Background: Phenytoin is a strongly protein-bound anticonvulsant requiring free drug monitoring for certain patients. Recently, free phenytoin assay on the TDx analyzer was discontinued by the Abbott Laboratories, but Roche Diagnostics has an assay for the determination of free phenytoin.
Methods: Because a clinical laboratory may not have an instrument for using Roche free phenytoin assay, we studied the possibility of determining free phenytoin using particle-enhanced turbidimetric inhibition immunoassay (PETINIA) total phenytoin assay on the Vista 1500 analyzer by comparing free phenytoin values obtained by Roche assay and Hitachi 917 analyzer with values obtained by using PETINIA assay.
Results: When protein-free ultrafiltrates were prepared at room temperature from 41 different patients taking phenytoin, using Roche assay (x axis) and PETINIA (y axis) assay, the following regression equation was observed: y = 0.8526x + 0.0661 (r = 0.85, n = 41). The regression equation indicated significant negative bias with the reference method. However, when additional 30 specimens were studied where ultrafiltration was performed at 37°C, lesser negative bias was observed as revealed by the regression equation: y = 0.922x - 0.117 (r = 0.90, n = 30).
Conclusions: We conclude that PETINIA assay on the Vista 1500 analyzer is not suitable for monitoring free phenytoin concentration if ultrafiltration is performed at room temperature but may be used with caution if ultrafiltration is performed at physiological temperature of 37°C, which is the desirable temperature for preparation of ultrafiltrate.