A creatinine result and a cystatin C result can give different estimates of kidney function. That does not mean one laboratory is wrong. The tests are affected by different things, and a combined estimate can be more useful when an accurate answer will change a clinical decision. Neither result should be interpreted without your previous tests, urine findings and medical history.
If you train regularly, take creatine or have lost muscle during illness, an unexpected kidney result can be particularly confusing. The aim is to find out whether the number reflects a lasting kidney problem, a temporary change or a limitation of the test, not to buy an increasingly large panel without a question to answer.
What is the difference between creatinine and cystatin C?
Creatinine is a waste product related to muscle metabolism. A blood test measures its concentration; an equation then estimates how well the kidneys filter blood. This is the estimated glomerular filtration rate, or eGFR. It is an estimate, not a direct measurement of filtration.
Cystatin C is another substance that can be measured in blood and used to estimate filtration. It is less dependent on muscle mass than creatinine, which can make it helpful when muscle-related factors make the creatinine estimate difficult to interpret. It is not free of other influences. Inflammation, thyroid disease and steroid treatment can affect interpretation. A doctor needs to know the clinical context. [1,2]
Ask The Wellness about a kidney-results review if you have conflicting results or an unexplained change. Bring the complete laboratory reports with units and reference ranges.
Why might creatinine look high in someone who trains?
Muscle mass, recent strenuous exercise, diet and creatine supplements can affect the interpretation of creatinine. This does not make an abnormal result automatically harmless. Exercise-related muscle injury, dehydration or an unrelated kidney problem can also matter.
Tell the clinician what you take, including powders, pre-workout products and non-prescription painkillers. Give an honest account of training before the blood test. A planned repeat under agreed conditions may be useful, but do not simply repeat until a preferred number appears. The question is whether the underlying kidney function is satisfactory. [1,2]
People with very low muscle mass can face the opposite problem: a reassuring-looking creatinine may overestimate filtration. Age, frailty, amputation or substantial muscle loss can therefore change which estimate is most useful.
When does a combined estimate help?
The KDIGO 2024 guideline supports using creatinine and cystatin C together when greater accuracy is needed and cystatin C is available. Its primary-care summary explains that the combined estimate improves accuracy and can help when there is uncertainty about creatinine. [1]
This matters when a result could change a medicine dose, a referral or another important decision. It does not mean every healthy person needs cystatin C added to every screening panel. Before paying for it, ask: what would we do differently if this result were higher or lower?
Occasionally, neither estimate answers the question well enough. A specialist may consider a measured filtration test. That is a different procedure, not another name for a routine eGFR blood result.
Does one low eGFR mean chronic kidney disease?
Not necessarily. Chronic kidney disease involves abnormalities persisting for at least three months, or other evidence establishing chronic disease. A single unexpected result needs assessment, including whether an acute problem is possible. You should not wait three months if the result suggests a sudden deterioration or you are unwell. [1]
Previous results are useful, but only when their dates and circumstances are clear. A result during vomiting, infection or a hospital admission is not the same as a stable baseline. The clinician may need to repeat testing sooner, assess medicines or arrange urgent care.
Why does a urine test still matter?
Kidney assessment is not a competition between two blood markers. Urine albumin-to-creatinine ratio, usually called ACR, checks for albumin leakage and helps assess risk alongside eGFR. Blood pressure, diabetes, urine blood and the pattern of change also matter. [1]
A person with a preserved eGFR can still have clinically important albumin in the urine. Conversely, a borderline estimate without urine abnormalities may need a different discussion from rapidly worsening function with substantial albumin leakage. Read our guide to urine ACR and eGFR before your review.
What should you bring to an appointment?
- All recent kidney results, including previous normal results.
- A full medicine and supplement list, with doses where known.
- Details of recent illness, dehydration, training or muscle injury.
- Urine-test reports and home blood-pressure readings if available.
- Any history of kidney stones, diabetes, urinary problems or kidney disease in the family.
For people whose care is split between London and another country, keeping one dated record avoids repeated investigations that never reach a clear conclusion. Ask the reviewing clinician to document which estimate will guide future decisions and when it should be checked again.
When should you seek urgent care?
Seek urgent medical assessment for markedly reduced urine output, severe dehydration, new significant swelling, breathlessness, confusion or feeling very unwell with an abnormal kidney result. Severe muscle pain or weakness with dark urine after exercise also needs urgent assessment. Do not assume that a training-related explanation makes these symptoms safe to watch at home.
What makes a private review worthwhile?
The useful service is interpretation: why the estimates differ, whether urine testing or a repeat is needed, and who will act on the result. Contact The Online GP, part of The Wellness, to ask which consultation fits your situation and confirm any investigation or specialist fees before booking. A more expensive panel is not automatically a better investigation.
Questions patients often ask
Is cystatin C always better? No. Its value depends on the clinical question and factors affecting both markers. The combined estimate is often more informative than treating either result as a perfect answer.
Should I stop creatine before testing? Discuss the product and test plan with your clinician. Do not use a generic internet washout period or change prescribed treatment yourself.
Can normal eGFR rule out kidney disease? No. Urine findings and other evidence can identify problems even when filtration appears preserved.
Will I need a kidney specialist? That depends on the trend, urine results, symptoms and overall risk. Ask for a written next step, rather than a diagnosis based on one isolated number.
Arrange a results-review consultation or contact the team with your complete reports. This article is general education, not a diagnosis or an urgent-care service.
Ask the team on WhatsApp, call +44 20 3951 3429 or email team@thewellnesslondon.com to discuss an appropriate consultation.
For a separate discussion of broader prevention, The Wellness executive programmes are published at £34,995, £14,995 and £6,995 as of 31 October 2026. These are programme prices, not fees for investigating this finding. A focused consultation or indicated specialist assessment may be the right next step instead.
Evidence and further reading
- KDIGO. 2024 CKD guideline: evaluation takeaways for primary care. International clinical guideline summary, including combined estimates and urine ACR.
- My Kidney Is Fine, Can't You Cystatin C?. Clinical discussion of limitations of creatinine and interpretation of cystatin C. This is supporting clinical literature, not proof that everyone needs additional testing.
Evidence checked 31 October 2026. KDIGO's 2027 medication-management update is currently a public-review draft, not a replacement final guideline for this article's evaluation advice.