To the Editor.—The recent article on Variability of Serum Phenytoin Concentrations in Nursing Home Patients by Mooradian et al1 adds some insight into the unpredictable nature of phenytoin kinetics in otherwise stable patients. Their finding of marked variability in phenytoin levels of at least 50% in all patients reaffirms the difficulties involved in clinically interpreting serum phenytoin levels.2 It is disturbing, however, that 15 patients had phenytoin levels drawn at least monthly with no apparent clinical indication for this frequency of monitoring.
Over the duration of this study, there were more than 170 levels drawn, with at least 10% being outside the therapeutic range, and no correlation with either therapeutic failure (breakthrough seizures) or toxicity (ataxia, falls, lethargy, confusion). Twelve of 15 patients had at least one subtherapeutic level with no dosage changes and apparently no seizures resulted. Seven of 15 patients had at least one value