Analytical performance of a monoclonal digoxin assay with increased specificity on the ACS:180

Significant Improvement in Digoxin Immunoassays Over Four Decades: Newer Assays are Less Affected by Interferences

BACKGROUND

Digitalis glycosides derived from foxglove plants have been used for medicinal purposes since the 16th century. Currently, digoxin derived from foxgloves is used clinically. Owing to the narrow therapeutic range, therapeutic drug monitoring is essential; however, digoxin immunoassays suffer from interference.

METHODS

The issue of interference was reviewed for both older polyclonal antibody-based digoxin assays and newer monoclonal antibody-based digoxin assays. A literature search was conducted using PubMed, ScienceDirect, Scopus, Web of Science, and ResearchGate for studies on digoxin immunoassays published in the English language from 1969 to the present.

RESULTS

Radioimmunoassays for digoxin in the 1970s and, later, first-generation nonradioimmunoassay methods were liable to several interferences, including digoxin-like immunoreactive substances, spironolactone, potassium canrenoate, and various digoxin metabolites. However, for the last 10-15 years, next next-generation digoxin immunoassays have been virtually free from such interferences. Nevertheless, certain herbal supplements, as well as both Digibind and DigiFab, interfere with serum digoxin measurement, even with the more recently developed digoxin assays.

CONCLUSIONS

More recently introduced monoclonal antibody-based digoxin assays are superior to the older polyclonal antibody-based digoxin assays.

Effect of Asian ginseng, Siberian ginseng, and Indian ayurvedic medicine Ashwagandha on serum digoxin measurement by Digoxin III, a new digoxin immunoassay

Asian ginseng, Siberian ginseng, and Indian Ayurvedic medicine Ashwagandha demonstrated modest interference with serum digoxin measurements by the fluorescent polarization immunoassay (FPIA). Recently, Abbott Laboratories marketed a new digoxin immunoassay, Digoxin III for application on the AxSYM analyzer. We studied potential interference of these herbal supplements on serum digoxin measurement by Digoxin III assay in vitro and compared our results with the values obtained by Tina-quant assay. Aliquots of drug-free serum pool were supplemented with various amounts of Asian ginseng, Siberian ginseng, or Ashwagandha approximating expected concentrations after recommended doses and overdoses of these herbal supplements in serum. Then digoxin concentrations were measured by the Digoxin III and Tina-quant (Roche Diagnostics) assay. We also supplemented aliquots of a digoxin pool prepared from patients receiving digoxin with various amounts of these herbal supplements and then measured digoxin concentrations again using both digoxin immunoassays. We observed modest apparent digoxin concentrations when aliquots of drug-free serum pool were supplemented with all three herbal supplements using Digoxin III assay (apparent digoxin in the range of 0.31-0.57 ng/ml), but no apparent digoxin concentration (except with the highest concentration of Ashwagandha supplement for both brands) was observed using the Tina-quant assay. When aliquots of digoxin pool were further supplemented with these herbal supplements, digoxin concentrations were falsely elevated when measured by the new Digoxin III assay. For example, we observed 48.2% (1.63 ng/ml digoxin) increase in digoxin concentration when an aliquot of Digoxin pool 1 (1.10 ng/ml digoxin) was supplemented with 50 microl of Asian ginseng extract (Brand 2). Measuring free digoxin does not eliminate the modest interferences of these herbal supplements in serum digoxin measurement by the Digoxin III assay.

Therapeutic drug monitoring of digoxin: impact of endogenous and exogenous digoxin-like immunoreactive substances

Digoxin is a cardioactive drug with a narrow therapeutic range. Therapeutic drug monitoring is essential in clinical practice for efficacy as well as to avoid digoxin toxicity. Immunoassays are commonly used in clinical laboratories for determination of serum or plasma digoxin concentrations. Unfortunately, digoxin immunoassays are affected by both endogenous and exogenous compounds. Endogenous compounds are termed 'digoxin-like immunoreactive substances' (DLIS), which are found in elevated concentrations in volume-expanded patients. Exogenous compounds that interfere with digoxin assays are various drugs such as spironolactone, potassium canrenoate as well as Digibind (Fab fragment of antidigoxin antibody), which is used in treating life-threatening digoxin overdose. Moreover, various Chinese medicines such as Chan Su, Lu-Shen Wan and oleander-containing herbal preparations also interfere with serum digoxin measurements by immunoassays. Monitoring unbound (free) digoxin concentration may under certain circumstances eliminate such interferences. Clinicians should be aware of limitations of therapeutic drug monitoring of digoxin using immunoassays.

