Bidirectional (positive/negative) interference in a digoxin immunoassay: importance of antibody specificity

The importance of high specificity in immunoassays used in therapeutic monitoring is highlighted by a case study in which therapeutic-to-toxic borderline digoxin levels were measured by a digoxin immunoassay in the serum sample from a patient administered digitoxin rather than digoxin. The sample, mistakenly sent to the laboratory for digoxin analysis, gave discordant results in three digoxin immunoassays: 1.99 and 0.79 ng/ml in assays using polyclonal antibodies (fluorescence-polarization immunoassay and microparticle enzyme immunoassay, respectively), and <0.1 ng/ml in a chemiluminescent immunoassay using more specific monoclonal antibody. The presence of digitoxin (approximately 40 ng/ml) in the sample was confirmed by three different digitoxin immunoassays. Based on these results, the interference of different levels of digitoxin was studied in the presence of 0, 0.85, 1.9, and 4.7 ng/ml digoxin in all three digoxin assays. The chemiluminescent assay showed no significant interference. The fluorescence-polarization immunoassay showed positive interference in all cases; however, the microparticle enzyme immunoassay showed a bidirectional interference: a positive interference observed at digoxin level <1.8 ng/ml, changing to a negative interference at higher digoxin concentrations. The authors conclude that in countries such as Germany, where both digoxin and digitoxin may be prescribed, caution should be used to interpret digoxin immunoassay results. Digoxin assays, with cross-reactivity to digitoxin <0.1% should be used.