Validity of unbound digoxin measurements by immunoassays in presence of antidote (Digibind (R))

Measurement of unbound digoxin in presence of Fab fragments may be useful i n management of overdoses. The analysis can be performed on serum directly or on ultrafiltrate of serum. The architecture of the immunoassay may influ ence the validity of results obtained using these two approaches. We tested this hypothesis by preparing serum mixtures containing various concentrati ons of digoxin and Digibind(R) and analyzed them by the immunoassays before and after ultrafiltration. Four samples collected from Digibind(R)-treated patients were also analyzed before and after ultrafiltration. The slopes a nd the y-intercepts of the measured versus the expected values for serum an d its ultrafiltrate overlapped for the MEIA digoxin assay. I;or other three immunoassays tested (ACS:180, Stratus, and On-Line), either the slope or t he intercept for measured versus the expected results for serum were signif icantly different (P<0.05) than those for ultrafiltrate. Following addition of digoxin and Digibind(R), differences in results for serum analyzed dire ctly or after ultrafiltration were <0.50 ng/ml. Comparable samples from dig oxin-overdosed patients treated with Digibind(R) had differences of >1.0 ng /ml. Previous claims reporting direct analysis of digoxin in presence of an tidote but not having used patient samples for validation should be revisit ed. To date, analysis of serum ultrafiltrate by an immunoassay proven not t o have matrix bias remains the most accurate approach in measuring unbound digoxin in presence of antidote. (C) 1999 Elsevier Science B.V. All rights reserved.