Creatine and Blood Tests: The Creatinine Problem

This is the most practically useful post on this blog. It takes one sentence to act on and it can save you an unnecessary referral.

If you take creatine and are having blood work, tell the person ordering it.

Here's why.

How kidney function is usually estimated

Routine blood panels include serum creatinine — a waste product produced continuously by muscle and cleared by the kidneys.

The logic is straightforward: production is assumed to be relatively constant, so if creatinine accumulates in blood, the kidneys must not be clearing it well. That makes it a convenient proxy for filtration.

Most labs then feed that value into an equation to calculate estimated glomerular filtration rate (eGFR), the number clinicians actually use to stage kidney function. Common equations take serum creatinine plus age and sex.

Note what that means: eGFR is calculated from creatinine. Anything that raises creatinine lowers the calculated eGFR automatically, regardless of what the kidneys are doing.

Why creatine raises it

Creatinine is the breakdown product of creatine. Roughly 1.5–2% of your body's creatine pool spontaneously converts to creatinine every day and is excreted.

Supplementation enlarges the creatine pool. A bigger pool produces proportionally more creatinine daily. Serum creatinine rises — because there's more of its precursor entering circulation, not because clearance has failed.

The kidneys are filtering normally. The assumption underlying the test (constant production) has been violated.

What this looks like in practice

A healthy adult supplementing creatine may show a modestly elevated serum creatinine and a correspondingly reduced calculated eGFR. Depending on where they started, that can push the number into a range flagged as abnormal.

A clinician who doesn't know about the supplementation may reasonably order repeat testing, a urinalysis, imaging, or a nephrology referral. None of it is unreasonable — they're responding correctly to the data available.

Which is why the fix is information, not avoidance.

What to say

Before the blood draw, or when discussing results:

"I take 3 grams of creatine monohydrate daily. I understand that can raise serum creatinine independently of kidney function."

Many clinicians know this. Many haven't encountered it, particularly outside sports medicine. Either way it costs you ten seconds and gives them the context to interpret the number correctly.

If there's genuine uncertainty

Two ways to distinguish a supplementation artefact from actual impairment. Both are your doctor's call, not yours — but knowing they exist helps the conversation.

Cystatin C. An alternative filtration marker produced by all nucleated cells rather than by muscle. It isn't affected by muscle mass, meat intake or creatine supplementation, which makes it useful precisely here. Equations exist that estimate GFR from cystatin C alone or from both markers together.

Stopping temporarily. Muscle creatine stores wash out over four to six weeks, and serum creatinine follows. Retesting after a break gives an unsupplemented baseline. Fuller detail in what happens when you stop.

Two other markers worth knowing about

Creatine kinase (CK). An enzyme released when muscle fibres are damaged, used clinically to investigate muscle injury and some cardiac events. Hard exercise raises it substantially — often well above the reference range for a day or two after unaccustomed training. This has nothing to do with creatine supplementation despite the similar name, but it's another reason to mention training habits before testing.

Urea and BUN. Sometimes measured alongside creatinine. Creatine supplementation doesn't consistently affect these, which is part of how clinicians can triangulate.

Who should be more careful

The interpretive issue above applies to healthy people. For some, the underlying question is more than an interpretive one:

• Existing kidney disease or reduced function
• A single functioning kidney
• Diabetes with kidney involvement
• Medications affecting kidney function, including sustained high-dose NSAIDs

In those cases the conversation is with your doctor before starting, not about how to interpret a result afterward. See does creatine damage your kidneys and creatine and kidney disease.

The one-line version

Creatine raises the marker used to estimate kidney function, without impairing kidney function. Say so before you're tested.

General information about a natural health product, not medical advice.