Creatinine-d5 is the deuterium labeled Creatinine. Creatinine (NSC13123) is an orally active, blood-brain barrier-permeable modulator of GABAA and NMDA receptors, with activities of antioxidation and metabolic regulation. Creatinine is generated via non-enzymatic conversion of creatine and phosphocreatine in muscle. Creatinine serum levels correlate with muscle mass, glomerular filtration rate, and extrarenal clearance, serving as an evaluation biomarker for renal function, muscle mass, and clinical outcomes, and used for perioperative renal risk assessment. In addition, Creatinine can induce specific Cryptococcus species to produce creatinine deiminase and act as their nitrogen source, while it can be secreted via renal tubules. Creatinine is widely used in research related to various diseases such as neurodegenerative diseases, psychiatric disorders, acute kidney injury, chronic kidney disease, and renal failure[1][2][3][4].