Interpretive Handbook
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Test 8621:
Primidone and Phenobarbital, Serum
Clinical Information 
Discusses physiology, pathophysiology, and general clinical aspects, as they relate to a laboratory test
Primidone is used for control of grand mal seizures that are refractory to other antiepileptics and seizures of psychomotor or focal origin.
Primidone is initially dosed in progressively increasing amounts starting with 100 mg at bedtime to 250 mg t.i.d. after 10 days of therapy in adults.
Primidone exhibits a volume of distribution of 0.6 L/kg and a half-life of 8 hours.
When monitoring primidone and phenobarbital levels simultaneously, the specimen should be drawn just before the next dose is administered.
Primidone is not highly protein bound, approximately 10%. Phenobarbital is a metabolite of primidone. Like phenobarbital, there are no known major drug-drug interactions that affect the pharmacology of primidone. Toxicity associated with primidone is primarily due to the accumulation of phenobarbital. Diagnosis and treatment are as described for PBAR Phenobarbital, Serum.
Useful For Suggests clinical disorders or settings where the test may be helpful
Assessing compliance
Monitoring for appropriate therapeutic level
Assessing toxicity
Interpretation Provides information to assist in interpretation of the test results
At steady-state, which is achieved approximately 2 weeks after therapy is initiated, blood levels of primidone that correlate with optimal response to the drug range from 9.0 to 12.5 mcg/mL for adults and 7.0 to 10.0 mcg/mL for children <5 years of age.
The corresponding levels for phenobarbital are 20 to 40 mcg/mL for adults and 15 to 30 mcg/mL for children <5 years of age.
Dosage adjustment based on blood level information is the best way to obtain optimal response to the drug.
Cautions Discusses conditions that may cause diagnostic confusion, including improper specimen collection and handling, inappropriate test selection, and interfering substances
At the same time that the primidone level is monitored, one should also monitor the phenobarbital level, as phenobarbital is a metabolite of primidone.
Reference Values Describes reference intervals and additional information for interpretation of test results. May include intervals based on age and sex when appropriate. Intervals are Mayo-derived, unless otherwise designated. If an interpretive report is provided, the reference value field will state this.
PRIMIDONE
Therapeutic concentration
Children (<5 years): 7.0-10.0 mcg/mL
Adults: 9.0-12.5 mcg/mL
Toxic concentration: > or =15.0 mcg/mL
PHENOBARBITAL
Therapeutic concentration
Infants and children: 15.0-30.0 mcg/mL
Adults: 20.0-40.0 mcg/mL
Toxic concentration: > or =60.0 mcg/mL
Clinical References Provides recommendations for further in-depth reading of a clinical nature
Rall TW, Schleifer LS: Drugs effective in the therapy of the epilepsies: primidone. In Goodman and Gilman's The Pharmacological Basis of Therapeutics. 8th edition. Edited by AG Gilman, TW Rall, AS Nies, P Taylor. New York, Pergamon Press, 1990, pp 446-447
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