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Metabolite and matrix interference in phenytoin immunoassays

P M Rainey1, K E Rogers, W L Roberts

  • 1Department of Laboratory Medicine, Yale University School of Medicine, New Haven, CT 06504, USA. petrie.rainey@yale.edu

Clinical Chemistry
|October 1, 1996
PubMed

Insights

The Abbott TDx II assay accurately measures free phenytoin in uremic patients, overcoming bias caused by metabolites. This new assay shows no significant cross-reactivity, ensuring reliable phenytoin level monitoring.

Area of Science:

  • Clinical Chemistry
  • Immunoassay Development
  • Pharmacokinetics

Background:

  • The Abbott TDx Free Phenytoin immunoassay showed a positive bias in uremic specimens.
  • This bias was attributed to the major phenytoin metabolite, 5-(p-hydroxyphenyl)-5-phenylhydantoin glucuronide (HPPG).
  • Bias levels correlated with both HPPG and phenytoin concentrations.

Purpose of the Study:

  • To evaluate the performance of new Abbott TDx II assays for phenytoin and free phenytoin.
  • To determine if the new assays eliminate the bias observed with previous assays in uremic samples.

Main Methods:

  • Assessed cross-reactivity of TDx II assays with HPPG and HPPH.
  • Tested TDx II assays on clinical specimens from uremic patients.
  • Correlated TDx II assay results with Emit-based assays.
  • Determined assay precision (CVs) and detection limits.
  • Evaluated calibration curve stability.

Main Results:

  • The new Abbott TDx II phenytoin and free phenytoin assays showed no significant bias in uremic patient specimens.
  • No significant cross-reactivity with HPPG was observed in the TDx II assays.
  • Assays demonstrated excellent correlation with Emit-based methods (r >0.98).
  • Assays exhibited low coefficients of variation (<3.5%) and sensitive detection limits (<0.1 mg/L).
  • Calibration curves remained stable for at least 6 weeks.

Conclusions:

  • The Abbott TDx II assays for phenytoin and free phenytoin effectively address the bias caused by HPPG in uremic samples.
  • These improved assays provide accurate and reliable measurement of phenytoin concentrations in diverse patient populations.
  • The TDx II assays offer enhanced stability and performance characteristics for therapeutic drug monitoring.

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