Used as a stable isotope-labeled internal standard in mass spectrometry and clinical diagnostics to accurately measure creatinine levels in biological samples such as blood and urine. Enables precise monitoring of kidney function and glomerular filtration rate (GFR) by correcting for analytical variability during assays. Also employed in metabolic studies to trace creatinine production and clearance in research settings. Its isotopic labeling allows clear distinction from endogenous creatinine, improving measurement accuracy and reliability in pharmacokinetic and nephrology studies.