Advantage of Body Surface Area on Dose Adjustment of Antiepileptic Drugs

The relationships of the serum concentration with daily dose/body weight (Dw) or daily dose/body surface area (Ds) of sodium valproate, phenobarbital and carbamazepine were investigated in epileptic children. The serum concentrations of these drugs significantly correlated with both Dw or Ds. The correlation coefficients (r) of the serum concentrations of sodium valproate (VPA), phenobarbital (PB), and carbamazepine (CBZ) to daily dose were 0.472, 0.638, and 0.812, respectively, using the body weight as a normalizing factor. While those were 0.602, 0.731, and 0.870, respectively, using the body surface area as a normalizing factor. In all drugs, r was higher with using Ds, rather than Dw. These results indicate that it would be better to use Ds than Dw in adjustments of the dosages of these drugs. We also examined the influence of patient age on the ratio of the serum level to the daily dose (L/Dw or L/Ds) for each drug. We found no significant correlation between age and the L/Ds ratio for all antiepileptic drugs studied. However, significant positive correlations were observed between age and the L/Dw ratio. The serum concentrations of the drugs studied well correlated with Ds, and the L/Ds ratio showed no significant difference among all age groups. These results indicate that dosage regimens based on a body surface area more reliable than those on a body weight basis.