Phenobarbital, Serum or Plasma

Phenobarbital, Serum or Plasma

CPT Test code: 80184

Specimen: Serum or plasma
Volume: 1 mL
Minimum Volume: 0.3 mL
Container: Red-top tube or green-top (heparin) tube. Do not use a gel-barrier tube. The use of gel-barrier tubes is not recommended due to slow absorption of the drug by the gel. Depending on the specimen volume and storage time, the decrease in drug level due to absorption may be clinically significant.
Collection: Transfer separated serum or plasma to a plastic transport tube. Peak: 4 to 12 hours after dose; trough: immediately prior to next dose.
Storage Instructions: Refrigerate
Causes for Rejection: Gel-barrier tube; hemolysis; severe lipemia; icteric specimen
Reference Interval: Therapeutic: 15-40 μg/mL
Critical Values: Potentially toxic: >40 μg/mL
Use: Phenobarbital, a long-acting barbiturate, is effective in generalized tonic-clonic and simple partial seizures. Higher plasma concentrations may be required to control the latter. Complex partial seizures do not respond as well, and absence seizures are not relieved and may be exacerbated. Phenobarbital frequently is used in the treatment of neonatal seizures and may be the initial drug employed in young children; however, because of increasing concern about adverse neuropsychological reactions to sedative/hypnotic antiepileptic drugs, many neurologists prefer less sedating drugs, such as carbamazepine, phenytoin, or valproate. The prophylactic use of phenobarbital in infants with febrile seizures has been challenged. Phenobarbital also is useful in seizures caused by barbiturate withdrawal in dependent individuals. The sodium salt is administered parenterally as part of the treatment regimen for status epilepticus.
Additional Information: Phenobarbital has a half-life of 84 to 108 hours. Phenobarbital can affect the metabolism of phenytoin, ethosuximide, and increase the clearance and elimination of chloramphenicol, theophylline, oral anticoagulants (warfarin), cyclosporine, and oral contraceptives; where appropriate, the use of these drugs in patients on phenobarbital should be monitored clinically and through the laboratory.
References: AMA, Division of Drugs and Toxicology, Drug Evaluations Subscription, Chicago, IL: American Medical Association, Fall 1992.