About Creatinine
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.