Creatinine

What is Creatinine?

Creatinine is a waste product from normal muscle activity that healthy kidneys filter into urine.

Creatinine forms when creatine phosphate in muscle breaks down during normal energy use. The amount your body makes each day depends mostly on how much muscle you carry, not on what you ate that morning, which is part of why it's a steadier signal than many blood markers. Healthy kidneys filter creatinine out of the blood almost entirely through the glomeruli, with a small additional amount actively secreted by the kidney's tubules. When filtering slows, creatinine backs up in the blood, so clinicians use it as a rough proxy for how well the kidneys are clearing waste. That proxy has real limits. Because muscle mass drives production, two people with identical kidney function can have different creatinine numbers, and the same person's number can shift with muscle loss, high-protein meals, dehydration, or a hard workout the day before the blood draw, according to MedlinePlus and Mayo Clinic Laboratories. A handful of common medications, including trimethoprim (found in the antibiotic combination trimethoprim-sulfamethoxazole) and cimetidine, partly block the kidney's tubular secretion of creatinine. That raises the blood level by a modest, predictable amount without any true drop in filtering, a pattern documented in clinical pharmacology literature going back decades and still referenced in current nephrology teaching. Labs also measure creatinine two different ways. Older Jaffe-based methods are more prone to interference from other blood compounds, while enzymatic assays tend to run cleaner. Since the mid-2000s, most US labs have standardized to isotope-dilution mass spectrometry (IDMS)-traceable methods, which matters because the eGFR equations doctors use were calibrated against that standard, and an uncalibrated assay can throw off the calculated result. A large, recent high-protein meal or a hard workout can also cause a temporary rise in blood creatinine, according to MedlinePlus and Mayo Clinic Laboratories. It's a short-term effect, not a change in underlying kidney function, which is one reason single results are best read in context rather than in isolation.

What changed recently

A systematic review and meta-analysis published in BMC Nephrology in November 2025, covering 21 studies, found that creatine monohydrate supplementation produces a small, statistically detectable rise in serum creatinine without a corresponding drop in measured GFR. The authors framed this as expected metabolic turnover, since supplemental creatine itself gets converted to creatinine, rather than a sign of kidney injury, and cautioned against reading creatinine alone as a kidney-function marker in people taking the supplement. Given how common creatine use has become among gym-goers and other health-engaged adults, it's a documented confounder worth mentioning to a doctor rather than interpreting from the number alone. Separately, a January 2025 review in Biomolecules revisited creatinine's role as both a kidney marker and a lean-body-mass indicator, reinforcing that individual variation in creatinine generation belongs in the conversation whenever the result gets interpreted.

Tests often checked alongside this

• eGFR — a calculated estimate that converts your creatinine, age, and sex into a filtering-rate figure; see the eGFR glossary entry for how the calculation and its equation updates work.
• BUN (blood urea nitrogen) — another kidney-clearance marker, often paired with creatinine as a ratio; see the BUN/creatinine ratio entry for how that comparison is read.
• Cystatin C — an alternative filtering marker that doesn't depend on muscle mass, sometimes ordered when creatinine alone is hard to interpret; see the cystatin C entry for its specific role.
• Urinalysis — checks for protein or blood in urine, giving a different angle on kidney health than a blood creatinine number alone.

What to ask your doctor

• Does my creatinine number reflect my muscle mass, or should I ask about a marker that doesn't?
• Am I on any medication, such as trimethoprim or cimetidine, that could raise creatinine without affecting true kidney function?
• If I take a creatine supplement, should that be factored into how this result gets read?
• Should this be tracked alongside eGFR over time rather than as a single value?

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