In suspected primidone overdose, the serum concentration of which compound is normal, prompting testing of an additional metabolite?

In suspected primidone overdose, the serum concentration of which compound is normal, prompting testing of an additional metabolite?

• A. Test for phenobarbital concentration
• B. Test for phenytoin concentration
• C. Test for procainamide concentration
• D. Eliminate primidone overdose as a possible diagnosis

Answer: (A)

Primidone is a prodrug that is metabolized in the body to phenobarbital and to another active metabolite called phenylethylmalonamide (PEMA). In suspected primidone overdose, measuring the serum phenobarbital concentration is a standard first step because phenobarbital is a major product of primidone metabolism and its level helps screen for primidone exposure. However, phenobarbital levels can be normal in some primidone overdoses due to timing of sampling or individual variability, so if the phenobarbital result doesn’t explain the clinical picture, you would pursue testing for the other metabolite (PEMA) to confirm primidone involvement. Phenytoin and procainamide are unrelated to primidone metabolism, so their concentrations are not helpful for diagnosing primidone overdose, and you wouldn’t prematurely exclude primidone overdose based on a single normal result.

Primidone is a prodrug that is metabolized in the body to phenobarbital and to another active metabolite called phenylethylmalonamide (PEMA). In suspected primidone overdose, measuring the serum phenobarbital concentration is a standard first step because phenobarbital is a major product of primidone metabolism and its level helps screen for primidone exposure. However, phenobarbital levels can be normal in some primidone overdoses due to timing of sampling or individual variability, so if the phenobarbital result doesn’t explain the clinical picture, you would pursue testing for the other metabolite (PEMA) to confirm primidone involvement. Phenytoin and procainamide are unrelated to primidone metabolism, so their concentrations are not helpful for diagnosing primidone overdose, and you wouldn’t prematurely exclude primidone overdose based on a single normal result.