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:

Thank you,

Dr. Catherine Ross, MD, MSc, FRCPC

Medical Director, ON

T   647-943-2413 Ext: 42980

E catherine.ross@lifelabs.com

References:

• 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.