Usefulness of monitoring free (unbound) concentrations of therapeutic drugs in patient management

Drugs are bound to various serum proteins in different degrees and only unbound or free drug is pharmacologically active. Although free drug concentration can be estimated from total concentration, for strongly bound drugs, prediction of free level is not always possible. Conditions like uremia, liver disease and hypoalbuminemia can lead to significant increases in free drug resulting in drug toxicity even if the concentration of total drug is within therapeutic range. Drug-drug interactions may also lead to a disproportionate increase in free drug concentrations. Elderly patients may have increased free drug concentrations due to hypoalbuminemia. Elevated free phenytoin concentrations have also been reported in patients with AIDS and pregnancy. Currently free drug concentrations of anticonvulsants such as phenytoin, carbamazepine and valproic acid are widely measured in clinical laboratories. Newer drugs such as mycophenolic acid mofetil and certain protease inhibitors are also considered as candidates for monitoring free drug concentration.

New Enzyme-Linked Immunosorbent Digoxin Assay on the ADVIA?? IMS??? 800i System Is Virtually Free from Interference of Endogenous Digoxin-like Immunoreactive Factors

Endogenous digoxin-like immunoreactive factors (DLIF) may crossreact with antidigoxin antibody and falsely elevate immunoassay results. Recently, a new enzyme-linked immunosorbent chemiluminescent assay for digoxin has been available for use on the ADVIA IMS (Integrated Modular System) 800i analyzer (Bayer Diagnostics). We studied potential interference of DLIF with this new digoxin assay. We analyzed 30 serum specimens from patients who have pathologic conditions that may increase serum DLIF concentrations. These patients were never exposed to digoxin or other agents that may lead to a measurable digoxin concentration. We also analyzed 10 specimens from neonates, 10 cord blood specimens, and 10 amniotic fluid specimens. Apparent digoxin concentrations were measured using the new enzyme-linked immunosorbent digoxin assay (IMS-Digoxin), a fluorescence polarization immunoassay (FPIA), and also a chemiluminescent immunoassay (CLIA, run on ACS:180(R) system from Bayer Diagnostics). We observed measurable apparent digoxin levels with the FPIA in 4 uremic patients (range 0.21-0.36 ng/mL, digoxin equivalent), 7 patients with liver disease (range 0.21-0.72 ng/mL), and 3 patients in the third trimester of pregnancy (0.22-0.66 ng/mL). We also observed measurable DLIF concentrations with the FPIA in 2 neonates (0.22 and 0.36 ng/mL), 5 cord blood specimens (range 0.21-1.18 ng/mL), and 5 amniotic fluid specimens (0.21-0.50 ng/mL). None of these DLIF-positive specimens showed any measurable digoxin concentration using the IMS-Digoxin or the CLIA assay. When serum specimens containing elevated concentrations of DLIF but no digoxin (as measured by FPIA) were supplemented with known concentrations of digoxin, we observed falsely elevated digoxin concentrations, as expected, only by the FPIA. In contrast, we observed a good agreement between the target and observed concentrations when the new IMS-Digoxin or the CLIA assay was used. We conclude that the IMS-Digoxin assay is free from interference of DLIF.

Fab antibody fragments: some applications in clinical toxicology

This review provides current information on the use of antigen-binding fragments (Fab) from cleaved antibodies to treat poisoning with digoxin and other potent, low formula mass poisons, such as colchicine and tricyclic antidepressants. Anti-digoxin Fab fragments have been used successfully for many years in the management of severe poisoning with digoxin, digitoxin, and a range of other structurally related compounds, including cardiotoxins from Nerium and Thevetia sp. (oleander) and Bufo sp. (toads). However, their main use remains treating digoxin poisoning. Equimolar doses of anti-digoxin Fab fragments completely bind digoxin in vivo. The approximate dose of Fab fragments (mg) is 80 times the digoxin body burden (mg). If neither the dose ingested nor the plasma digoxin/digitoxin concentration is known, in an adult 380 mg of anti-digoxin Fab fragments should be given. The dose for elderly patients or those with renal impairment should be similar to that for those with normal renal function. Fab fragments have a plasma half-life of 12-20 hours, but this can be prolonged in patients with renal impairment. Analysis of serum ultrafiltrate using an immunoassay shown not to have matrix bias remains the most accurate approach to measuring free digoxin in the presence of anti-digoxin Fab fragments. The antibody fragments are given intravenously over 15-30 minutes after dilution to at least 250 mL with plasma protein solution, 0.9% (w/v) sodium chloride, or deionised water, except in infants where the volume infused can be reduced. Factors limiting the efficacy of Fab fragments are the dose, the duration of the infusion and any delay in administration. Guidelines for Fab fragment administration in children include (i) dilution to a final Fab concentration of 10 g/L in either 5% (w/v) dextrose or 0.9% (w/v) sodium chloride; (ii) infusion through a 0.22 microm filter; (iii) administration of the total dose over a minimum of 30 minutes; and (iv) avoiding coadministration of other drugs and/or electrolyte solutions. Fab fragments are generally well tolerated. Adverse effects attributable to Fab treatment include hypokalaemia and exacerbation of congestive cardiac failure; renal function could be impaired in some patients. Fab fragment preparations for treating acute colchicine and tricyclic antidepressant poisoning have been developed, but are not available commercially. Colchicine poisoning is rare in Western countries, and pharmacological management together with supportive care is usually effective even in severe tricyclic antidepressant overdosage. Attempts have been made to produce anti-paraquat antibodies capable of enhancing paraquat elimination from the lung, but thus far all such attempts have proved unsuccessful.

