Drug Test, General Toxicology, Blood - Toxicology screens are used to identify substances affecting a patient and to guide the clinician to predict future toxic effects, to confirm the differential diagnosis or to guide therapy. Accurate diagnosis of clinical intoxication secondary to illicit drug use based on clinical history and physical examination may be difficult without laboratory confirmation. This testing is also necessary when multiple drug ingestion is involved, as the effects of one drug may mask the clinical signs and symptoms of the effects of other drugs.