Variability among commercially available digoxin radioimmunoassay kits in cross reactivity to dihydrodigoxin

We evaluated four commercially available 125I-digoxin radioimmunoassay kits with regard to their ability to cross react with the digoxin metabolite dehydrodigoxin. We prepared dihydrodigoxin serum samples in digoxin-free serum over the concentration range 0.4 to 5.0 microgram/liter and assayed them with each kit according to the manufacturer's instructions. The metabolite was able to displace the 125I-labeled digoxin derivative from the antibody supplied with all four kits. However, the extent of the cross reactivity depended on the kit, ranging from essentially zero to a high degree of interference. Dihydrodigoxin is the only metabolite of digoxin to have been quantitiated in human serum, and may comprise up to 30% of total glycosides. Over the clinical and therapeutic range of serum digoxin concentrations, enough dihydrodigoxin can be produced to interfere in the determination of serum digoxin concentrations by this method. We suggest that laboratories evaluate their specific kit with regard to cross reactivity to this metabolite.

Radioimmunoassay for digoxin with a fully automated continuous-flow system

We describe a fully automated continuous-flow radioimmunoassay for digoxin with use of a Technicon AutoAnalyzer system. It makes use of antibodies linked to magnetizable particles; a magnet separates the bound and free fractions. The precise dispensing of reagents by the AutoAnalyzer system permits a small incubation volume (about 160 microliter) and the reproducible timing enables non-equilibrium conditions to be used. Thus a single sample is assayed in 15 min, with a sample throughput of 30/h. The standard curve ranges from 0.5 to 8.0 microgram/liter and is most precise between 1 and 3 microgram/liter, the between-assay coefficient of variation being less than 3%. There is no significant carryover between samples of high and low concentration, and results by the method correlate closely (r = 0.969) with those by an established manual assay in which charcoal separation is used.

Some factors affecting a commercial kit for radioimmunoassay of digoxin using tritiated digoxin

Some factors affecting results of digoxin determinations using one commercially available radioimmunoassay kit are described and discussed. Serum of pregnant women, cord blood, amniotic fluid and serum of patients taking spironolactone may show erroneously high digoxin activity due to lack of specificity of the antiserum. Cross-reaction with digitoxin was found to vary substantially with antibody-lot. Haemaccel (5 g/1) in the sample leads to too low results. When ethanol (100 g/1) is present results are too high. The need for testing the specifity of every new lot of antiserum before use is stressed.

Suitability of antidigoxin antiserum for digoxin in plant extracts

Antiserum specificity for digoxin was demonstrated by means of three tests: cross-reactivities test, dilutions test and thin-layer chromatography test. The cross-reactivities test examined quantitatively the cross-reactivities with specific compounds and provided an explanation for the basis of the dilutions test. The dilutions test was used to demonstrate the suitability or unsuitability of the antiserum for use in the radioimmunoassay of digoxin in a crude biological sample. The tlc test examined for antiserum cross-reactivity with digoxin and related compounds in a crude extract.

Effects of sheep digoxin-specific antibodies and their Fab fragments on digoxin pharmacokinetics in dogs

Intact sheep antidigoxin antibodies and their Fab fragments have both been found to exert profound effects on digoxin pharmacokinetics in [3H] digoxin-treated dogs. Both classes of molecule remove digoxin from the extravascular space and sequester it in the circulation in protein-bound form, a form in which the digoxin is presumably inactive. These two classes of molecule differ, however, in that the intact antibody molecules interfere with digoxin excretion, thereby promoting the retention of the glycoside; this retained digoxin is eventually released in free, active form when the administered antibody is metabolically degraded. In contrast, urinary excretion of digoxin continues in Fab-treated dogs, with significant quantities of digoxin being excreted promptly in the urine in complex with Fab fragments. These differences in urinary excretion, together with the probable decreased immunogenicity of sheep antidigoxin Fab fragments, suggest that such fragments possess potential advantages over intact antibody molecules for use in the therapy of life-threatening digoxin intoxication in man.

Measurement of digoxin by radioimmunoassay

A sensitive, specific and precise procedure for the measurement of digoxin by radioimmunoassay is presented. The method is rapid, convenient and highly reliable for this very important measurement. Studies designed to evaluate the validity and reproducibility of the assay are presented and discussed.

Radioimmunoassay and enzyme immunoassay compared for determination of digoxin

Patients' sera were analyzed for digoxin by using two different radioimmunoassays and an enzyme immunoassay. Quantitative results obtained by enzyme immunoassay (I) were compared to results obtained on aliquots of the same sample by the radioimmunoassays (II and III). The correlation coefficients were: I vs. II 0.90, n=108; I vs. III 0.94, n=102; and II vs. III 0.95, n=158. Day-to-day precision (10 days) on a low control (1.3 mug/liter) and a high control 3.0 mg/liter), expressed as coefficients of variation, were: I, 13% and 7.8%, II, 4.0% and 4.7%; and III, 8.9% and 4.2%. Ten digoxin-supplemented samples (0-8 mug/liter) were analyzed by the three methods. Correlation coefficients were: supplemented sample vs. I, O.99; supplemented sample vs. II, 0.97; supplemented sample vs. III, 0.98.

