Creatinine provides validity measure in urine drug tests
Muscle contraction leads to the spontaneous cyclization of creatine and creatine phosphate to yield creatinine, a metabolic degradation product that is excreted by the kidneys (Figure 1). The amount of creatine per unit of muscle is relatively consistent (about 0.5 mmol creatinine per kg muscle), and it breaks down at a constant rate, so serum and urinary creatinine concentrations change little in the absence of disease.
Ropero Miller, J. (2002). Creatinine provides validity measure in urine drug tests. Clinical & Forensic Toxicology News, Sept, 4-8.