Improvements to the SYVA fluorescence energy transfer immunoassay for digoxin

During an extensive evaluation of the SYVA Advance Digoxin assay in this laboratory, it was found that there was a positive bias with respect to the in-house RIA method currently being used, which is accentuated with serum containing hemoglobin at concentrations above 1.0 g/L and heparinised or EDTA plasma. This occurred even though the SYVA method uses a pre-treatment (oxidizing) reagent for the purpose of minimising this interference. The positive bias was such that the method was considered unsuitable for routine use. A modification to the pre-treatment reagent, consisting of a threefold increase in concentration and the addition of 1 mol/L urea, is proposed which enables either plasma or serum to be used, reduces the bias and eliminates the interference caused by hemolysis. An evaluation was performed using patients' specimens obtained from routine submissions to this laboratory for digoxin analysis and, therefore, reflects the performance of the improved assay in routine use.