Metabolite and matrix interference in phenytoin immunoassays

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.