Analytical performance of a new chemiluminescent phenytoin (ACS:180) assay

The authors, as a beta testing site, evaluated the ACS:180-phenytoin chemiluminescent assay (Ciba-Corning Diagnostics Corp., Medfield, MA, U.S.A.) by comparing its performance with a widely used fluorescence assay for phenytoin (Abbott Laboratories, Abbott Park, IL, U.S.A.). The ACS:180-phenytoin assays were run on a ACS-180 analyzer and fluorescence polarization assays on a TDx analyzer. The within-run precision for ACS-phenytoin assay was determined using controls obtained from Ciba-Corning. The CVs were 2.9% for low control (mean = 5.5, SD = 0.16 microgram/ml, n = 10), 2.8% for the medium control (mean = 13.4, SD = 0.37 microgram/ml, n = 10), and 2.7% for the high control (mean = 24.6, SD = 0.66 microgram/ml, n = 10). The corresponding between run precisions were 4.1% for the low control (mean = 5.4, SD = 0.22 mg/ml, n = 10), 3.1% for the medium control (mean = 13.8, SD = 0.43 mg/ml, n = 10), and 2.9% (mean = 24.5, SD = 0.70 mg/ml, n = 10) for the high control. The assay was linear from 0.5 to 40 micrograms/ml of serum phenytoin concentrations with a detection limit of 0.24 microgram/ml. The recoveries were 93-97% for concentrations of phenytoin of 5-30 micrograms/ml. They also compared 111 serum specimens collected from patients receiving phenytoin. The concentrations of phenytoin ranged from none detected to 32.4 micrograms/ml. Using fluorescence polarization assay as x-axis (reference method) and ACS:180-phenytoin assay as y-axis, they obtained the following regression line: y = 1.0x - 0.26, r = 0.993. They conclude that the ACS-phenytoin assay has a good precision and that the results correlate well with the fluorescence polarization assay.