Creatinine is a waste product produced by the breakdown of creatine in muscle cells. Larger muscles produce more creatinine. Athletes using creatine supplementation produce even more. The standard upper limit for serum creatinine in males is around 106 µmol/L — but trained athletes routinely sit at 110–130 µmol/L with perfectly healthy kidneys. The standard reference ranges were not derived from populations with significant muscle mass.
eGFR is the marker to watch
When your creatinine sits slightly above range and your eGFR — the calculated estimate of kidney filtration rate — is above 90 mL/min/1.73m², your kidneys are functioning well. The elevated creatinine is simply a reflection of your muscle mass and creatine metabolism, not kidney stress. If eGFR drops below 60 mL/min/1.73m² — and stays there across multiple tests — that's a genuine signal. A single elevated creatinine in a trained athlete is almost never that.
BUN and protein intake
BUN (blood urea nitrogen) is another kidney marker that confuses athletes. High protein intake elevates BUN independent of kidney function. If you're eating 2g of protein per kilogram of bodyweight, your BUN will sit at the upper end of the range. This is expected and not a concern in isolation. The BUN/creatinine ratio is a useful tool. A ratio above 20 suggests dehydration or very high protein intake. A ratio below 10 with both values elevated might suggest kidney disease. Most athletes sit between 10 and 20 — perfectly normal.
Uric acid
Uric acid is worth including in your kidney panel if you're following a very high protein diet or experience joint discomfort. Elevated uric acid is associated with gout and, at chronically high levels, kidney stone risk. MarkerX flags elevated creatinine in athletes with the specific context that muscle mass and creatine supplementation are likely contributors — rather than treating it the same way a standard report would treat the same value in a sedentary individual.