Variability of phenytoin, carbamazepine and valproate concentrations in a clinic population

The Comprehensive Epilepsy Program of Minnesota database was searched for patients who had no dose change of carbamazepine (CBZ), phenytoin (PHT) or valproate (VPA) for 3 consecutive visits. Data from 860 patients were available. The coefficient of variation (CV) was calculated from 3 or more visits without dose change. For CBZ, 290 CV values from 206 individuals were available; the mode CV was 23.3%. For PHT 293 CVs were collected from 192 individuals; the mode CV was 25.2%. For VPA 244 CVs was 181 persons; the mode CV was 27.1%. If one uses the standard of a CV of less than 20% representing compliance with PHT, and a CV of less than 25% for CBZ and VPA, it appears that even in a specialty clinic, compliance with medication is a significant problem.