title: Effect of ultrafiltrate volume on determination of free phenytoin concentration.
abstract: Monitoring free phenytoin concentration is clinically useful for patients with uremia, hepatic disease, hypoalbuminemia, and related conditions. Free phenytoin is commonly measured by immunoassay in the protein-free ultrafiltrate prepared by centrifuging serum for 20-30 minutes, using an appropriate ultrafiltration device. We studied the effect of centrifugation time (15-40 minutes) and protein concentrations on ultrafiltration volume, and the related effects on measured free phenytoin concentrations. Temperature was ambient for all studies. The ultrafiltration volumes were directly proportional to centrifugation time and were inversely proportional to the protein concentrations. Although ultrafiltration volume significantly increased with longer centrifugation time, the measured free phenytoin concentrations did not increase proportionately. The concentration of phenytoin in the residual serum retained in the ultrafiltration device did not change proportionally either. Therefore, equilibrium of phenytoin concentrations between the ultrafiltrate and retentate was maintained, regardless of centrifugation time or protein concentration.
authors.affiliation: ARUP Institute for Clinical and Experimental Pathology, University of Utah School of Medicine, Salt Lake City, Utah, USA.
authors.forename: Gwen A
authors.initials: GA
authors.lastname: McMillin
authors.forename: Joetta
authors.initials: J
authors.lastname: Juenke
authors.forename: Amitava
authors.initials: A
authors.lastname: Dasgupta
mesh_terms: D001798:Blood Proteins / Q000032:analysis; D006801:Humans; D010672:Phenytoin / Q000097:blood*; D012709:Serum Albumin / Q000032:analysis; D014462:Ultrafiltration
chemical_list: D001798:Blood Proteins; D012709:Serum Albumin; D010672:Phenytoin