Cystatin C: What This Kidney Function Blood Test Can Tell You

Quick Facts:

 

• Cystatin C is a small protein produced by cells throughout the body.
• It is filtered from the bloodstream by the kidneys.
• It is much less influenced by muscle mass than creatinine.
• Combining creatinine and cystatin C can provide a more accurate estimate of GFR in many situations.
• Cystatin C still has its own potential confounding factors, so it should not be interpreted completely in isolation.
• Kidney assessment often also requires urine testing, particularly urine albumin ratio (ACR).

 

 

What is Cystatin C?

 

Cystatin C is a small protein produced by nucleated cells throughout the body.

It enters the bloodstream at a relatively consistent rate and is freely filtered through the glomeruli – the microscopic filtration units within your kidneys.

Because the amount remaining in the blood is closely related to how effectively the kidneys are filtering it, cystatin C can be used as an endogenous marker of glomerular filtration rate, or GFR.

Put more simply:

If kidney filtration falls, cystatin C will generally rise.

This principle is similar to creatinine. The difference lies in what else influences the two markers.

 

What is Creatinine?

 

Creatinine is a waste product generated largely through normal muscle metabolism.

Your kidneys filter creatinine from your bloodstream, so measuring blood creatinine provides useful information about renal filtration.

Creatinine is then incorporated into an equation to produce your estimated glomerular filtration rate – eGFR.

eGFR does not directly measure how many millilitres of blood your kidneys physically filter every minute. It is an estimate, calculated using blood markers and other variables.

This distinction matters.

An abnormal eGFR can be clinically important, but it needs to be interpreted in context.

The UK Kidney Association specifically notes that eGFR is an estimate and that significant error can occur at extremes of body composition, including in people with very high muscle mass, muscle wasting, amputations or severe malnutrition.

Trends in eGFR can therefore sometimes be more informative than one isolated result.

 

 

Why can muscle mass affect creatinine?

 

Because creatinine is closely connected to skeletal muscle metabolism, someone carrying substantially more muscle may naturally produce more creatinine.

That means a muscular individual can sometimes have:

Higher blood creatinine → lower calculated eGFR

without the reduction necessarily representing the same degree of kidney dysfunction that the number might suggest at first glance.

NICE specifically advises caution when interpreting creatinine-based eGFR in people at extremes of muscle mass, including bodybuilders. Increased muscle mass can lead to an underestimate of true GFR, while reduced muscle mass can cause GFR to be overestimated.

This doesn’t mean high creatinine should simply be ignored in someone who trains.

It means the result needs context.

 

Can exercise affect creatinine?

 

Potentially, yes.

Exercise – particularly strenuous resistance training – can influence creatinine through increased muscle metabolism and, in some situations, temporary changes related to hydration and muscle breakdown.

The timing of the blood test therefore matters.

 

If someone has completed a particularly hard training session shortly before testing, an unexpectedly elevated creatinine result may warrant interpretation alongside:

• Previous results
• Training history
• Hydration
• Muscle mass
• Dietary intake
• Supplementation
• Urine findings
• Other kidney markers

 

It is one reason that repeat testing under more standardised conditions can sometimes be clinically useful.

 

What about Creatine supplements?

 

There used to be initial concerns around creatine and down-regulation of kidney function which we now know to not be the case and it is noted as a safe supplement with emerging studies on muscle health, cognitive and performances benefits.

This is particularly relevant to people who train.

Creatine and creatinine are not the same substance, but creatine can ultimately be converted into creatinine.

Supplementation can therefore alter serum creatinine in some people, potentially making creatinine-based kidney estimates more difficult to interpret.

KDIGO’s 2024 chronic kidney disease guidance specifically identifies high-protein diets and creatine supplementation among situations where non-kidney influences on serum creatinine need to be considered.

That does not mean somebody taking creatine should assume a high creatinine result is harmless.

Rather, creatine use becomes another piece of information that should be considered when deciding what that result means.

 

How is Cystatin C different from creatinine?

 

The biggest practical difference is that cystatin C is far less dependent on skeletal muscle mass.

For someone who is:

• Particularly muscular
• A bodybuilder
• An endurance athlete
• Experiencing significant muscle loss
• Underweight or malnourished
• Living with a condition affecting muscle mass
• Creatinine may not behave exactly as expected.

 

Cystatin C can therefore provide a useful alternative perspective.

KDIGO specifically identifies extreme exercise and bodybuilding as situations where cystatin C-based estimation may be appropriate when increased muscle mass is the primary factor making creatinine difficult to interpret.

 

Is Cystatin C more accurate than creatinine?

