Detection of Barbiturates in Urine

Barbiturates have been known since 1864 when Dr. A. von Bayer synthesized barbituric acid. In 1903, barbital was introduced as a hypnotic for routine medicinal use. They were reclassified as Schedule 2 drugs in 1979 requiring a triplicate prescription to reduce the abuse of barbiturates. The benzodiazepines (Valium, Librium, Xanax) are generally safer in overdose and have largely replaced the use of barbiturates in medicinal pharmacology. Barbiturates are numerous, chemically derived from a common 2,4,6 trioxohexahydropyrimidine (barbituric acid) nucleus. Some of the common names are: phenobarbital (Luminol®), secobarbital (Seconal®), pentobarbital (Nembutal®), butalbital (Fiorinal®), amobarbital (Amytal®), and many others.

Pharmacological Effects

Barbiturates reversibly depress the activity of all excitable tissues. The use of phenobarbital in epilepsy, one of the few major remaining useful areas of barbiturate pharmacology, is due to its selective anticonvulsant activity depressing low frequency electrical activity in the cortex. Tolerance to barbiturates occurs with continued use. At first, a generalized sedative effect gives way to tolerance, especially toward effects on mood, sedation, and hypnosis. Like other central nervous system depressant drugs, barbiturates are abused and some individuals develop a dependence on them. Dependence upon and tolerance to barbiturates are closely related. The former, generating the drug seeking behavior that leads to increased usage and consequent higher levels of tolerance and hence the extent, duration, and continuity of abuse prior to withdrawal.

Laboratory Methods

Some labratories utilize immunoassay (EIA) for detecting barbiturates in urine. The immunoassays provide a cost effective, sensitive method for detection and reacts with a number of barbiturates. Gas chromatography/mass spectrometry (GC/MS) is used to further identify and confirm the presence of a particular barbiturate in the sample.

Cutoff and Detection Post Dose

The 200 ng/ml cutoff for the screening immunoassay allows detection of barbiturate use for up to 72 hours post dose. The cutoff level for GC/MS is 200 ng/ml for all barbiturate metabolites (amobarbital, butabarbital, butalbital, pentobarbital, phenobarbital, secobarbital). Various barbiturates have differing half lives of clearance post dose. For instance, the short acting barbiturate, secobarbital, has a half life of 29-34 hours, while phenobarbital, a long acting barbiturate, has a half life of 24-140 hours. Thus, use of phenobarbital may be detected much longer than secobarbital.

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Written by Toxicology Laboratories

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