Frequency of S-mephenytoin hydroxylation deficiency in 373 Spanish subjects compared to other Caucasian populations.

We have investigated the prevalence of poor metabolisers (PM) of S-mephenytoin in 373 unrelated, healthy Spanish Caucasian subjects, based on the enantiomeric S/R mephenytoin ratio in urine collected 0-8 h and 24-32 h after intake of the racemic drug. Five of the subjects were PM (1.34%, 95% confidence interval 0.18-2.59%), a prevalence lower than in 6 other Caucasian populations, but only significantly lower than in studies in France and Switzerland (P < 0.01). We suggest that this difference might be due to the use of different phenotyping procedures.