Forensic urine drug-testing laboratories routinely evaluate the suitability of urine specimens to assess potential adulteration, substitution, or dilution. For example, the measurement of urinary creatinine and specific gravity is performed by these laboratories to determine whether a specimen is abnormally dilute (1).
Creatine is synthesized endogenously and is stored in skeletal muscle in a high-energy phosphorylated form. During muscle contraction, creatine and creatine phosphate are spontaneously converted to creatinine. Creatinine is eliminated from the body by renal excretion at a relatively constant rate, making it a clinically chosen measurement to indicate renal function and a forensically chosen measurement to detect dilute urine and potentially adulterated specimens (2).