Short answer: creatine supplements and a lot of muscle raise the creatinine in your blood, so the standard creatinine-based eGFR can look falsely low even when your kidneys are fine. Cystatin C is much less affected by muscle and supplements, so a cystatin C eGFR, or a combined creatinine and cystatin C eGFR, is the better check if you train and take creatine. In healthy people at usual doses, research reviews have not found that creatine damages the kidneys. A low eGFR that stays low on repeat testing, low on cystatin C too, or comes with protein in the urine or high blood pressure, is a GP conversation.
Sources: NICE NG203 (chronic kidney disease, 2021); KDIGO 2024 CKD guideline; Kabiri Naeini et al., BMC Nephrology 2025; de Souza e Silva et al., Journal of Renal Nutrition 2019; Gualano et al., Eur J Appl Physiol 2008; Antonio et al., JISSN 2021. Written by James Smillie, TrueVitals, reviewed by the TrueVitals clinical review team. Sources checked November 2026.
Creatine is the compound your muscles use to recycle energy for short, hard efforts. You make some, you eat some in meat and fish, and many people who train take 3 to 5 g a day as a supplement.
Creatinine is the waste product creatine breaks down into. Muscles release it into the blood at a fairly steady rate, and the kidneys filter it out. The more muscle you carry and the more creatine you take in, the more creatinine there is to clear.
eGFR (estimated glomerular filtration rate) is the number most UK reports use for kidney function. NICE asks UK labs to calculate it from serum creatinine, age and sex using the CKD-EPI creatinine equation. The equation assumes average muscle and average creatinine production. If you make more creatinine than average, it reads that as kidneys clearing less, and the eGFR comes out lower than your real filtration.
That is the whole problem in one line: the test measures the waste product of muscle, and you have more muscle (and more creatine) than the equation expects. NICE NG203 says so directly: interpret the creatinine eGFR with caution in adults with extremes of muscle mass, "for example, in bodybuilders", because increased muscle mass leads to an underestimate of GFR. It also lists adults "who have higher muscle mass or use protein supplements" among the situations where the creatinine eGFR may be less reliable.
KDIGO's 2024 guideline lists "high-protein diets and creatine supplements" among the non-kidney reasons serum creatinine rises, and says interpreting creatinine "requires consideration of dietary intake". The common ones for people who train:
| Factor | What it does | What to do before a test |
|---|---|---|
| Creatine supplements | Add to the creatine pool, so more creatinine is made. KDIGO names them as a non-kidney cause of raised creatinine. | Tell whoever reads the result. Either pause for a few days before a repeat test (a common clinical suggestion, not a guideline) or check cystatin C. |
| High muscle mass | More muscle, more creatinine. NICE says increased muscle mass leads to an underestimate of GFR. | Nothing to change. Use cystatin C to cross-check. |
| Cooked meat | Cooking turns some of the creatine in meat into creatinine, which you absorb. | NICE: no meat in the 12 hours before a creatinine eGFR test. |
| Protein supplements and high-protein diets | Named by NICE (protein supplements) and KDIGO (high-protein diets) as reasons the creatinine eGFR can mislead. | Keep protein ordinary the day before; no shake on the morning of the test. |
| Hard training | KDIGO lists extreme exercise and bodybuilding as situations where a cystatin C eGFR may be appropriate. A heavy session also raises creatine kinase (CK), a muscle enzyme, for days. | Avoid a hard or new session for two days before the draw, as a sensible precaution. |
| Dehydration | Less fluid in the blood concentrates creatinine and can make kidney numbers look worse for a day. | Drink normally. Water is fine during the fast. |
None of these explain away a result that stays low on repeat testing, low on cystatin C as well, or comes with protein or blood in the urine. They explain a borderline creatinine eGFR in someone who is otherwise well.
Cystatin C is a small protein made by the body's cells and filtered by the kidneys. Because it doesn't come from muscle, a lot of muscle or a creatine supplement doesn't push it up the way it pushes creatinine up. That makes it the natural cross-check when a gym-goer's creatinine eGFR comes back borderline. In a randomised, placebo-controlled trial of healthy men taking about 10 g of creatine a day for three months, cystatin C did not rise; it fell over time in both groups, and the authors concluded high-dose creatine "does not provoke any renal dysfunction" in that group (Gualano et al., European Journal of Applied Physiology 2008).
The international KDIGO 2024 guideline on chronic kidney disease recommends using a combined creatinine and cystatin C eGFR (eGFRcr-cys) "in clinical situations when eGFRcr is less accurate and GFR affects clinical decision-making". Its table of non-kidney influences names creatine supplements and high-protein diets, and says a cystatin C eGFR may be appropriate in extreme exercise or bodybuilding when increased muscle mass is the only abnormality.
NICE NG203 tells clinicians to interpret the creatinine eGFR with caution in bodybuilders and in adults who use protein supplements. But in its 2021 update NICE removed its earlier (2014) recommendation that cystatin C equations be considered during diagnosis, saying evidence for their accuracy was lacking and that they might reduce false positives but increase false negatives. It made cystatin C a research question instead. In practice, many GPs won't routinely offer it, and some NHS labs only run it for kidney specialists.