Rapid Detection of Oleander Poisoning Using Digoxin Immunoassays

Oleander is an ornamental shrub that grows in the United States, Australia, India, Sri Lanka, China, and other parts of the world. All parts of the plant are poisonous because the presence of cardiac glycoside oleandrin. Despite its toxicity, oleander extract is used in folk medicines. Because of its structural similarity, oleandrin cross-reacts with the fluorescence polarization immunoassay (FPIA) for digoxin. We studied the potential of detecting oleandrin in serum using 5 common digoxin immunoassays (FPIA, MEIA, both from Abbott; Beckman digoxin assay on Synchron LX, Chemiluminescent assay, CLIA from Bayer Diagnostics) and a recently FDA-approved turbidimetric assay on the ADVIA 1650 analyzer (Bayer). Aliquots of drug-free and digoxin-like immunoreactive substances (DLIS)-free serum pools were supplemented with ethanol extract of oleander leaves or oleandrin (Sigma Chemicals) in amounts expected in vivo after severe overdose. We observed significant apparent digoxin concentration with FPIA, Beckman, and the new turbidimetric assay (1 mL drug-free serum supplemented with 5.0 microL of oleander extract: apparent digoxin 2.36 ng/mL by the FPIA, 0.32 ng/mL by the MEIA, 0.93 ng/mL by the Beckman, 0.82 ng/mL by the new turbidimetric assay). The CLIA showed no cross-reactivity. Similar observations were made when serum pools were supplemented with oleandrin. Because cross reactivity should be tested in the presence of the primary analyte, we supplemented serum pools prepared from patients receiving digoxin with oleander extract or oleandrin. The measured digoxin concentrations were falsely elevated with the FPIA, Beckman, and turbidimetric assays, the highest false elevation being observed with the FPIA. Surprisingly, apparent digoxin concentrations were falsely lowered when MEIA was used. Digibind neutralizes free apparent digoxin concentration in vitro in serum pools supplemented with oleander extract, and this effect can be measured by the FPIA. We conclude that FPIA is most sensitive to detect the presence of oleander in serum. In contrast, the CLIA (no cross-reactivity) should be used for monitoring digoxin in a patient receiving digoxin and self-medicated with a herbal remedy containing oleander.

Interference of Endogenous Digoxin-Like Immunoreactive Factors in Serum Digoxin Measurement is Minimized in a New Turbidimetric Digoxin Immunoassay on ADVIA 1650 Analyzer

Endogenous digoxin-like immunoreactive factors (DLIF) cross-react with antidigoxin antibody and falsely elevate or lower measured serum digoxin concentrations, depending on the assay design. Recently, Bayer Diagnostics released a turbidimetric assay for digoxin on the ADVIA 1650 analyzer. We studied potential interference of DLIF with this new digoxin assay. We analyzed 40 serum specimens from patients who have pathologic conditions that may increase serum DLIF concentrations. These patients were never exposed to digoxin or other agents that may lead to a measurable digoxin concentration. We also analyzed five specimens from autopsy and five specimens from neonates. Apparent digoxin concentrations were measured using the new turbidimetric digoxin assay, the fluorescence polarization immunoassay (FPIA, Abbott Laboratories, Abbott Park, IL), and also the chemiluminescent immunoassay (CLIA, Bayer Diagnostics). We observed measurable apparent digoxin levels with the FPIA in 5 uremic patients (range 0.24-0.86 ng/mL), 6 patients with liver disease (range 0.21-0.72 ng/mL), in 3 patients in the third trimester of pregnancy (0.21-26 ng/mL), and in 3 neonates (range 0.21-0.46 ng/mL). Four out of 5 autopsy specimens showed measurable apparent digoxin concentrations (0.23-0.81 ng/mL). In contrast, only 1 specimen (a uremic patient) showed an apparent digoxin concentration of 0.26 ng/mL with the turbidimetric digoxin immunoassay (FPIA value 0.86 ng/mL, CLIA value 0.32 ng/mL). Because DLIF is absent in the protein-free ultrafiltrate, we also measured free digoxin concentrations in DLIF-positive patients to ensure that the apparent digoxin concentrations were caused by DLIF. We observed no apparent digoxin concentrations in the protein-free ultrafiltrate in any DLIF-positive specimens. When serum specimens containing elevated concentrations of DLIF but no digoxin were supplemented with a known concentration of digoxin, we observed falsely elevated digoxin concentrations by the FPIA, as expected. In contrast, we observed a good agreement between the target and observed concentrations when the new turbidimetric assay was used. We conclude that DLIF has minimal effect on serum digoxin measurements by the new turbidimetric assay.

