Used primarily as a biomarker for kidney function, creatinine levels in blood and urine help assess glomerular filtration rate (GFR), indicating how well the kidneys filter waste. Elevated serum creatinine suggests impaired kidney function, commonly seen in acute or chronic kidney disease. It is also used to monitor patients with hypertension, diabetes, or those on nephrotoxic medications. In clinical labs, creatinine measurements standardize urine drug tests and protein excretion (e.g., urine albumin-to-creatinine ratio) to correct for variations in urine concentration. Additionally, creatinine is involved in the creatine-phosphate energy system in muscles and serves as a reference in metabolic studies involving muscle mass and exercise physiology.