Cross-reactivity of fosphenytoin in two human plasma phenytoin immunoassays

• Published in: Clinical chemistry (Baltimore, Md.) Vol. 44; no. 7; pp. 1474 - 1480
• Main Authors: Kugler, Alan R, Annesley, Thomas M, Nordblom, Gerald D, Koup, Jeffrey R, Olson, Stephen C
• Format: Journal Article
• Language: English
• Published: Washington, DC Am Assoc Clin Chem 01.07.1998; American Association for Clinical Chemistry
• Subjects: Analysis; Anticonvulsants - administration & dosage; Anticonvulsants - blood; Biological and medical sciences; Chromatography, High Pressure Liquid; Cross Reactions; Fluorescence Polarization Immunoassay; General pharmacology; Humans; Immunoenzyme Techniques; Injections, Intraperitoneal; Injections, Intravenous; Medical sciences; Models, Biological; Pharmacology. Drug treatments; Phenytoin - administration & dosage; Phenytoin - analogs & derivatives; Phenytoin - blood; Predictive Value of Tests; Reproducibility of Results; Human; Biological fluid; Fosphenytoin; Intravenous administration; Temporal study; Fluorescence polarization immunoassay; Anticonvulsant; Hydantoins; Intramuscular administration; Quantitative analysis; Immunological method; Blood plasma
• ISSN: 0009-9147; 1530-8561
• DOI: 10.1093/clinchem/44.7.1474

The cross-reactivity of fosphenytoin, a phosphate ester prodrug of phenytoin, was investigated in the Abbott phenytoin TDx/TDxFLx fluorescence polarization immunoassay (TDx) and the Behring Diagnostics phenytoin Emit 2000 enzyme-multiplied immunoassay (Emit). The first part of our study investigating cross-reactivity utilized in vitro correlation of the two immunoassays with a validated and specific phenytoin HPLC method used to assay plasma samples prepared in several phenytoin and fosphenytoin concentration combinations. Fosphenytoin cross-reacted with both immunoassays, but to a greater extent with TDx. In the second part of the study, empirically-derived models that best explained the in vitro data were used to predict "immunoassay-derived" phenytoin concentrations in plasma samples collected from actual patients after intravenous (i.v.) or intramuscular (i.m.) fosphenytoin dosing. The greatest degree of phenytoin concentration overestimation occurred at times when fosphenytoin concentrations were highest: within 1 to 2 h after i.v. infusion or during the first 2 to 4 h after i.m. injection. It is recommended that phenytoin concentrations not be monitored using these or other potentially nonspecific immunoanalytical methods for at least 2 h after i.v. fosphenytoin infusion or 4 h after i.m. fosphenytoin injection.