Impaired protein binding of Chinese medicine DanShen in uremic sera and sera with hyperbilirubinemia: rapid assessment of total and free DanShen concentrations using the fluorescence polarization immunoassay for digoxin

DanShen is a Chinese medicine that is used to treat cardiovascular disorders. DanShen is moderately to strongly protein bound, mainly to albumin. Because impaired protein binding of albumin-bound drugs in uremia has been reported, we studied protein binding of DanShen by measuring the digoxin-like immunoreactive component of this Chinese medicine. We observed a significantly higher percentage of free fraction of DanShen in uremic sera in vitro. Impaired protein binding of DanShen was also observed in sera from patients with liver disease, who had elevated concentrations of bilirubin. Treating uremic sera with activated charcoal significantly improved the protein binding of DanShen, indicating that uremic compounds are responsible for the impaired protein binding of DanShen. On the other hand, when various amounts of bilirubin were added to aliquots of the normal pool supplemented with DanShen, we observed only a modest displacement of DanShen from the protein-binding sites by bilirubin, indicating that hypoalbuminemia may play a major role in impaired protein binding of DanShen in sera with elevated bilirubin concentrations. We conclude that protein binding of DanShen is lower in uremic sera and in sera with elevated bilirubin concentrations.

Effect of the traditional Chinese medicines Chan Su, Lu-Shen-Wan, Dan Shen, and Asian ginseng on serum digoxin measurement by Tina-quant (Roche) and Synchron LX system (Beckman) digoxin immunoassays

Chan Su, Lu-Shen-Wan, Dan Shen, and Asian ginseng are traditionally used to treat a number of conditions, including cardiovascular disease. All of these traditional Chinese medicines exhibit cardioactive properties. Digoxin is a cardioactive drug with a narrow therapeutic range (0.8-1.9 ng/mL). A patient taking digoxin may also take these Chinese medicines for their cardiotonic effects. Moreover, the active components of these medicines that are responsible for cardiotonic effects bear structural similarities to digoxin. Therefore, we studied the potential interference of these Chinese medicines with two digoxin immunoassays--the Tina-quant (Roche Diagnostics) and the Beckman (Synchron LX system)--and compared the values with the fluorescence polarization immunoassay (FPIA; Abbott Laboratories). When very small amounts (2-5 microL) of aqueous extract of Chan Su or Lu-Shen-Wan were added to drug-free serum, we observed high digoxin-like immunoreactivity with the FPIA. In contrast, when ethyl acetate extract of Dan Shen or microliter amounts of ginseng extract were added to drug-free serum, we observed modest digoxin-like immunoreactivity with the FPIA, but no apparent digoxin activity with the Roche and Beckman digoxin immunoassays. When aliquots of a digoxin pool prepared from patients receiving digoxin were supplemented with these Chinese medicines, we observed the most significant interference with the FPIA. The presence of endogenous digoxin-like immunoreactive substances can have additive effects with these Chinese medicines and falsely increase apparent digoxin levels by the FPIA. On the other hand, the Roche and Beckman assays were free from interference from DLIS but showed significant interference from Chan Su and Lu-Shen-Wan. We conclude that the FPIA showed the most significant interference from all four of the Chinese medicines we studied. However, the Roche and Beckman assays showed no interference from two (Dan Shen and Asian ginseng) of the four Chinese medicines we studied.

