Validation of a Free Phenytoin Assay on C obas c501 Analyzer Using Calibrators From C obas I ntegra Free Phenytoin Assay by Comparing its Performance With Fluorescence Polarization Immunoassay for Free Phenytoin on the TD x Analyzer

For many years, fluorescence polarization immunoassay ( FPIA ) on the TD x analyzer has been used for determination of free phenytoin concentration. Recently A bbott L aboratories decided to discontinue the TD x analyzer and related assays on this analyzer. Free phenytoin assay is also available from R oche D iagnostics for application on the C obas I ntegra analyzer (fluorescence polarization assay) but not on C obas c510 analyzer. Free phenytoin calibrators from the C obas I ntegra free phenytoin assay and the reagents from the KIMS phenytoin assay were used for the determination of free phenytoin on the C obas c501 analyzer. The intra‐run and inter‐run precisions were both <7.2%. The assay was linear from 0.2 to 4 μg/ml. The free phenytoin assay on the C obas c501 was compared with the FPIA assay on the TD x analyzer using sera from 25 patients receiving phenytoin (phenytoin concentration between 0.3 and 3.7 μg/ml). The following regression equation was observed: y = 0.9899 x + 0.0408 ( r = 0.98, n = 25). In conclusion, the free phenytoin assay on the C obas c501 analyzer is a valid alternative to free phenytoin assay on the TD x analyzer. J. Clin. Lab. Anal. 27:1–4, 2013. © 2012 Wiley Periodicals, Inc.