Abbott ARCHITECT iPhenytoin assay versus similar assays for measuring free phenytoin concentrations.

OBJECTIVE To measure free phenytoin (FP) concentrations in filtered specimens using the Abbott ARCHITECT iPhenytoin assay and to compare results from this method with results from the Abbott TDx/FLx assays. METHODS We verified accuracy, analytic measurement range, and precision for FP measurements. For correlation and therapeutic interval studies, we used filtered calibrators, controls, proficiency-testing materials, and surplus clinical samples. After implementation, we determined proficiency testing results. RESULTS The analytic measurement range was 2.0 to 25.0 micromol/L. Quality control materials (6.1, 12.6, and 20.1 micromol/L) provided mean (SD) recoveries of 96.1 (5.0%), 99.2 (5.0%), and 99.3 (5.7%), respectively, and coefficients of variation of 5.2%, 5.0%, and 5.8%, respectively. Clinical specimens produced mean (SD) FP recovery levels of 103.7 (10.6%) (bias, 0.1 [0.3] micromol/L). Altering the FP therapeutic range (4.0-8.0 micromol/L) was unnecessary. Proficiency testing yielded consistently acceptable results. CONCLUSION Our accuracy, precision, and correlation results were similar for the TDx/FLx and ARCHITECT assays, which demonstrates that the ARCHITECT iPhenytoin assay is acceptable for clinical FP measurements.