Anthony Ofori | Medical Graduate in General Medicine | Founder, WellNest Central
Last updated: 24 September 2026
Two people can have the same creatinine result and very different kidney function. One may have a lot of muscle; the other may have lost muscle during illness. If the number on your report does not seem to fit the rest of your health picture, could a cystatin C test help?
Sometimes, yes. It offers another way to estimate how well the kidneys filter blood. It is especially useful when a creatinine-based eGFR is uncertain or when a more accurate estimate could change a clinical decision.
What is cystatin C?
Cystatin C is a protein made by cells throughout the body. Your kidneys filter it from the blood. A blood sample can be used to estimate your glomerular filtration rate, or eGFR. This is an estimate of filtration, not a direct measurement of every kidney function.
Most routine eGFR reports use creatinine, a substance closely tied to muscle. Cystatin C is generally less affected by muscle mass. The National Kidney Foundation explains that a cystatin C test may be helpful when an earlier kidney test gave an unclear answer or when muscle mass makes creatinine harder to interpret.
Why might creatinine give an incomplete picture?
Creatinine levels can change with muscle mass, recent intense exercise, creatine supplements and cooked meat. A person who lifts weights may start with more creatinine. An older adult who has lost muscle may produce less. Either situation can make a creatinine-based eGFR less representative of the person’s true filtration rate.
This does not make the usual test useless. It means the result deserves context. If food might have influenced a blood draw, see our article on cooked meat, creatinine and eGFR.
Three ways your report may estimate eGFR
- eGFRcr: calculated from serum creatinine. This is the common starting point.
- eGFRcys: calculated from cystatin C. It can help when creatinine may be misleading.
- eGFRcr-cys: calculated using both blood markers. The combined result is often more accurate than creatinine alone.
According to the NIDDK’s adult eGFR guidance, the combined equation is especially valuable when an estimate sits near a clinical decision point, such as a medication-dosing threshold. Your healthcare professional decides which estimate fits your situation.
WELLNEST CENTRAL CONSULTATIONS
Make sense of your kidney-test questions
Bring your creatinine, eGFR and urine results. We can help you organise your questions before discussing them with your clinician.
When is it worth asking about cystatin C?
Consider asking your clinician if any of the following applies:
- Your creatinine-based eGFR does not match your other test results or health history.
- You have unusually high or low muscle mass, an amputation, or substantial muscle loss.
- A result is close to a threshold that could change a medicine dose, diagnosis or referral.
- Previous blood tests have changed in a way that remains unexplained after considering illness, diet, exercise and medicines.
The KDIGO 2024 kidney guideline recommends using a combined creatinine and cystatin C estimate when creatinine-based eGFR is less accurate and the result will affect a clinical decision. Availability and cost can differ by laboratory and health system.
Does cystatin C give a perfect answer?
No. It is less influenced by muscle mass, but it can be affected by other factors, including steroid medicines, thyroid problems, inflammation and body fat. Tell the clinician about your medicines and medical conditions when discussing the result.
If creatinine and cystatin C give substantially different estimates, neither number should be chosen automatically. Your clinician may consider both, review the full history and, in special situations, arrange a measured GFR test.
Do you still need a urine test?
Yes, when kidney disease is being assessed. Cystatin C estimates filtration; it does not show whether albumin is leaking into urine. The urine albumin-to-creatinine ratio, or uACR, answers that different question. Our guide to three kidney tests that work together explains how eGFR, uACR and urinalysis fit into one assessment.
A single result usually cannot establish chronic kidney disease. Clinicians look for changes that persist for at least three months or other evidence of kidney damage. A sudden large change, however, may need assessment promptly rather than a routine repeat months later.
Questions to bring to your appointment
- Is my eGFR based on creatinine, cystatin C or both?
- Could my muscle mass, diet, recent exercise or supplements have affected creatinine?
- Would a cystatin C test change a decision about my care?
- What was my uACR, and how do these results compare with earlier tests?
- Should any test be repeated, and when?
The bottom line
A cystatin C test can add clarity when creatinine-based eGFR is uncertain. It is most useful as part of a wider assessment that includes previous results, urine albumin, medicines, health conditions and the decision at hand. It is not a “better kidney test” for every person in every situation.
If you would like help preparing your questions, book a WellNest Central consultation. A consultation offers health education and preparation for medical care; diagnosis and treatment decisions belong with your healthcare professional.
This article is for general education and does not replace personal medical advice, diagnosis or emergency care. Seek prompt medical assessment for a sudden change in kidney results, reduced urine output, marked swelling, breathlessness or severe illness.