[Analysis of digoxin by a radio-immunologic]

A more accurate knowledge of the effects of cardiotonic heterosides can be gained using radiocompetition. The present investigation aimed at: -comparing two commercial kits, the usual quality criteria being applied; -estimating cross-reaction between the immune systems and the endogenous as well as exogenous structural analogues; -establishing the therapeutical or toxic levels in man.

Reversal of advanced digoxin intoxication with Fab fragments of digoxin-specific antibodies

Purified Fab fragments of ovine digoxin-specific antibodies reversed severe digoxin intoxication in a patient who had taken 22.5 mg of the drug with suicidal intent. Atrioventricular block with extreme bradycardia was temporarily managed by pacing, but progressive, intractable hyperkalemia (serum potassium of 8.7 meq per liter) with increasing pacing threshold and progressive intraventricular conduction delay was controlled only after infusion of 1100 mg of Fab. Sinus rhythm returned 10 minutes after completion of Fab infusion. Within five hours, the serum potassium concentration fell to 4.0 meq per liter. Free digoxin concentrations in serum fell sharply to undetectable levels, whereas total serum digoxin concentration concomitantly increased 12-fold. Renal excretion of digoxin bound to Fab was documented. Reversal of toxicity was not accompanied by hemodynamic instability, and antibodies to sheep Fab fragments were not detected in the patient's serum after treatment. Thus, purified digoxin-specific Fab fragments are capable of rapid reversal of advanced digoxin toxicity.

Anti-drug-related antibodies in nonthrombocytopenic cardiac patients

Drug-related antibodies (quinidine, hydrochlorothiazide, and digoxin) were studied using the 51Cr platelet lysis test and a recently introduced simplified platelet factor 3 assay in a total of 109 patients admitted to a coronary care unit. Quinidine antibodies were found in 16 (14.7%), hydrochlorothiazide antibodies in six (5.5%) and digoxin antibodies in two (1.8%) of 109 patients. All patients had normal hematologic data, including platelet counts. There was a significantly high incidence of quinidine antibodies in male patients (p less than 0.02), despite the previous report that quinidine-induced purpura was seen predominantly in female patients. Ten patients (9.2%) had antiplatelet antibodies. The simplified platelet factor 3 assay can easily be done in most laboratories and appears as sensitive as the 51Cr platelet lysis test. The high incidence of drug-related or antiplatelet antibodies in hematologically asymptomatic patients may indicate the presence of an insidious, compensated thrombolytic state in many patients.

Hapten concentrations in the circulation of animals actively immunized with hapten-protein conjugates

Animals immunized with hapten-protein conjugates subsequently circulate high concentrations of hapten bound by antibody. The levels of hapten detected are capable of significantly reducing antibody titer in the sera immunized animals. In the case of steroid-protein conjugates, the main source of increased plasma steroid concentration is the immunizing conjugate, although a contribution from increased host secretion may also occur. The results for rabbits immunized with digoxin-BSA indicate that the appearance of circulating digoxin followed the appearance of circulating antibody to digoxin. Appearance of digoxin in circulation appears to coincide with the operation of the immune response and may be related to macrophage activity. Similar conclusions are drawn from results obtained for circulating morphine in the serum of a sheep immunized with morphine-BSA. Injected hapten-protein antigens are probably processed by macrophage to produce low molecular weight haptenic fragments which are maintained in circulation for prolonged periods in the form of antibody-hapten complexes.

Adaptation of a T3-uptake test and of radioimmunoassays for serum digoxin, thyroxine, and triiodothyronine to an automated radioimmunoassay system--"Centria"

We report the adaptation of four radioassays to the prototype of an automated radioimmunoassay system ("Centria," Union Carbide). The system consists of three integrated modules: (a) an automated pipettor, which dispenses samples and reagents; (b) the key module, an incubator/separator, in which centrifugal force is used to initiate and terminate multiple radioassay incubations and separations simultaneously; and (c) a gamma-counter/computer, which counts three tubes simultaneously and converts counts into concentration units. Radioimmunoassays for thyroxine, triiodothyronine, and digoxin were developed with use of well-characterized antibodies and of prepackaged Sephadex-containing columns to separate bound and free radioactive ligand. A triiodothyronine-uptake test in which the same kind of columns were used was also adapted to the instrument. Results for clinical samples compared favorably with those obtained by manual procedures. We report data on correlation between different methods and preliminary data on precision of the prototype system.

A rapid, simple assay for digoxin

An extremely rapid radioimmunoassay for digoxin is described which is precise over the range of concentrations required to determine whether, or not, a patient has digoxin toxicity. The assay is based on the use of (125)-iodine-labelled digoxin and of a gel equilibration technique for the separation of antibody-bound and free digoxin. The results obtained compare closely with those by a conventional radioimmunoassay and the technique is sufficiently simple to enable its performance by relatively inexperienced laboratory staff.