rate, or eGFR. It is often useful when creatinine is harder to interpret because muscle mass is unusually high or low. [1]
Before the test
- Sample
- A blood sample from a vein.
- Mention
- List steroid medicines and any thyroid condition; these can affect interpretation.
What can the result mean?
A higher blood cystatin C often points to lower filtration; a lower value often points to higher filtration. It still does not diagnose chronic kidney disease from one measurement. The estimate and other kidney findings need to fit together over time. [1]
When is it useful, and how is it read?
This can help when creatinine-based filtration estimates are uncertain, for example with unusual muscle mass. Compare the resulting estimate with creatinine, urine albumin, clinical history, and changes over time; a combined equation may be useful when appropriate. [1] [3]
Reading the results together
| Marker | What it adds |
|---|---|
| Creatinine | A blood marker used in common eGFR equations. |
| Cystatin C | Another blood marker that can be combined with creatinine. |
| eGFR | The calculated estimate of kidney filtration. |
These blood tests inform filtration estimates; they do not replace checking urine albumin when indicated. [3] [4]
What can affect it?
Cystatin C is less affected by muscle mass than creatinine, but it is not free of other influences. Thyroid function, inflammation, and some medicines can affect the result. The equation and assay used also matter.
Myth: «Cystatin C is completely unaffected by anything except the kidneys»
It is less tied to muscle mass than creatinine, but thyroid function, steroids, and other factors can still affect it. Its value comes from adding a different perspective, not from being a perfect measurement. [1]
Questions you might have
Is cystatin C itself the eGFR?
No. It is a blood measurement used in an equation to estimate filtration. [1] [3]
Why might a clinician use both cystatin C and creatinine?
Combining them can improve the filtration estimate, especially when a more accurate estimate affects a decision. [3]
What comes next?
Your clinician may combine creatinine and cystatin C in an eGFR equation for a more reliable estimate, particularly near an important treatment decision. Read it with urine albumin and your clinical history. [2]
Would a combined creatinine–cystatin C estimate clarify my kidney function?
The reference interval and units on your report are part of understanding your result. Understand units, reference intervals, and treatment targets.
Sources & references
Medical references & original research
- Mayo Clinic Laboratories — Cystatin C with Estimated GFR
- NIDDK — Demographics and eGFR Accuracy
- NIDDK — eGFR Equations for Adults
- NIDDK — Chronic Kidney Disease Tests & Diagnosis
- National Kidney Foundation — Cystatin C
Books we consulted
References that informed our explanations of the test and its context.
- Rifai N, Chiu RWK, Young I, Burnham CAD, Wittwer CT, editors. Tietz Textbook of Laboratory Medicine. 7th ed. Elsevier; 2023.
Practical-section sources accessed: .
For general health education. Interpreting your result and deciding on tests or treatment requires your health context and a qualified clinician.
