You started creatine. A few weeks later a routine blood panel comes back with serum creatinine slightly up and eGFR slightly down, and the report flags it. That sequence is common enough to have its own literature, and understanding it before the appointment is worth doing — both so the number does not frighten you unnecessarily, and so you do not wave away a result that deserves a proper look. This article is background for that conversation, not a substitute for one.

Why creatine raises creatinine

The similarity in the names is not a coincidence. Creatinine is a breakdown product of creatine. Your muscles hold creatine and phosphocreatine, a small fraction of that pool converts to creatinine every day, and your kidneys clear it into urine. Blood creatinine is therefore a function of two things: how much is being produced, and how well it is being cleared.

Clinical labs use it as a proxy for the second. That works because the first is normally stable — your muscle mass and dietary creatine intake do not swing much week to week, so a rise usually means reduced clearance.

Take a creatine supplement and you change the input. More substrate means more of the daily breakdown product, so serum creatinine drifts upward while clearance is doing exactly what it was doing before.

A 2025 systematic review and meta-analysis in BMC Nephrology by Kabiri Naeini and colleagues put numbers on this. Across 21 studies, with 12 pooled, creatine supplementation produced a small but statistically significant increase in serum creatinine — and no statistically significant difference in GFR compared with controls. The authors conclude the rise reflects metabolic turnover rather than renal impairment.

The International Society of Sports Nutrition position stand on creatine reaches the same place from the safety side: no compelling evidence that creatine supplementation negatively affects renal function in healthy or clinical populations, and the studies measuring filtration directly found no effect on creatinine clearance or glomerular filtration rate.

Why eGFR moves too

This follows mechanically from the above.

You do not measure GFR in a routine panel. Measuring it directly requires clearance testing that is slow and expensive, so labs calculate an estimate — eGFR — from serum creatinine plus age and sex, using an equation such as CKD-EPI.

So when creatine supplementation pushes serum creatinine up, the equation faithfully converts that into a lower eGFR. Nothing malfunctioned. What the equation cannot know is why creatinine is higher — it assumes production is typical for your age and sex, and in a creatine user that no longer holds.

The result is a calculated eGFR that can underestimate true filtration in someone supplementing creatine, or with substantially more muscle mass than the equation expects. This is a recognised limitation of creatinine-based estimation, not a fringe claim.

Cystatin C: the marker that separates the two

There is a second way to estimate filtration, and it does not run through creatinine at all.

Cystatin C is a small protein produced at a fairly constant rate by essentially all nucleated cells and cleared by the kidneys. Because its production is not tied to muscle mass, meat intake, or creatine supplementation, a cystatin C-based eGFR is not moved by the thing that moved your creatinine. The two markers disagreeing is itself informative: a low creatinine-based eGFR alongside a normal cystatin C-based one points toward production rather than filtration as the source of the difference.

This is mainstream nephrology, not a workaround. The KDIGO 2024 clinical practice guideline for chronic kidney disease recommends estimating GFR from the combination of creatinine and cystatin C (eGFRcr-cys) when cystatin C is available, and supports its use where creatinine-based estimation is less accurate and the GFR value affects a decision.

Two honest caveats. Cystatin C is not universally stocked, costs more, and has its own confounders — thyroid dysfunction, corticosteroids, and some inflammatory states can shift it. And a normal result does not guarantee that every future creatinine reading is an artifact. It is a way of asking a sharper question, not of dismissing the first answer.

Where the GLP-1 comes in

Two things stack on top of the creatine story if you are on semaglutide, tirzepatide, or a similar medication.

Fluid intake falls. These medications reduce appetite, and for many people thirst drifts down alongside it. Less spontaneous drinking means a mild tendency toward underhydration — and volume status affects creatinine. A dehydrated draw can read higher than a well-hydrated one in the same person on the same kidneys.

GI side effects can cause real volume depletion. This is not speculative. The FDA prescribing information for Ozempic (semaglutide) carries postmarketing reports of acute kidney injury, some requiring haemodialysis, and states that the majority occurred in patients who experienced gastrointestinal reactions leading to dehydration — nausea, vomiting, or diarrhoea. The label advises monitoring renal function in patients reporting reactions that could lead to volume depletion.

That is why this article does not end with “so the number is nothing.” On a GLP-1 you have a supplement that raises creatinine for benign reasons and a medication whose known GI reactions can raise it for reasons that are not benign at all. Those two possibilities look identical on a lab report. Telling them apart is a clinical job, and it needs the information only you have.

What to bring to the appointment

The single most useful thing you can do is make sure your clinician is not working from an incomplete picture:

  • Say that you take creatine, and for how long. This is the piece most often missing. A clinician who does not know can reasonably read a rising creatinine as a filtration problem and start a workup that a sentence would have redirected.
  • Say the dose and the form — 3 to 5 grams a day of monohydrate is the usual pattern, covered in our dosing guide.
  • Say which GLP-1 you are on, the current dose, and whether you have had nausea, vomiting, or diarrhoea recently. Recent GI symptoms change the interpretation of the same number substantially.
  • Describe your fluid intake honestly, including whether thirst has changed since starting.
  • Ask about a cystatin C-based eGFR if the number is going to influence a decision. Phrasing it as a question — “would a cystatin C-based eGFR help clarify this?” — is accurate, because whether it is warranted depends on your history.

What not to do: do not stop the creatine on your own the day before a draw to make the number look better. A clinician comparing panels needs to know what was actually true when the blood was drawn.

What this article is not saying

Worth being explicit, because this topic attracts overconfident writing.

It is not saying your lab result is wrong. The assay measured what it measured, and the equation calculated correctly from that input. The question is what the value means for you, and that is interpretation.

It is not saying you can ignore a flagged eGFR. A creatine supplement being a plausible explanation is not the same as it being the explanation. People who take creatine also develop kidney disease, and the artifact story is the kind of reassuring narrative that makes a real finding easy to dismiss.

It is not general permission. If you already have reduced kidney function, a single kidney, or known kidney disease, the calculus is different, and that conversation belongs with your nephrologist before you start.

Common questions

How long does the creatinine elevation last?

It tracks the supplementation, settling back within weeks of stopping as the elevated muscle creatine pool drains down. This is one reason a clinician may suggest rechecking after a defined period — the pattern over time is more informative than a single reading.

How large is the change?

Small in the studies — the BMC Nephrology pooled estimate is a modest increase, not a dramatic one. Large or rapidly rising creatinine is not the expected picture from a 3 to 5 gram daily dose, and is worth taking seriously rather than attributing to the supplement.

The short version

Creatine raises serum creatinine because creatinine is what creatine breaks down into. Because eGFR is calculated from serum creatinine, a creatine supplement can lower your calculated eGFR while measured filtration is unchanged — which is what the pooled trial data shows. A cystatin C-based estimate does not depend on that input and is the standard way to ask the question again more precisely.

On a GLP-1 there is a second thread: reduced thirst and GI side effects can cause genuine volume depletion, which the FDA label ties to reports of acute kidney injury. Both threads produce a similar-looking number. Tell your clinician about the supplement and the medication, ask whether a cystatin C eGFR would clarify things, and let someone with your full chart decide what the result means.

For the wider picture — dosing, hydration, and who should have that conversation before starting — see our complete guide to creatine on a GLP-1.