Table 1: Precision of the determination of 4′-OH-mephenytoin (mean ± SD, ).

Joined concentration (ng/mL) Intraday (ng/mL) RSD (%) Interday (ng/mL) RSD (%) 230 3.75% 4.41% 2300 2.57% 3.40% 5800 2.09% 2.45%