Phenytoin and plasmapheresis: Importance of sampling times and impact of obesity

Keith M. Olsen, Michael A. Marx, Michael S. Monaghan, Edilissa Barnes, Bruce H. Ackerman, Alex A. Pappas

Research output: Contribution to journalArticlepeer-review

9 Scopus citations


The effect of plasmapheresis (PP) on total and free phenytoin clearance is reported. An obese patient with the diagnosis of thrombotic thrombocytopenic purpura (TTP) was treated with PP. Twelve episodes of PP, having exchange volumes of 1.5–2.25 times the plasma volume with a mean ± 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 phenytoin concentrations from pre- to end-PP of 7.32 ± 2.5 mg/L compared to 1.98 ± 0.7 mg/L observed pre-PP to 1 h post-PP. These values corresponded to 48.4 ± 11.6 and 15.0 ± 6.7% decreases in phenytoin concentrations at the two aforementioned time periods. To prevent misinterpretation of plasma phenytoin concentrations, samples should not be obtained for at least 2 h after PP.

Original languageEnglish (US)
Pages (from-to)624-628
Number of pages5
JournalTherapeutic Drug Monitoring
Issue number6
StatePublished - Dec 1994


  • Free phenytoin
  • Phenytoin
  • Plasmapheresis

ASJC Scopus subject areas

  • Pharmacology
  • Pharmacology (medical)


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