Phenobarbital, Serum or Plasma

CPT: 80184
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  • Luminal®
  • Phenobarbital Level, Blood

Expected Turnaround Time

Within 1 day

Related Documents

Specimen Requirements


Serum or plasma


1 mL

Minimum Volume

0.3 mL


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.


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

Room temperature

Stability Requirements



Room temperature

14 days


14 days


14 days

Freeze/thaw cycles

Stable x3

Causes for Rejection

Gel-barrier tube; hemolysis; severe lipemia; icteric specimen

Test Details


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.



Reference Interval

Therapeutic: 15−40 μg/mL

Critical Value

Potentially toxic: >40 μg/mL

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.


AMA, Division of Drugs and Toxicology, Drug Evaluations Subscription, Chicago, IL: American Medical Association, Fall 1992.


Order Code Order Code Name Order Loinc Result Code Result Code Name UofM Result LOINC
007823 Phenobarbital, Serum 3948-7 007828 Phenobarbital, Serum ug/mL 3948-7

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