Primidone, Serum or Plasma

Serum or plasma

1 mL

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.

This deoxybarbiturate is closely related chemically to the barbiturates. It is converted to two active metabolites, phenobarbital and phenylethylmalonamide (PEMA). Primidone is used principally in generalized tonic-clonic and complex and simple partial seizures; some clinicians believe that the drug has specific usefulness for complex partial seizures. It is as effective as carbamazepine or phenytoin in controlling partial or generalized tonic-clonic seizures, although a greater incidence of adverse reactions, especially during initial therapy, limits patient acceptance. Primidone is commonly given with phenytoin but monotherapy is preferred. The conversion of primidone to phenobarbital is significantly increased when this drug is used with other antiepileptic medication. It is not effective in absence seizures.

Immunoassay

Therapeutic: primidone: 5.0−12.0 μg/mL, phenobarbital: 15−40 μg/mL

Plasma half-life of primidone usually ranges from six to eight hours with rapid elimination (24 to 40 hours). Since phenobarbital requires a longer interval (48 hours) to achieve therapeutic blood levels, checking both levels can be used to determine chronic compliance. The phenobarbital:primidone ratio normally is 2.5, can be higher (4.3 mean) in patients on other anticonvulsants (phenytoin, carbamazepine) and lower than normal among patients discontinued from those medicines or who are chronically noncompliant. Primidone decreases the effects of oral anticoagulants, carbamazepine, valproate, ethosuximide, felbamate, lamotrigine, topiramate, and oxcarbazepine.¹