What is Creatinine?
Creatinine is a breakdown product of creatine, a compound muscles use for rapid energy. As creatine breaks down in muscle cells, creatinine is formed and excreted almost entirely through the kidneys. Because creatinine production per person is fairly constant — determined by total muscle mass — and excretion occurs via glomerular filtration, serum creatinine is an indirect measure of how well the kidneys filter. It is reported in µmol/L (internationally also mg/dL; 1 mg/dL ≈ 88.4 µmol/L). The limitations of creatinine as a kidney marker are important to know. First, production varies strongly with muscle mass: a muscular athlete naturally has a higher serum creatinine than an older woman with little muscle mass — both can have identical kidney function. Second, creatine supplementation, high protein intake, and recent intense training temporarily raise the value. Third, creatinine only falls measurably once kidney function has declined by roughly 50% — it is a late marker. That is why creatinine is always reported together with eGFR, and why context is essential.
Why is Creatinine relevant?
Creatinine is the most widely used baseline marker for kidney function and is part of virtually every blood panel. When kidney function declines — from diabetes, high blood pressure, chronic kidney disease, drug-induced injury, or infection — filtration slows and creatinine rises in the blood. A rising creatinine across multiple measurements is therefore an early signal to investigate further, even if the absolute value still falls within the reference range. For athletes and people supplementing with creatine, creatinine has an added dimension: creatine supplementation temporarily increases creatinine production and can raise the value without any implication for kidney function. Dehydration also concentrates creatinine in the blood and transiently elevates the value — an effect that reverses with good hydration. In both cases, eGFR (when properly calculated) is a more robust marker.
Creatinine high or low — what it means
Always interpret creatinine alongside eGFR — the estimated glomerular filtration rate — and in the context of age, sex, body composition, and current hydration. Reference ranges are sex- and lab-specific: broadly 60–110 µmol/L for women and 70–120 µmol/L for men, but always check your own laboratory's bounds. A value just above the upper limit in a muscular, well-hydrated person is often normal for that individual; a value sitting just within the range in an older person with little muscle mass can sometimes already indicate a significant decline in kidney function. For a fair measurement, wait at least 48 hours after heavy training or stopping creatine supplementation, and ensure you are well hydrated. Trends across multiple measurements provide the most reliable information: a gradually rising creatinine with stable muscle mass and hydration is a clear signal to investigate kidney function further, even if each individual reading still falls within the reference range.
Educational information only — not medical advice. Consult a healthcare professional for clinical decisions.
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