Effective Oct 1, 2022, we will discontinue Primidone Testing. Any Primidone order will be substituted with a phenobarbital measurement and reported with the following comment: “Phenobarbital is the major active metabolite of Primidone.”
Primidone tends to be unbound to serum proteins and demonstrates significant inter-dosage variability in measured concentration.1 A product of the hepatic metabolism of primidone is phenobarbital, which is believed to be the predominant active metabolite.2 Phenobarbital tends to be higher than primidone when the concentration is at a steady state and demonstrates lower inter-dosage variability in measured concentration.1 This makes it a better target for therapeutic drug monitoring.
For any clinical or technical questions regarding this change please contact the following LifeLabs biochemists:
Uvaraj Uddayasankar, PhD FCACB
T 416-675-4530 Ext. 42211
Kika Veljkovic (she/her) PhD FCACB
Discipline Head, High Volume Chemistry
T 416-675-4530 Ext. 42832
Dr. Catherine Ross, MD, MSc, FRCPC
Medical Director, ON
T 647-943-2413 Ext: 42980
- Eadie, M. J. (1991). Formation of active metabolites of anticonvulsant drugs. Clinical pharmacokinetics, 21(1), 27-41.
- Johannessen, S. I., & Johannessen Landmark, C. (2020). Anti-Convulsant Agents: Phenobarbital and Primidone. NeuroPsychopharmacotherapy, 1-10.