Kidney values are the blood values that show how well your kidneys filter your blood. The four you meet most often are creatinine, eGFR, urea and cystatin C. In athletes carrying a lot of muscle the standard calculation often reads too pessimistically, because creatinine comes from your muscles and not only from your kidneys.
I wrote this because I see the pattern too often. A 95 kilo strength athlete gets an eGFR of 68 back, reads somewhere that this is called stage 2 kidney disease, and panics.
Usually nothing is wrong. Sometimes something is. The difference sits in which value you measure alongside it.
What are kidney values and what do they measure?
Kidney values are waste products and proteins in your blood that your kidneys normally filter out. If they build up, your kidney is filtering less. Creatinine and urea are the classic waste products, cystatin C is a protein every nucleated cell makes, and eGFR is not a measurement but a calculation.
Your kidneys process roughly 180 litres of blood a day. From that they keep about one and a half litres of urine. The rest goes back into your bloodstream, including the water, salts and glucose you still need.
You cannot measure that filtering directly without an infusion and half a day in hospital. So we estimate it, using substances whose normal clearance rate we know.
The Nierstichting describes creatinine as a waste product from your muscles that leaves your body through your kidneys. That single sentence contains the exact problem the rest of this article deals with.
Why is your creatinine higher when you carry more muscle?
Creatinine forms in your muscles, as a breakdown product of creatine phosphate. The more muscle mass you carry, the more creatinine you produce per day, and the higher your blood value sits at exactly the same kidney function. Your kidney is doing nothing wrong. It is simply being sent more.
Say two men of 40 have kidneys that work equally well. One weighs 70 kilos and does not train. The other weighs 95 kilos with 15 kilos more muscle and squats three times a week.
The second man has structurally more creatinine in his blood. Not because his kidneys are weaker, but because his muscles produce more.
The eGFR formula tries to correct for this using age and sex. Muscle mass is not in it. That is the heart of the problem, worked out further in high creatinine from muscle mass.
If you also supplement creatine, a small extra amount comes on top. Creatine converts spontaneously into creatinine. A narrative review in Nutrients concluded that this rise does not in itself indicate kidney dysfunction (PMID 36986197), which I work through further in is creatine bad for your kidneys.
What is a normal eGFR and when is it wrong?
An eGFR above 90 counts as normal, 60 to 89 as mildly reduced and below 60 as a reason to look further. Those cut-offs come from population research on average bodies. In a heavily muscled body the calculated value can land tens of points below your actual filtration rate.
Worth stressing: eGFR stands for estimated glomerular filtration rate. The word estimated is doing a lot of work. The number is the output of a formula using your creatinine, your age and your sex.
Feed in a higher creatinine and the formula returns a lower eGFR. It has no way of knowing whether that creatinine came from a failing kidney or from a back full of muscle.
Many labs also report anything above 90 as a category rather than a number, and give an exact figure only below it. A jump from 91 to 88 therefore often means less than it appears.
The individual marker is explained on our eGFR page.
Which kidney value gives athletes the fairest picture?
Cystatin C. Almost every nucleated cell in your body produces this protein, at a rate that barely tracks your muscle mass. That lets you test a low creatinine-based result against an eGFR that does not care about your build.
A study in the Journal of the American Geriatrics Society looked at exactly that difference, the gap between a cystatin C eGFR and a creatinine eGFR, and linked it to muscle mass (PMID 38819605). The gap is measurable and it is not random.
In practice it works like this. If your creatinine eGFR reads 65 and your cystatin C eGFR reads 95, muscle mass is the strong explanation.
If both sit around 65, something else is going on and that belongs with your GP. How this plays out is covered in cystatin C, the kidney value that fits muscular athletes.
Which kidney values are in a blood test?
A standard kidney panel holds creatinine and the eGFR calculated from it, often with urea added. Broader panels add cystatin C plus the urine albumin to creatinine ratio. Each of those values can move up or down for reasons that have nothing to do with your kidneys.
This table is why I wanted to write this article. Elsewhere you can find what each value measures. What you rarely find is the column next to it: what distorts that value in an athlete.
| Value | What it measures | What distorts it in athletes | What you want alongside it |
|---|---|---|---|
| Creatinine | Waste product of muscle breakdown cleared by your kidney | Muscle mass, creatine use, hard training in the preceding days, a meat-heavy meal | Your muscle mass and when your last hard session was |
| eGFR | Filtration rate calculated from creatinine, age and sex | Everything that distorts creatinine, because it is the same measurement inside a formula | Whether your lab can also calculate the cystatin C version |
| Cystatin C | Protein from all nucleated cells, filtered by your kidney | Barely by muscle mass, somewhat by thyroid function and significant inflammation | Your TSH if the value is unexpectedly off |
| Urea | Waste product of protein processing in your liver | Protein intake and fluid deficit, both common in athletes | What you ate and drank that week |
| Urine albumin | Whether protein is leaking through your kidney filter | A hard endurance effort can cause temporary protein leakage | Whether you trained hard within 24 hours of the test |
Urea deserves one more note. Eat 200 grams of protein a day and your urea almost certainly sits at the high end of normal. That is processing, not damage.
