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Creatinine is a waste product of muscle metabolism that healthy kidneys continuously filter from the blood at a near-constant rate, making it a reliable proxy for kidney function. Importantly, creatinine levels don't start rising until kidney function has already declined by 30–50% — meaning significant kidney damage can be present before this marker flags. It's most useful when combined with age and sex to calculate eGFR, which provides a more accurate and actionable measure of kidney filtration capacity.
Also known as: Crea, Creat, Creatinine, Creatinine, Serum, Serum Creatinine
High creatinine indicates impaired kidney filtration from CKD, acute kidney injury, or dehydration. Very muscular individuals may run high without kidney disease. Low creatinine suggests low muscle mass or advanced liver disease.
Standard labs accept up to 1.2 mg/dL for men and 1.1 for women. Functional practitioners flag above 1.0 in men and 0.8 in women. Trending over time is essential.