Creatinine is the kidney marker most of us meet on a routine blood test. It is useful, cheap and embedded in clinical practice. But it is also partly a measure of muscle: a highly active person, a frail older adult and someone recovering from illness can arrive at the same creatinine result for different physiological reasons. Cystatin C offers a second way to estimate filtration. The question is not whether it is a better longevity score. It is whether it can make a clinically important kidney estimate less uncertain.
A filtration estimate, not a direct reading
The kidneys filter blood continuously, and glomerular filtration rate (GFR) is the clinical shorthand for how much filtration is occurring. Because directly measuring GFR is cumbersome, laboratories normally estimate it from a blood marker, usually creatinine. The result is reported as eGFR.
Cystatin C is another blood marker that can be used for that estimate. It is produced by most cells and filtered by the kidneys. Crucially, it is less tied to muscle mass than creatinine, though it is not free of other influences. The 2024 KDIGO chronic kidney disease guideline recommends using the combined creatinine–cystatin C equation when cystatin C is available and the estimate would affect a clinical decision.
That is a narrower claim than the one made by many longevity panels. Cystatin C is not a direct photograph of the kidneys, and one result does not diagnose chronic kidney disease. It is an additional measurement that can improve an estimate in the right context.
Why creatinine can occasionally mislead
Creatinine comes from normal muscle metabolism. That relationship makes it a practical marker for millions of routine tests, but it creates a predictable problem at the margins. Someone with low muscle mass may have a reassuring creatinine level despite reduced filtration; someone unusually muscular may have a higher creatinine without equivalent kidney impairment.
The 2024 KDIGO chronic kidney disease guideline identifies extremes of muscle mass, high-protein diets and creatine supplements as situations in which creatinine has non-kidney determinants. These are not minor technicalities when a result sits near a prescribing, referral or monitoring threshold. A newly detected low eGFR should be confirmed with repeat testing rather than treated as a final verdict.
Diet and timing matter, too. A sensible comparison needs an ordinary, stable testing context, not a heroic workout, dehydration or abrupt dietary experiment.
What cystatin C adds
Cystatin C gives clinicians another filtration marker with a different set of blind spots. When the creatinine-based estimate and the cystatin-C-based estimate agree, confidence in the broad picture rises. When they diverge, the discrepancy is information: it invites a closer look at body composition, illness, medicines and whether a combined equation would better guide the decision.
The National Institute of Diabetes and Digestive and Kidney Diseases notes that the combined creatinine–cystatin C equation is particularly useful when eGFR is close to a consequential decision point, such as some drug-dosing or transplant evaluations. Its clinical eGFR guidance is a helpful corrective to the idea of a single ‘best’ biomarker: the equation is selected for the decision in front of the patient.
That also explains why cystatin C is becoming more visible in preventative care. It can be a reasonable question for a clinician when a creatinine-derived eGFR looks unexpectedly low or high for the person, when muscle mass makes creatinine harder to interpret, or when the number will influence care. It is not automatically a better annual screening test for everyone.
Risk signals need a second test: urine albumin
Kidney health is not captured by filtration alone. Urine albumin-to-creatinine ratio (ACR) looks for albumin leaking into the urine, a different and clinically meaningful signal. Current guidance uses both filtration and albuminuria to classify chronic kidney disease and estimate risk.
The National Kidney Foundation’s laboratory recommendations specifically encourage ordering urine ACR alongside serum creatinine when assessing or monitoring kidney disease risk. A cystatin C result without urine testing can therefore look more complete than it is. Conversely, a normal cystatin C result does not rule out all kidney problems.
For people with diabetes, high blood pressure, cardiovascular disease or a family history of kidney disease, the practical question is usually not “Which biomarker wins?” It is whether the appropriate combination of blood pressure, eGFR trend and urine ACR has been checked and interpreted together.
How to read a result without overreacting
First, identify what the laboratory actually reported. It may list cystatin C concentration, eGFR calculated from cystatin C, or a combined creatinine–cystatin C eGFR. These are related but not interchangeable. The unit and equation matter.
Second, look for a pattern rather than making a story from a single data point. Chronic kidney disease is defined by persistent abnormalities, not an isolated borderline number. The KDIGO guideline recommends repeating tests after incidental detection of a low eGFR, whilst the broader clinical context determines the next step.
Third, consider the reason for testing. A muscular 45-year-old with a borderline creatinine eGFR and no albuminuria presents a different problem from an older adult with diabetes, rising blood pressure and a falling eGFR trend. The test cannot supply that context on its own.
When it is worth discussing with a clinician
A cystatin C test is most useful when it will change how an existing result is interpreted or acted on. Discuss it with a clinician if your creatinine-based eGFR is near a treatment or referral threshold, if your muscle mass is atypical for your age and size, if results have moved unexpectedly, or if a clinician has raised uncertainty about kidney function.
Bring the whole record: prior creatinine and eGFR results, urine ACR, blood pressure, medication list, recent illness and any use of creatine or protein supplements. Cystatin C also has non-kidney determinants: the KDIGO guideline identifies smoking and chronic illness, including inflammation and malnutrition, as potential influences. That is another reason the number belongs in a clinical conversation rather than a personal leaderboard.
What this means in practice
- Ask which eGFR equation your laboratory used before comparing results across providers or years.
- If a creatinine eGFR is unexpected, ask whether cystatin C or a combined estimate would meaningfully clarify the result.
- Pair kidney blood tests with a urine ACR when assessing kidney risk; filtration and albumin leakage answer different questions.
- Repeat a surprising result under ordinary conditions rather than reacting to one test after illness, dehydration, a hard training block or a major dietary change.
- Keep blood pressure, glucose and medication review on the agenda: they are more actionable for kidney risk than chasing an isolated biomarker.
What we do not know
It is tempting to treat cystatin C as a superior longevity marker. That goes beyond what the test can show. Its established role is to refine an eGFR estimate when the estimate will affect a clinical decision; it does not tell a person which intervention to choose, nor does a favourable result prove that a lifestyle intervention protected the kidneys.
Access and laboratory practice also vary. UK guidance still centres creatinine eGFR and ACR for routine assessment, while international guidance increasingly supports combined estimates in selected decisions. The right test sequence depends on availability, cost, medical history and the decision at stake.
Cystatin C is best understood as a way to reduce uncertainty, not manufacture it. When a kidney result is genuinely ambiguous, a second marker can sharpen the picture. When it is not, the basics—reliable follow-up, urine testing where indicated, and attention to blood pressure and diabetes risk—remain the more useful form of prevention.
Photo: Robina Weermeijer on Unsplash.