The creatinine test is a validity test that is done systematically to screen for attempts to cheat on urine drug tests. Creatinine is a waste product from the muscles and its concentration in a urine sample largely reflects the amount of water that was ingested by an individual during the few hours prior to specimen collection. The more water you drink, the lower the level of creatinine (and drugs being tested).
Drug regulations stipulate that any specimen containing less than 2 mmol/L of creatinine shall be considered abnormally diluted, and negative test results should be issued with reservations. The same regulation states that any specimen containing less than 0.2 mmol/L is inconsistent with human urine, and the specimen should be considered not to be human urine (substituted specimen). Depending on the circumstances, when done on a diluted specimen, the test should be repeated on a new sample with voluntarily restricted water consumption or on a saliva sample. A substituted specimen is considered proof of an attempt to cheat and, therefore, equivalent to a positive specimen.