It isn't perfect. NHS lab guidance notes that cystatin C eGFR should be read with caution in uncontrolled thyroid disease (it can read falsely high in an underactive thyroid), and KDIGO notes very high inflammation as a source of error. That's why the combined creatinine and cystatin C eGFR, and the difference between the two, tend to tell you more than either on its own.
How to read the two together. If your creatinine eGFR is borderline (say 55 to 70) but your cystatin C eGFR is comfortably normal, muscle and creatine are the likely explanation, especially if your urine is clear of protein and your blood pressure is normal. If both are low, the result is more likely to reflect real kidney function and should be repeated and taken to your GP.
This is the question behind most searches, so here is what the reviews we opened actually found:
The limits of that evidence. Most of the trials are small, short and in healthy young adults. They tell you little about people who already have reduced kidney function, diabetes, high blood pressure, or who take medicines that affect the kidneys (regular anti-inflammatory painkillers such as ibuprofen, for example). If that's you, speak to your GP before taking creatine, and don't assume a low eGFR is "just the creatine".
The supplement isn't the only thing to think about. Products sold for muscle gain are sometimes contaminated or mislabelled, and very high-dose regimes go well beyond what has been studied. Stick to plain creatine monohydrate at studied doses from a reputable seller, and if you also use anabolic steroids or SARMs, they are a far bigger concern for your kidneys, heart and liver than creatine. See our blood test before steroids guide.
There is no NICE or KDIGO rule telling you to stop creatine before a kidney test. What exists is common clinical practice. In September 2026, a nephrologist at Scripps Health in California said her team commonly advises patients to "stop their supplementation for a few days before testing" to see whether creatinine returns to baseline, and pointed to cystatin C as a better marker in this setting. Treat that as a reasonable suggestion, not a standard.
You have two sensible options:
Test with cystatin C included, and tell whoever reads the results that you take creatine and train. The cystatin C eGFR then does the job the creatinine eGFR can't. Useful if you want to see your normal, on-supplement picture.
If a creatinine eGFR has already come back borderline, stop creatine for a few days (some clinicians suggest longer) and repeat the test under the same conditions. If creatinine falls back, that points to the supplement rather than the kidneys.
For any kidney test: no meat for 12 hours (NICE), no protein shake that morning, no hard or new training session for a couple of days, normal water intake, and keep a note of any supplements and medicines. If you're testing with TrueVitals, fast from 10pm the night before; water is fine.
Creatine is a good explanation for a slightly low creatinine eGFR in a fit, well person. It is not a reason to ignore a pattern. See your GP if:
Get urgent help if you pass much less urine than normal, have swollen legs or face, are confused or breathless, or have dark brown urine and severe muscle pain after extreme exercise. That last combination can mean rhabdomyolysis, which damages the kidneys and needs same-day medical care.
The Ultimate panel (140 biomarkers, 139 for women) measures creatinine, eGFR and cystatin C side by side, and calculates cystatin C eGFRs from them, along with urea, uric acid, electrolytes, creatine kinase and a urinalysis. So a creatinine eGFR pulled down by muscle and creatine is read against the cystatin C result rather than flagged on its own, and the report is written from your own lifestyle quiz, where you can tell us that you train and take creatine. Samples go to Nationwide Pathology, a UKAS-accredited lab, and the clinician-reviewed report arrives within 4 working days of the lab receiving your sample. The Advanced panel includes creatinine and eGFR but not cystatin C, so if this is your question, Ultimate is the one that answers it.
For more on the rest of a training-focused panel, see our blood test for gym-goers and athletes' blood test guides, and our kidney function blood test guide for the kidney markers in detail.
When we're not the right choice:
Written by James Smillie and reviewed by the TrueVitals clinical review team, 2 November 2026. Guidance from NICE NG203, Chronic kidney disease: assessment and management (recommendations 1.1.1, 1.1.3, 1.1.4, 1.1.8, 1.1.12 to 1.1.13 and 1.5.5, and the committee's rationale on cystatin C), and the KDIGO 2024 Clinical Practice Guideline for the Evaluation and Management of CKD (recommendation 1.2.2.1, practice points 1.2.2.3 to 1.2.2.7 and table 8). Research: Kabiri Naeini et al., BMC Nephrology 2025; de Souza e Silva et al., Journal of Renal Nutrition 2019; Gualano et al., European Journal of Applied Physiology 2008; Antonio et al., JISSN 2021. Lab note on cystatin C from York and Scarborough Teaching Hospitals NHS Trust; pausing advice from Scripps Health (September 2026). All checked in November 2026. TrueVitals is a UK blood testing company, so we have an interest in you testing; we've tried to say plainly when you don't need us.
Creatinine, eGFR and cystatin C read together, with a clinician-reviewed report within 4 working days.
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