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Estimation of Serum Digoxin by Radioimmunoassay

Requests for digoxin estimation have become frequent since the introduction of radioimmunoassay kits for its measurement. Results are often required promptly, and consequently assays are set up for small numbers of samples. The Lanoxitest Gamma Kit (Wellcome Reagents Ltd.), which utilizes a "binding reagent" and a 1251 iodinated digoxin derivative, contains reagents sufficient for 60 tubes. During the past 36 months this laboratory has found the kit to give inconsistent intra-assay precision (usually better with freshly reconstituted reagents). It has been necessary to set up standards and samples in triplicate, and even then assays sometimes had to be repeated because of wide variation between replicates. Errors may arise when the microprecipitate containing antiserum-bound digoxin is separated by centrifugation and decantation from free digoxin, which remains in solution. It is difficult to decant all tubes in a uniform manner and avoid bubble formation with possible disturbance of the invisible microprecipitate. The Lanoxitest Beta Kit with 3H labelled digoxin utilizes a charcoal separation procedure (Zeegers et al., 1973), but the kit itself is inconvenient in laboratories lacking their own beta counting facilities. This method was adapted for use with 1~51 labelled digoxin, and because charcoal is visible it became relatively easy to see if disturbance had inadvertently occurred at the separation stage. The assay protocol is shown in the Table. Rabbit