The diagnosis of substance use disorder, like most other medical diagnoses, is primarily clinical, with the patient's history and the mental status examination playing central roles in the diagnostic process. Nevertheless, laboratory testing to identify recent drug use is increasingly important in clinical settings, ranging from the initial diagnosis to treatment management and from research and epidemiology to health care assessment. Drug testing identifies the recent use of specific abused substances and in some settings can help to differentiate chronic or repetitive ingestions from single or low-frequency uses. Laboratory testing is especially helpful in medical settings as part of the screening process to identify patients for evaluation for substance use disorder.