Limitation to the use of the urinary S‐/R‐mephenytoin ratio in pharmacogenetic studies.

The reproducibility of the S-/R-mephenytoin ratio was examined in urines stored at -20 degrees C over 2 years. Large changes in this ratio were observed in some urine samples stored for only a few months under these conditions. The changes observed in the S-/R-mephenytoin ratio are attributed to the decomposition of an acid labile metabolite of S-mephenytoin which is eliminated in the urine. The instability of this metabolite makes it desirable to process urine shortly after its collection in order to avoid inaccurate phenotype assignments based upon the urinary S-/R-mephenytoin ratio. If rapid processing of urines is impractical, additional methods are described for preventing improper phenotype assignment of subjects.