Why does creatinine mislead in muscular men?
Creatinine is a waste product of muscle, so the more muscle you carry, the more creatinine you make. Labs turn creatinine into an estimated kidney filtering rate (eGFR) using a formula that cannot see your muscle mass, so a big, strong man can look like he has weaker kidneys than he does.
The National Kidney Foundation lists the usual culprits that can raise creatinine without kidney disease:
- Taking creatine supplements
- High muscle mass, such as a very muscular frame or bodybuilding
- Recent high-intensity exercise
- Eating large amounts of cooked meat
A man who checks three or four of those boxes can end up with an eGFR that looks worse than his kidneys really are, and walk away worried about a disease he does not have. Our page on eGFR and creatinine explains how that number is calculated.
What is cystatin C, and why does muscle not move it?
Cystatin C is a small protein made by cells throughout your body, not just muscle, and healthy kidneys filter it out. When filtering slows, it builds up in the blood, much as creatinine does.
The difference is what else moves it. MedlinePlus notes that cystatin C levels are not affected by muscle size, age or diet, and lists having a lot of muscle mass as one reason a provider may measure it instead of creatinine. The National Kidney Foundation gives the same example, saying cystatin C can give a more accurate result in people with lots of muscle, such as bodybuilders.
Creatinine and cystatin C side by side
| Feature | Creatinine | Cystatin C |
|---|---|---|
| Where it comes from | Muscle breakdown and dietary protein | Cells throughout the body |
| Affected by muscle size | Yes | No |
| Affected by diet | Yes, including large amounts of cooked meat | No |
| Creatine supplements | Can raise it | Not listed as a factor |
| Known accuracy limits | Muscle mass, diet, recent hard exercise | Untreated thyroid conditions, steroid medicines |
| Typical use | Routine eGFR on most panels | Confirming an unclear or borderline eGFR |
How is a cystatin C result read?
The raw cystatin C value, reported in milligrams per liter, matters less than the eGFR calculated from it. Lab ranges for the raw number vary, so compare it with the range printed on your report.
The National Kidney Foundation’s eGFR calculator uses the race-free CKD-EPI equations, including a 2021 equation that combines creatinine and cystatin C and a 2012 equation that uses cystatin C alone. Whichever one your lab uses, the result is read on the same kidney stage scale: 90 or higher is normal, 60 to 89 may mean early loss of function, and below 60 suggests kidney disease, with stage confirmed over time rather than from one test.
Here is a reassuring example. A lifter whose creatinine-based eGFR reads in the low 60s, whose cystatin C-based eGFR reads above 90 and whose urine albumin is normal has a pattern that points to muscle rather than kidney disease.
When can cystatin C mislead too?
No marker is perfect. The National Kidney Foundation notes that cystatin C may not be accurate in people with untreated thyroid conditions or in those taking steroid medicines. If either applies to you, your provider will weigh both markers together, and may check thyroid function first. Our TSH test page explains that screen.
Cystatin C also does not replace the urine test for albumin, which picks up kidney damage that filtering rates can miss in early stages. A full kidney picture uses both.
Where creatine and TRT fit in
Many men on testosterone therapy also train hard and use creatine, which stacks several creatinine-raising factors at once. That makes a creatinine-only eGFR easy to misread on follow-up labs. Our page on taking creatine while on TRT covers the supplement itself, and our creatine kinase test page covers the separate muscle-damage marker that hard training raises.
How Ultimate Male approaches a kidney question
Kidney function is part of our Targeted Preventative Health Panel and Comprehensive Health Optimization Panel. Cystatin C is not one of the markers named on those panels, so if you are muscular, take creatine or have had a borderline eGFR before, ask on the free 10-minute call whether adding it makes sense for you.
At your consultation in San Gabriel or Downey, a PA-C or MD reads your kidney numbers against your training, supplements and medicines rather than against the range alone. The liver and kidney function panel page explains that panel, and the full list sits on our preventative blood testing page.
