This test measures the amount of barbiturates in blood. It is used to help monitor barbiturate blood levels during treatment or if an overdose is suspected. Barbiturates are central nervous system depressant. Any of a group of drugs that act as depressants of the central nervous system, are highly addictive, and are used primarily as sedatives and anticonvulsants. Phenobarbital and pentobarbital are examples of barbiturates.