Evaluation of the Coat-A-Count 125I fentanyl RIA: comparison of 125I RIA and GC/MS-SIM for quantification of fentanyl...

The Coat-A-Count solid phase 125I Fentanyl Radioimmunoassay was evaluated with respect to linearity and precision using equine urine fortified with fentanyl and then compared with a gas chromatographic/mass spectrometric method for quantification of fentanyl in urine. The RIA assay was found to be linear over the urine fentanyl concentration range of 0.25 to 7.5 ng/mL and precise with coefficients of variation (CV) ranging from 9.6 to 19.3%. The RIA calibrators, ranging in fentanyl concentrations from 0.25 to 7.5 ng/mL, and controls, at mean fentanyl concentrations of 0.46 and 1.32 ng/mL, were compared by both the RIA and GC/MS methods. The cross-reactivity with the 125I RIA test was determined for the fentanyl metabolites, norfentanyl and hydroxyfentanyl, and found to be 5% and 35%, respectively. The illicit fentanyl analogs were found to show significant cross-reactivity, ranging from 20 to 100%. The 125I RIA was compared to GC/MS quantifications of fentanyl in 35 positive and 20 negative case urine specimens.