 

Sometimes – but this question needs a little nuance.

Cystatin C is not universally “better” than creatinine.

Creatinine is cheap, widely available and clinically valuable. For many people, it performs perfectly well as an initial kidney filtration marker.

Cystatin C becomes especially useful when there is reason to suspect that creatinine may be biased by something unrelated to kidney filtration.

And increasingly, the strongest option is not necessarily choosing one over the other.

It is using both.

A combined equation using creatinine and cystatin C – often written as eGFRcr-cys – can improve estimation of GFR because the limitations affecting each marker are different.

KDIGO’s 2024 guidance recommends creatinine-based eGFR initially and, where cystatin C is available, using a combined creatinine-cystatin C estimate when greater accuracy is required for clinical decision-making.

 

Think of it less as:

Creatinine VS Cystatin C

and more as:

What does each marker add to the overall picture?

 

 

 

Can Cystatin C be affected by anything other than kidney function?

 

Yes.

This is an important part of the conversation that can sometimes be missed when cystatin C is described as a “better kidney marker”.

Cystatin C is much less influenced by muscle mass and dietary creatinine, but that does not mean it is completely unaffected by factors outside kidney filtration.

Research and kidney guidance identify potential influences including:

• Thyroid dysfunction
• Glucocorticoid medication
• Significant obesity/adiposity
• Smoking
• Some chronic illnesses
• Cancer and high cell-turnover states

For example, studies suggest hyperthyroidism can raise cystatin C while hypothyroidism may reduce it independently of kidney filtration.

Glucocorticoid treatment can also increase Cystatin C production in some circumstances.

This is exactly why no biomarker should be interpreted without knowing the person behind the number, their clinical context and any relevant previous medical history that can be relevant to understanding a marker and the relevance of interpreting the report in from of you.

 

High Creatinine but normal Cystatin C: what could that mean?

 

This is one of the scenarios where cystatin C becomes particularly interesting.

Imagine somebody with:

• Above-average muscle mass
• Regular resistance training
• Creatine supplementation
• Raised creatinine
• A calculated creatinine eGFR that appears lower than expected

If cystatin C and eGFR suggest better filtration, that discrepancy can indicate that creatinine may be being influenced by factors other than reduced renal filtration.

It does not automatically prove that kidney function is normal.

But it gives the clinician additional information to work with. The pattern often matters more than any individual result.

 

What if both Creatinine and Cystatin C are abnormal?

 

If both markers suggest reduced kidney filtration, this may increase confidence that the finding reflects a genuine change in GFR rather than an isolated creatinine-related effect.

At that point, the next question becomes “What else is happening?”

This is where urine markers, medical history, medication use, blood pressure, previous results and clinical symptoms become important.

 

 

Kidney health is not just an eGFR

 

It is very easy to think about kidney function as a single number.

In reality, a meaningful kidney assessment may include several different pieces of information.

 

• Creatinine and eGFR – Provides an estimate of glomerular filtration and remains the standard starting point for most kidney function assessments.

 

• Cystatin C and eGFRcys – Provides another estimate of filtration that is less dependent on muscle mass.

 

• Combined eGFRcr-cys – Uses both markers and can improve accuracy where a more reliable GFR estimate is needed.

 

• Urine albumin ratio – ACR – Measures the amount of albumin leaking into urine.

 

• Urea Blood Nitrogen (BUN) – Measures the amount of urea nitrogen – a waste product from protein breakdown.

 

• Urinalysis – Measures the physical, chemical, and microscopic properties of a urine sample to screen for infections, kidney disease, and diabetes

 

This tells us something different.

Someone can have relatively preserved filtration while still having abnormal albumin loss through the kidneys.

NICE therefore recommends assessing chronic kidney disease using both GFR and urine ACR categories, rather than GFR alone.

 

Urinalysis

 

Can identify findings such as:

• Red blood cells
• White blood cells
• Protein
• Glucose
• Leukocytes
• Nitrites

 

Previous Results

 

One of the most important tests is sometimes not a new biomarker at all.

It is the trend.

• Has creatinine always sat at this level?
• Has eGFR gradually changed over several years?
• Was the previous result dramatically different?
• Have any lifestyle factors changed?
• Have any supplements been introduced since the previous test?

 

Kidney function should generally be interpreted longitudinally wherever previous data are available.

 

 

Does one low eGFR mean Chronic Kidney Disease (CKD)?

 

No.

Chronic kidney disease requires evidence that kidney abnormalities are persistent.

NICE defines CKD as abnormalities of kidney function or structure present for more than three months with implications for health.

