PHENYTOIN AND PLASMAPHERESIS - IMPORTANCE OF SAMPLING TIMES AND IMPACT OF OBESITY

The effect of plasmapheresis (PP) on total and free phenytoin clearanc e is reported. An obese patient with the diagnosis of thrombotic throm bocytopenic purpura (TTP) was treated with PP. Twelve episodes of PP, having exchange volumes of 1.5-2.25 times the plasma volume with a mea n +/- SD 7.7 +/- 0.8 L of plasma removed, were studied. A significant (p < 0.05) difference was observed with a mean change in plasma phenyt oin concentrations from pre- to end-PP of 7.32 +/- 2.5 mg/L compared t o 1.98 +/- 0.7 mg/L observed pre-PP to 1 h post-PP. These values corre sponded to 48.4 +/- 11.6 and 15.0 +/- 6.7% decreases in phenytoin conc entrations at the two aforementioned time periods. To prevent misinter pretation of plasma phenytoin concentrations, samples should not be ob tained for at least 2 h after PP.