You have trained consistently for years, you visibly carry muscle, and then your GP comes back with: "Your creatinine is a bit on the high side." For many lifters that feels like a line through their work. But in someone with a lot of muscle, a raised creatinine is usually exactly what you would expect, and not the same as poor kidneys.
My stance: the eGFR formula is built on an average population, and you are not that. Before you worry about your kidneys, it is worth understanding where that creatinine comes from.
Why muscle mass raises your creatinine
Creatinine arises from the breakdown of creatine in your muscles. The more muscle tissue you have, the more creatinine you produce per day, and so the higher your baseline in the blood. That is not a sign of damage but a consequence of your body composition. A bodybuilder and a non-training peer can have the same healthy kidneys and still show very different creatinine.
Creatine supplementation and a high-protein diet can lift that value a little further, as can a heavy training session in the days before the draw. Each of these is an explainable cause that has nothing to do with kidney function.
Creatinine, eGFR and why the formula underestimates you
Your eGFR is estimated from your creatinine. Because the formula assumes a higher creatinine means "less filtration", your estimated kidney function can come out too low while your kidneys work fine. In muscular people that is a well-known pitfall.
The Dutch Kidney Foundation (Nierstichting) points out that eGFR is an estimate, not a direct measurement of your kidney function, and that the result should always be read in the context of the person. For a lifter that means: do not get rattled by an eGFR just below the line if you have no other complaints or risk factors.
| Cause of high creatinine | Type | What it says about your kidneys |
|---|---|---|
| A lot of muscle mass | Benign | Nothing, higher production |
| Creatine supplementation | Benign | Nothing, arithmetic consequence |
| Heavy training shortly before the draw | Temporary | Nothing, retest after rest |
| Dehydration | Temporary | Little, drink and retest |
| Genuine kidney damage | Concerning | Confirm with cystatin C and a doctor |
Cystatin C: the marker that ignores your muscle mass
To find out whether it really is your kidneys, cystatin C is the marker you need. Cystatin C is produced by almost all cells in your body and does not depend on your muscle mass, your diet or your creatine use. In muscular people, an eGFR calculated from cystatin C therefore often gives a fairer picture (Baxmann, 2008).
The Dutch College of General Practitioners (NHG) names cystatin C as an additional test when the creatinine-based eGFR may be unreliable, for example with an atypical body build. Exactly the situation of many strength athletes.
When is a high creatinine actually a signal?
In muscular athletes a slightly raised creatinine is rarely concerning, but there are situations where you do take it seriously. A rapidly rising value over a short time, a creatinine well above the range that your muscle mass or training does not explain, or a deviation that comes with symptoms deserves attention. Think of persistent fatigue, foamy or noticeably changed urine, fluid retention, or high blood pressure.
If you have risk factors such as diabetes, raised blood pressure or kidney problems in the family, an off result weighs more heavily. In those cases it is wise not to lean on cystatin C alone, but to discuss the result with your GP, who can judge the whole picture. The Dutch College of General Practitioners (NHG) guidelines advise doctors to use exactly that combination of markers and context when kidney function is in doubt.
How to test your kidney values well
Test on a calm day, not within 48 to 72 hours of a heavy session, and drink enough so dehydration does not artificially lift your value. Measure cystatin C and urea alongside creatinine, so you can judge your kidneys apart from your muscle mass. Our kidney health blood test bundles those markers.
Read on: the pillar blood values for strength athletes, plus creatine and your blood values and protein intake and your kidneys.
My advice: a raised creatinine in a trained lifter is usually your muscle mass talking. Confirm it with cystatin C, look at the trend over time, and discuss a persistent deviation or symptoms with your GP.
References
- Baxmann AC, Ahmed MS, Marques NC, et al. Influence of muscle mass and physical activity on serum and urinary creatinine and serum cystatin C. Clinical Journal of the American Society of Nephrology. 2008. PMID: 18235143.
- NHG (Dutch College of General Practitioners), guideline and public information on kidney function and cystatin C.
- Dutch Kidney Foundation (Nierstichting), public information on eGFR and kidney function.
Disclaimer
Every blood test result includes a professional assessment by a BIG-registered doctor. This article gives general information and is not a substitute for medical advice. A blood test is a tool to walk into the conversation with your GP better informed, not a diagnosis in itself. For treatment decisions, discuss your results with your GP.
To see what your creatinine means for your estimated filtration rate, the kidney function calculator works out your eGFR.
Autor
Enhanced Health
Dr. Naimi, lekarz wpisany do holenderskiego rejestru BIG, nadzoruje standardy medyczne naszych treści i ocen. Przeczytaj naszą politykę medyczną
Powiązane badania
Powiązane artykuły
MPV blood value: what the size of your platelets says
Total protein blood test: what it says about your protein intake