This includes a GFR below 60 mL/min/1.73 m² on at least two occasions separated by at least 90 days, or other evidence of kidney damage.

A single unexpectedly low eGFR therefore needs appropriate clinical follow-up rather than an immediate assumption of chronic kidney disease.

NICE also recommends repeating a newly identified eGFR below 60 within two weeks in an adult who has not previously been tested, while taking biological and analytical variation into account.

Clinical urgency will, of course, depend on how abnormal the result is and whether there are other concerning findings or symptoms.

 

 

Who might benefit from Cystatin C testing?

 

Cystatin C may be particularly worth considering when:

• Creatinine is unexpectedly high
• eGFR appears unexpectedly low
• Muscle mass is significantly above average
• Muscle mass is significantly reduced
• Creatine supplementation may be affecting interpretation
• Creatinine and the wider clinical picture do not appear to match
• A more accurate estimate of kidney filtration would change a clinical decision

It should not simply be added to every blood test without a reason, good testing is not about measuring the largest possible number of biomarkers.

It is about selecting markers that answer a meaningful clinical question, or are relevant to a persons lifestyle.

 

 

What does a High Cystatin C result mean?

 

Generally, higher blood cystatin C can indicate reduced glomerular filtration.

But just as with creatinine, the number should not be interpreted completely on its own.

The laboratory’s reference range, your eGFRcys, creatinine, medications, health history, thyroid status and other potential confounding factors may all be relevant.

This is why simply labelling cystatin C as “high” or “low” provides far less information than understanding why it sits where it does.

 

 

What is a normal cystatin C level?

 

Reference intervals vary between laboratories, methods and populations.

For kidney assessment, the cystatin C concentration itself can be useful, but its use within an eGFRcys or combined eGFRcr-cys calculation often provides greater clinical context.

Your result should therefore be interpreted against the reference interval supplied by the laboratory that performed the analysis rather than using a generic online “optimal range”.

 

 

Cystatin C vs Creatinine: the takeaway

 

Creatinine has not become obsolete.

Cystatin C has not replaced it.

Instead, cystatin C gives us another tool for situations where creatinine may not tell the entire story.

For someone with significant muscle mass, unusual body composition, creatine supplementation or an unexpected creatinine/eGFR result, cystatin C may help us distinguish between:

• A creatinine result influenced by factors outside the kidneys

and

• A genuine reduction in kidney filtration that warrants further assessment.

 

And sometimes the clearest answer comes from combining both, because kidney health is not captured by one result.

It is found in the relationship between blood markers, urine markers, clinical context and trends over time.

A flagged result is a starting point, not a diagnosis.

When kidney markers do not seem to fit the person in front of us, the answer is not to ignore the result or automatically assume disease.

It is to ask better questions:

• Could muscle mass be affecting creatinine?
• Has the person trained heavily before testing?
• Are they taking creatine?
• What does Cystatin C show?
• What is happening in their urine?
• Is this new, or has it always been their baseline?

That is where testing becomes useful: not simply producing numbers, but putting those numbers into context.

 

Frequently Asked Questions

 

Is Cystatin C a blood test?

Yes.

Cystatin C is usually measured from a blood sample and can be used to calculate a cystatin C-based estimated GFR.

 

 

Is Cystatin C better than creatinine for bodybuilders?

 

It can be particularly useful in people with significantly increased muscle mass because cystatin C is much less influenced by skeletal muscle than creatinine.

Kidney guidance specifically recognises bodybuilding/extreme exercise as situations where cystatin C may add value.

 

 

Can taking Creatine cause high Creatinine?

 

Creatine supplementation can influence serum creatinine in some people, meaning the result needs to be interpreted alongside supplementation, muscle mass, training and other kidney markers.

A high creatinine result should not automatically be dismissed as “just creatine”, however.

 

 

Does High Creatinine always mean kidney problems?

 

No. Reduced kidney filtration is one cause of elevated creatinine, but muscle mass, dietary intake, supplementation, exercise and some medications can also influence it.

An unexplained elevation still warrants appropriate interpretation and follow-up.

 

 

Can Cystatin C be high when kidney function is normal?

 

Potentially. Cystatin C has non-kidney influences of its own, including thyroid dysfunction, glucocorticoid exposure and some chronic disease states.

It should therefore be interpreted within the wider clinical picture.

 

 

What tests should be done alongside Cystatin C?

 

Depending on why kidney function is being assessed, this may include creatinine, eGFR, a combined creatinine-cystatin C eGFR, urine ACR, urinalysis, electrolytes, blood pressure and comparison with previous results.

The appropriate investigation depends on the individual.