Phenytoin immunoassay measurements in serum samples from patients with renal insufficiency: Comparison with high-performance liquid chromatography

The debate continues regarding the possible interference of phenytoin metabolites in phenytoin immunoassays, and its clinical importance for patients with renal failure. The aim of this study was to compare the results obtained using the Abbott fluorescence polarization immunoassay (FPIA), Dade enzyme-multiplied immunoassay technique (EMIT), and high-performance liquid chromatography (HPLC) to establish the significance of the differences in conditions of renal failure. Thirty-six adult patients who had been treated with phenytoin and whose renal function ranged from normal to severely impaired were chosen for this study. In accordance with previously established validation criteria for analytical methods for the determination of drugs, a 15% bias from the HPLC phenytoin values was considered an acceptable limit. The mean (+/- SEM) glomerular filtration rate (GFR) of the patients was 37.5 +/- 4.6 mL/min (range=10-102 mL/min). The mean values found using FPIA (10.8 +/- 1.2 mu g/mL) and EMIT (10.8 +/- 1.3 mu g/mL) presented acceptable deviations with respect to HPLC (10.5 +/- 1.2 mu g/mL), and a high correlation was found among the results (N = 36) of the different methods (r >= 0.987, P < 0.001). An FPIA deviation above the 15% bias limit with respect to HPLC was found only in two cases with very low serum phenytoin concentrations and low GFR values (< 20 mL/ min), although it does not appear to be important in terms of adjusting drug dosage. According to our data, FPIA and EMIT gave accurate results for total phenytoin in serum samples from patients with renal failure.