Cystatin C – The Better Kidney Test Athletes Should Be Using

Why This Matters More Than You Think

Most doctors still use creatinine tests for kidney health. But here’s what they aren’t considering: creatinine blood testing can be misleading.

If you work out hard, take creatine supplements, consume a lot of red meat, or have more muscle than average, your creatinine levels might look bad even when your kidneys work fine. Even worse, some herbs like astragalus supplements can make your kidney function appear better than it actually is by affecting how creatinine is cleared from your blood.

There’s a better test called Cystatin C. This test gives more accurate results for active people, athletes, bodybuilders, and anyone using kidney support supplements. Yet most doctors don’t use it. This article explains why Cystatin C matters for kidney health in athletes and when you should ask for this test instead.

Your kidneys filter waste from your blood 24/7. When they start failing, you need to know early. The wrong test could miss problems or cause unnecessary worry and expense.

Quick Facts

• Main benefit: More accurate kidney function testing for athletes and lifters with high muscle mass

• Research backing: Numerous studies show Cystatin C is more accurate than creatinine testing for certain populations

• Best for: Athletes, bodybuilders, people taking creatine or testosterone, anyone with above-average muscle mass, or those already using liver & kidney support supplements

• Testing: Simple blood test, same as creatinine. Most New Zealand labs need to send samples to a lab in Christchurch for analysis.
  
  
  

The Science: How Cystatin C Works

Cystatin C is a protein made by all your cells. Your kidneys filter it from your blood at a steady rate. When kidney function drops, Cystatin C blood levels go up. It’s like a waste product that builds up when your body’s filtration slows down.

Here’s why Cystatin C is better than creatinine: muscle mass doesn’t affect Cystatin C results. Creatinine comes from muscle breakdown, so bigger muscles or creatine use mean higher creatinine. This can make healthy athletes look like they have kidney problems. Creatinine is also elevated by consuming creatine, or by training really hard.

An article in the New England Journal of Medicine (“Cystatin C versus Creatinine in Determining Risk Based on Kidney Function”) shows how Cystatin C provides more accurate risk assessment for kidney disease, especially in athletes.

Think of creatinine like measuring exhaust from different-sized engines – bigger engines or different fuels make more exhaust even when running perfectly. Cystatin C is like measuring how well the air filter works, regardless of engine size.

Key Biomarkers

• Cystatin C: Small protein produced by all cells, filtered by kidneys, not reabsorbed. Levels rise when kidney filtration decreases. Not affected by muscle mass, creatine supplementation, or gender.

• Creatinine (Traditional Test): Waste from muscle metabolism. Baseline higher in muscular people, affected by red meat and creatine supplements. Can miss early kidney issues.

• eGFR (Estimated Glomerular Filtration Rate): Calculation of kidney filtration. eGFR-Cys (Cystatin C based) is more accurate for athletes and bodybuilders.

Pro Tip: Creatinine tests can miss kidney disease in low muscle mass and falsely suggest kidney problems in muscular athletes.

Who Should Ask for Cystatin C Testing

• Athletes and bodybuilders with high muscle mass

• Individuals using creatine monohydrate or other muscle-building supplements

• Enhanced athletes using testosterone or PEDs

• Combat athletes manipulating water weight

• Anyone with abnormal creatinine test results

• People following high-protein diets

• Older adults or anyone with a family history of kidney disease

Practical Tips for Getting Tested

Getting Tested Through Strom Sports

We’ve made Cystatin C testing accessible in New Zealand through Strom Sports’ partnership with i-screen. You can order comprehensive athlete blood testing in NZ, including Cystatin C alongside other biomarkers for performance and longevity:

👉 Click Here for Strom Athlete Blood Testing 

This removes the hassle of convincing your GP and ensures your results are interpreted in the context of athletic performance and supplement use.

FAQs

Q: Why don’t all doctors use Cystatin C instead of creatinine?
A: Education, habit, and cost. Creatinine is cheaper and has been used for decades. But research shows Cystatin C is the best kidney test for athletes and muscular individuals.

Q: Should I stop taking creatine before testing?
A: Not necessary for Cystatin C – creatine affects creatinine but not Cystatin C, making it ideal for supplement users.

Q: Can Cystatin C detect kidney problems earlier than creatinine?
A: Research suggests yes. Cystatin C may identify kidney issues earlier, especially in athletes or low muscle mass individuals.

Research References

• eGFR-cys below 60: Indicates kidney disease

• New England Journal of Medicine – “Cystatin C versus Creatinine in Determining Risk Based on Kidney Function”

• PMC (2024) – “Creatinine and Cystatin C: A Measure of Renal Function in Men With Testosterone-Induced Muscle Hypertrophy”

• National Kidney Foundation (June 2025) – “Cystatin C”

• ScienceDirect (April 2022) – “Creatinine: An Inaccurate Measure of Renal Function in Men with Testosterone-Induced Muscle Hypertrophy”

• Journal of the American Society of Nephrology (2005) – “Cystatin C and the Risk of Death and Cardiovascular Events among Elderly Persons”

• Kidney International (1999) – “A More Accurate Method to Estimate Glomerular Filtration Rate from Serum Creatinine: A New Prediction Equation”

• Clinical Chemistry (2005) – “Cystatin C as a Marker of GFR: History, Indications, and Future Research”
  

When requesting kidney function testing, especially if you’re an athlete, take supplements, or have higher muscle mass, ask your healthcare provider about Cystatin C testing alongside or instead of standard creatinine tests. This more accurate biomarker can provide better insight into your actual kidney health.