What temporarily raises your kidney values with nothing wrong?
Four things do this in athletes regularly: a hard session in the 48 hours before the draw, creatine use, a high protein diet and simply not having drunk enough. All four raise creatinine or urea without your filtration having changed.
Fluid deficit is the quiet one. Come in dehydrated from a summer endurance ride and draw blood straight away, and your blood is more concentrated, so nearly everything reads higher.
That same fluid deficit concentrates your urine, which is the leading risk factor for stone formation. What to do about it sits in preventing kidney stones.
Anti-inflammatories belong on this list too, though they work differently. Ibuprofen and related NSAIDs constrict the vessels feeding your kidney filter. In a healthy, well hydrated athlete that is short lived and recovers, but the combination of dehydration, a long endurance effort and a painkiller is one to avoid.
Protein itself is less guilty than its reputation. A meta-analysis of 28 trials with 1,358 participants found no difference in kidney function change between higher and normal protein intakes in healthy adults (PMID 30383278). We work through that in protein intake and your kidneys.
For a clean measurement, draw fasted, well hydrated, and at least two days after your hardest session.
What do your blood pressure and your kidneys have to do with each other?
High blood pressure is one of the two largest causes of chronic kidney damage worldwide, alongside type 2 diabetes. Your kidneys are packed with small filter units built from fine vessels, and sustained high pressure damages those vessels slowly. In the other direction your kidneys help regulate your blood pressure, so the traffic runs both ways.
For strength athletes this matters more than it looks. During a heavy set your blood pressure spikes enormously for a few seconds, and that is normal and harmless with healthy vessels.
What counts is your resting pressure, measured on an ordinary day. If that sits structurally above the target your GP uses, it matters more for your kidneys over time than any supplement does.
Compounds that push your blood pressure or your haematocrit up sharply belong in the same consideration. Thicker blood and higher pressure load the same filtering tissue, as covered in high haematocrit in athletes.
So measure your blood pressure at home occasionally, at rest, and not straight after training. It is the cheapest kidney protection there is.
When do you take your kidney values to a doctor?
Go when an eGFR below 60 stays low on repeat testing, when protein in your urine does not clear, when your cystatin C comes back as low as your creatinine eGFR, and always when there is visible blood in your urine. That last one never belongs in the wait-and-see category, and nor does cola coloured urine after a hard session, covered in dark urine after training.
A single off result is not a diagnosis. Kidney values move with your fluid balance, your food and your training.
What counts is a pattern. If a value stays off across two or three measurements, that deserves a conversation with your GP, even if you feel excellent.
Feeling fine tells you little here. Why that is sits in kidney damage symptoms.
How do you get your kidney values measured?
Through your GP if you have symptoms or risk factors, or yourself through a blood test without a referral if you simply want to know your baseline. For athletes the second route is often practical, because a GP rarely orders a kidney panel for someone without complaints.
Our 360 Health test includes creatinine, urea, eGFR and cystatin C, so you get the creatinine result and the check on it in the same draw.
Which values you need at minimum and how to arrange it is set out step by step in testing your kidneys.
If you measure repeatedly, keep the conditions identical. Same time of day, same distance from your last hard session, always well hydrated. A trend is only a trend if the measuring conditions are one too.
Why this matters to you as an athlete
Your kidneys do not complain. They lose capacity quietly and only send signals once a fair amount is gone. That makes measuring worthwhile, because you see something you cannot feel.
At the same time there is no need to panic over a number that reflects your build. I would not tell any lifter to drop creatine or protein on the basis of a single eGFR of 70.
My advice is simple. Measure under clean conditions, run cystatin C alongside if you carry a lot of muscle, and look at the pattern rather than one number. If the pattern is off, the conversation with your GP is about data instead of about a scare.
References
- Nierstichting. Blood testing for creatinine and kidney function. Accessed 2026.
- de Souza e Silva A, et al. Is It Time for a Requiem for Creatine Supplementation-Induced Kidney Failure? A Narrative Review. Nutrients. 2023. PMID 36986197.
- Devries MC, et al. Changes in Kidney Function Do Not Differ between Healthy Adults Consuming Higher- Compared with Lower- or Normal-Protein Diets. J Nutr. 2018. PMID 30383278.
- Difference between kidney function by cystatin C versus creatinine and association with muscle mass and frailty. J Am Geriatr Soc. 2024. PMID 38819605.
Disclaimer
Every blood test result includes a professional assessment from 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. Discuss any abnormal value with your GP.
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