Variability of total phenytoin serum concentrations within elderly nursing home residents

Background: Approximately 6% of all elderly nursing home residents receive phenytoin. Phenytoin concentrations are often measured to guide therapy. Objective: To evaluate the intraresident variability among multiple measurements of total phenytoin serum concentrations in nursing home residents. Methods: This was an observational study of 56 elderly (≥65 years) nursing home residents from 32 nursing homes who had at least 3 phenytoin concentrations measured while on the same dose of phenytoin for at least 4 weeks and who were not taking any interfering concomitant medications. These were a subset of 387 elderly nursing home residents from 112 nursing homes across the United States who had total phenytoin concentration measurements between June 1998 and December 2000. Results: The mean age was 80.1 years (range, 65 to 100 years) and 58.9% were women. The mean daily dose of phenytoin per resident was 4.9 ± 1.5 mg/kg. Total phenytoin concentrations within an elderly nursing home resident varied as much as two- to threefold, even though there was no change in dose. The person with the smallest variability had a minimum concentration of 10.0 μg/mL and a maximum of 10.4 μg/mL. The person with the largest variability had a minimum concentration of 9.7 μg/mL and a maximum of 28.8 μg/mL. Conclusions: There is considerable variability in the total phenytoin concentrations in the elderly nursing home resident and measurement of a single total phenytoin concentration should not be used to guide treatment.