Can muscle mass affect your kidney test results?
Why creatinine and cystatin C give different answers about your kidneys
A lower-than-expected kidney function result can be worrying, but it doesn't always mean your kidneys aren't working properly.
Most eGFR (estimated glomerular filtration rate) results are calculated using creatinine, a measure that can be thrown off by how much muscle you carry.
If you're muscular or exercise heavily, this calculation could be making your kidneys look worse than they actually are. In this article, we look at alternative measures and the best way to get a reliable result.
In this article, we cover:
- What does eGFR measure?
- What is creatinine?
- Creatinine and creatine: what's the difference?
- What's a normal eGFR?
- What is cystatin C?
- How do creatinine and cystatin C eGFR calculations compare?
- What should you do if your kidney results look abnormal?
- Not sure where to start?
What does eGFR measure?
Your kidneys filter waste and excess water from your blood. The rate at which they do this is called your glomerular filtration rate (GFR).
Measuring GFR directly is complicated, so labs usually estimate it from a blood test. This calculation is known as your estimated glomerular filtration rate (eGFR). In short, eGFR gives an estimate of how well your kidneys are working.
Most routine labs calculate eGFR using creatinine.
What is creatinine?
Creatinine (not to be confused with creatine) is a waste product from muscle. Your kidneys remove it from your blood, so if your kidneys aren't working well, creatinine levels increase.
But kidney function isn't the only thing that affects creatinine.
Causes of raised creatinine include [1–3]:
- Greater muscle mass
- Recent strenuous exercise
- Dehydration
- A recent meat-heavy meal
- Creatine supplements
- Certain medications, like trimethoprim
NICE advises caution when interpreting creatinine-based eGFR in people with unusually high or low muscle mass, including bodybuilders. Increased muscle mass can make eGFR appear lower than it really is [1].
The reverse can also happen in someone with very low muscle mass: a low creatinine may make kidney function appear better than it is.
Creatinine and creatine: what's the difference?
Creatine and creatinine are related, but they're not the same thing.
Creatine is stored in your muscles and helps supply energy during exercise. It's why some people take it as a gym supplement. Creatinine, on the other hand, is a waste product formed as creatine is broken down.
Taking creatine supplements can cause a modest increase in blood creatinine, which may lower your calculated eGFR. Research suggests this increase doesn't necessarily represent reduced kidney filtration [4]. However, an abnormal result shouldn't be ignored without further assessment.
What's a normal eGFR?
An eGFR of 90 mL/min/1.73 m2 or above is generally considered normal, although it doesn't rule out kidney disease if there are other signs of kidney damage.
A result between 60 and 89 can also be normal, particularly as you get older, provided there are no other signs of kidney damage.
An eGFR below 60 can point to reduced kidney function, but one result alone doesn't diagnose chronic kidney disease. The result generally needs to remain low for at least three months or be accompanied by other evidence of kidney damage [1,3].
| eGFR (mL/min/1.73 m2) | What it may mean |
| 90 or above | Normal or high filtration |
| 60–89 | Can be normal, especially if there are no other signs of kidney damage |
| 45–59 | Mild-to-moderate reduction |
| 30–44 | Moderate-to-severe reduction |
| 15–29 | Severe reduction |
| Below 15 | Kidney failure range |
These ranges are only useful if the eGFR calculation is reliable. When creatinine may be affected by muscle mass, exercise, or supplements, sometimes cystatin C can provide a more realistic estimate.
What is cystatin C?
Cystatin C is a protein made by cells throughout the body. Like creatinine, it's filtered from your blood by the kidneys. However, it's much less affected by muscle mass, meat intake, and creatine supplementation.
A cystatin C-based eGFR may therefore be useful if:
- You have unusually high or low muscle mass
- You take creatine supplements
- You exercise intensely
International kidney guidelines specifically mention that extreme exercise and bodybuilding may make creatinine-based eGFR calculations less reliable [3].
Cystatin C isn't a perfect measure either. It can be affected by things like inflammation, thyroid disorders, smoking, body fat, and medications like prednisolone [3].
How do creatinine and cystatin C eGFR calculations compare?
Creatinine-based eGFR remains the standard first-line test [3].
Cystatin C-based eGFR may be more useful when muscle mass, exercise, or creatine use might be distorting the creatinine result.
And when the two disagree, an equation that combines creatinine and cystatin C is generally more accurate than either estimate alone [3,5]. If you have both your creatinine and cystatin C results, you can use this eGFR calculator to work out your combined result.
What should you do if your kidney results look abnormal?
Don't ignore an abnormal result just because you're muscular, exercise frequently, or take creatine.
Check whether your eGFR was calculated using creatinine or cystatin C, and consider whether anything may have temporarily affected the result. Sometimes, other tests like a urine albumin test or blood pressure check are needed.
Share any persistent or significantly abnormal results with your GP, who can help arrange any further tests.
Not sure where to start?
If your result was based on creatinine alone and you fall into any of the groups above, our Cystatin C Blood Test can help give a clearer picture.
References
- National Institute for Health and Care Excellence. Chronic kidney disease: assessment and management. NICE guideline NG203. 2021. Available at: nice.org.uk/guidance/ng203.
- Delanaye P, Mariat C, Cavalier E, et al. Trimethoprim, creatinine and creatinine-based equations. Nephron Clinical Practice. 2011;119(3):c187–c193. doi: 10.1159/000328911.
- Kidney Disease: Improving Global Outcomes (KDIGO) CKD Work Group. KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of Chronic Kidney Disease. Kidney International. 2024;105(4S):S117–S314.
- Kabiri Naeini E, Eskandari M, Mortazavi M, et al. Effect of creatine supplementation on kidney function: a systematic review and meta-analysis. BMC Nephrology. 2025;26(1):622. doi: 10.1186/s12882-025-04558-6.
- Inker LA, Eneanya ND, Coresh J, et al. New creatinine- and cystatin C-based equations to estimate GFR without race. New England Journal of Medicine. 2021;385(19):1737–1749. doi: 10.1056/NEJMoa2102953.