Analytical performance of a monoclonal digoxin assay by dry chemistry on the Vitros 950

Digoxin assay in plasma/serum samples is used for therapeutic measurements as a guide to clinical management of cardiac patients. A thin dry film multilayer monoclonal immunoassay for digoxin, Vitros, was evaluated for analytical performance. The effect of digoxin-like immunoreactive substances (DLIS) was studied assaying plasma samples taken from 100 renal disease patients, 62 hepatic disease patients and 40 pregnant women not receiving digoxin. The Vitros digoxin assay was compared with the AxSym digoxin II assay using plasma samples from 180 patients treated with digoxin. The results revealed satisfactory precision and accuracy for therapeutic drug monitoring purposes: the coefficient of variation (CV%) was lower than 5%; results for dilutions were linear in the range 0.4-3.9 microlg/L and mean analytical recovery was 105%. Measurable DLIS concentrations were observed in 29% of hepatic disease patients and in 7% of renal disease patients with apparent digoxin concentration ranging from 0.4 to 0.75 microg/L. The incidence of DLIS was comparable to that observed with AxSym digoxin II. Comparative results from patient samples gave a regression line equation: Yvitros 950=0.96XAxym +0.14, r=0.89. The data revealed a mean difference of 0.09+/-0.26 microg/L significantly greater than zero (p=0.02). We concluded that Vitros digoxin assay for precision, accuracy and extent of DLIS interference may be a good method for therapeutic drug monitoring; care needs to be taken since assay results generated by Vitros and AxSym analysers are not necessarily interchangeable.