METABOLITE AND MATRIX INTERFERENCE IN PHENYTOIN IMMUNOASSAYS

The major phenytoin metabolite, 5-(p-hydroxyphenyl)-5-phenylhydantoin glucuronide (HPPG), was primarily responsible for the positive bias no ted when uremic specimens were assayed with the Abbott Tdx(R) Free Phe nytoin fluorescence polarization immunoassay. The amount of bias depen ded on both HPPG and phenytoin concentration, increasing with increase s in either concentration. The new Abbott TDx II assays for phenytoin and free phenytoin exhibited no significant cross-reactivity with HPPG and no bias in clinical specimens from uremic patients. Both assays c orrelated well with Emit-based assays (r > 0.98), had CVs of < 3.5%, a nd had minimum detection limits of < 0.1 mg/L. Calibration curves were stable for at least 6 weeks. All of the TDx assays cross-reacted with another metabolite, 5-(p-hydroxyphenyl)-5-phenylhydantoin (HPPH), but expected HPPH concentrations are too low to cause a clinically signif icant bias. The Emit-based phenytoin assay exhibited a significant mat rix effect when calibrators were prepared in defibrinated plasma proce ssed to resemble serum.