Variability of serum phenytoin concentrations in nursing home patients

A marked variability in serum phenytoin concentrations was observed in an elderly nursing home resident. To determine the prevalence of this problem, 15 frail nursing home patients who were receiving phenytoin therapy were studied over a mean follow-up period of 10.6 +/- 0.89 months. The mean number of serum phenytoin level measurements during this period was 13.1 +/- 1.5. For a given individual, there was no variation in phenytoin dose or preparation administered. All the patients had a difference of more than 50% between the highest and the lowest serum phenytoin levels, and in five patients (33%), the difference exceeded 150%. The change in serum phenytoin level was temporally related to influenza vaccination in only three patients. The form of phenytoin was not a significant determinant of the variability in this patient population, nor did enteral feeding have any effect. It is recommended that nursing home patients receiving phenytoin therapy have periodic serum phenytoin measurements obtained, even in the absence of seizures or classic signs of phenytoin toxicity.