Calculate your kidney function
Your eGFR from creatinine, age and sex using the CKD-EPI 2021 equation. Add cystatin C and you will see immediately whether muscle mass is skewing your result.
Kidney function calculator (eGFR)
Kidney function calculator (eGFR)
Enter your values from your lab report. Everything is calculated in your browser and never leaves your device.
Cystatin C is not affected by muscle mass. If you have the value, enter it for a more reliable result.
Your cystatin C based eGFR is clearly higher than your creatinine based one. That points to muscle mass, heavy training or creatine use pushing your creatinine up, rather than to reduced kidney function. In that case the combined estimate above is the most reliable.
Your creatinine based eGFR sits below 90. With a lot of muscle mass, a high-protein diet or creatine use, creatinine can run high without your kidneys working any less. Cystatin C is immune to all of that and gives a cleaner picture.
This calculation uses the CKD-EPI 2021 equations (Inker et al., New England Journal of Medicine, 2021) and is intended for education, not medical diagnosis. An eGFR is an estimate: it varies from one measurement to the next and is affected by muscle mass, hydration and diet. Always discuss an abnormal or persistently low result with your doctor.
Creatine, creatinine and why your eGFR can look worse than it is
Train seriously, eat a lot of protein or supplement with creatine? Then your creatinine is likely to sit on the high side, and your eGFR comes out low. It is one of the most common scares on a lab report, and usually nothing is wrong.
Creatinine is a waste product of your muscle metabolism. The more muscle mass you carry, the more creatinine you produce each day. Creatine supplementation raises that pool further still. Your kidneys filter exactly as well as before; there is simply more creatinine passing through. The eGFR equation does not know the difference and reads it as a lower filtration rate.
Cystatin C solves that. This protein is produced at a constant rate by almost every cell in your body and has nothing to do with muscle mass. If your cystatin C based eGFR comes out considerably higher than your creatinine based one, it was your muscles getting in the way, not your kidneys.
Important: this is not a free pass. A low eGFR can genuinely point to reduced kidney function, athletes included. If your value stays low, certainly on repeat measurements or together with protein in the urine, see your doctor. Muscle mass is an explanation you have to rule out, not a conclusion you get to assume.
What is eGFR?
The eGFR (estimated glomerular filtration rate) estimates how many millilitres of blood your kidneys filter per minute, adjusted for body surface area. It is the standard measure of kidney function. Above 90 counts as normal; below 60 is reason to check in with your doctor.
How is it calculated?
This tool uses the CKD-EPI 2021 equations: the current international standard, in which ethnicity deliberately no longer plays a role. Enter creatinine only and we use the creatinine equation. Add cystatin C and we use the combined equation, which is demonstrably more accurate than either one on its own.
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Frequently asked questions
Which values do I need?
Your age, sex and creatinine (in µmol/L). Cystatin C (in mg/L) is optional, but it makes the estimate clearly more accurate, especially if you carry a lot of muscle mass.
Does creatine raise my creatinine?
Yes, and that is normal. Creatine supplementation typically raises your creatinine by around 10 to 20 percent without changing your kidney function. Your creatinine based eGFR therefore comes out lower than it really is. Cystatin C is not affected by this.
Why do my two eGFR values differ?
Because they are based on different markers. Creatinine tracks your muscle mass; cystatin C does not. A large gap between the two usually points to a lot of muscle mass, or very little, rather than to a kidney problem. The combined equation accounts for both and is the most reliable.
Is an eGFR below 90 worrying?
Usually not. An eGFR between 60 and 89 is common in healthy people and drifts down gradually with age. Only repeatedly low values, or values combined with other abnormalities such as protein in the urine, are reason to discuss it with your doctor.
Are my entered values stored?
No. The calculation happens entirely in your browser: your entered values never leave your device and are not stored anywhere.
Want your kidney function measured?
A blood test measures your creatinine, cystatin C and more, so you know whether a low eGFR is down to your kidneys or your muscles.