Clinical Use: Clinicians use the test for Phenobarbitone to assess compliance for the drug and to monitor appropriate therapeutic level, and to check for toxicity.
Background: Phenobarbitone is a general central nervous system (CNS) suppressant that has proven effective in the control of generalized and partial seizures. It is frequently co – administered with phenytoin for control of complex seizure disorders and with valproic acid for complex parietal seizures.Phenobarbitone is slowly but completely absorbed, with bioavailability in the range of 100%. It is approximately 50% protein bound with a volume of distribution of 0.5 L/kg. Phenobarbital has a long half-life of 96 hours, with no known active metabolites.

Sedation is common at therapeutic concentrations for the first 2 to 3 weeks of therapy, but this side effect disappears with time.

Toxicity due to Phenobarbitone overdose is characterized by CNS sedation and reduced respiratory function. Mild symptoms characterized by ataxia, nystagmus, fatigue, or attention loss. Death usually occurs due to respiratory arrest when pulmonary support is not supplied manually.

There are no known drug interactions that significantly affect the pharmacokinetics of Phenobarbitone; conversely, Phenobarbitone affects the pharmacokinetics of other drugs significantly because it induces the synthesis of enzymes associated with the hepatic cytochrome P450 metabolic pathway.

Acute intermittent porphyria attacks may be induced by Phenobarbitone stimulation of hepatic cytochrome P450.

Reference Ranges: 0 – 170 µmol/L
Associated Diseases:
Patient Preparation: None required
Specimen Requirements: Serum (SST) sample
Turnaround Time: 1 week
Additional Information: Free Phenobarbitione is also available. Minimum sample volume is 1.0ml and price is £26.00
Referred Test: Referred test
Location: Therapeutic Drug Monitoring Unit