Clinical utility of free drug monitoring

Most drugs are bound to serum proteins to a various degree. Only unbound or free drug is pharmacologically active. Usually total drug is measured for therapeutic monitoring because there is equilibrium between bound and free drugs, and concentration of free drug can be predicted from total drug concentration. However, under certain conditions this equilibrium is disturbed and the measured free drug concentration can be significantly higher than expected from total drug concentrations, especially for strongly protein-bound drugs. In such case a patient may experience drug toxicity even if the total drug concentration is within the therapeutic range. Conditions like uremia, liver disease and hypoalbuminemia can lead to significant increases in free drug concentration. Therefore, monitoring free phenytoin and free valproic acid is recommended in these patients. Drug-drug interactions can also lead to a disproportionate increase in free drug concentration. One strongly protein-bound drug can significantly displace another strongly protein-bound drug if both drugs share the same binding site. Several over-the-counter pain medications such as salicylate, naproxen, and ibuprofen can cause significant displacement of both phenytoin and valproic acid from albumin binding site. Interestingly, such interactions are absent in uremic patients. Elderly patients may have increased free phenytoin or valproic acid due to hypoalbuminemia. Elevated free phenytoin concentrations have also been reported in patients with AIDS. Although digoxin is 25% bound to protein, monitoring free digoxin is useful in patients with elevated endogenous digoxin-like immunoreactive substances or in patients overdosed with digoxin and being treated with digibind. Monitoring free digoxin can also eliminate interference of Chinese medicines Chan Su and Danshen in serum digoxin measurement by certain immunoassays. However, free drug monitoring is not a routine procedure in clinical laboratories due to technical difficulties and lack of established reference ranges for free drugs.

Effect of Chinese medicines Chan Su and Danshen on EMIT 2000 and Randox digoxin immunoassays: wide variation in digoxin-like immunoreactivity and magnitude of interference in digoxin measurement by different brands of the same product

Chan Su is a Chinese medicine prepared from the skin gland of a Chinese toad and is used in treating arrhythmia and other heart diseases. Danshen is prepared from the Chinese medicinal plant and is used for various cardiovascular diseases including angina pectoris. The authors studied the potential interference of such medicines with the widely used EMIT 2000 (Dade Behring; Deerpark, IL) digoxin assay and the recently marketed Randox digoxin assay (Randox Laboratories Ltd, Antrim, United Kingdom) (both run on the Bayer ADVIA 1650 analyzer) (Bayer Diagnostics, Tarrytown, NY) and compared their results with an FPIA (Abbott Laboratories) and a chemiluminescent immunoassay (CLIA; Bayer Diagnostics) for digoxin. Aliquots of drug-free serum were supplemented with 1 microL ethyl acetate extract of Danshen or aqueous extract of Chan Su, and apparent digoxin concentrations were measured by all four digoxin immunoassays (FPIA, EMIT, Randox, CLIA). The authors also supplemented aliquots of several different serum pools prepared from patients taking digoxin with very small amounts of Chan Su or Danshen extract and compared digoxin values with the control digoxin values (serum pool containing no Chinese medicine). The authors observed no interference of Danshen in either EMIT, Randox, or CLIA assay but observed an interference with the FPIA assay. On the other hand, the authors observed high interference of Chan Su in the FPIA assay but moderate interference with the EMIT 2000 and Randox digoxin assays. CLIA assay was again free from any interference. The authors also observed a wide variation in digoxin-like immunoreactivity and magnitude of interference in digoxin immunoassay in different brands of Chan Su and Danshen, indicating poor quality control in manufacturing of these Chinese medicines. Taking advantage of the high protein binding of digoxin-like immunoreactive components of Chan Su, the authors further demonstrated that interference of Chan Su in EMIT 2000 and Randox assays can be mostly eliminated by monitoring free digoxin.

Endogenous and exogenous digoxin-like immunoreactive substances: impact on therapeutic drug monitoring of digoxin

Endogenous digoxin-like immunoreactive substance (DLIS) was first reported in volume-expanded dogs. Its presence has been confirmed in blood, urine, and other body fluids. Elevated DLIS concentrations are encountered in patients with volume-expanded conditions such as uremia, essential hypertension, liver disease, and preeclampsia. DLISs cross-react with antidigoxin antibodies and falsely elevate serum digoxin concentrations, interfering in interpretation of results for therapeutic digoxin monitoring. Falsely lower digoxin values due to the presence of DLISs have been reported. The association of DLISs with volume expansion led to speculation that they could be natriuretic hormones. Several structures have been proposed for DLISs, including nonesterified fatty acid, phospholipid, lysophospholipid, bile acid, bile salt, and steroid. Exogenous DLISs can be found in serum after ingestion of various Chinese medicines and therapy with spironolactone, canrenone, or potassium canrenoate. Like endogenous DLISs, exogenous DLISs interfere with serum digoxin assays, complicating therapeutic digoxin monitoring. However, most reported endogenous and exogenous DLISs are strongly protein-bound while digoxin is weakly protein-bound. Therefore, interference of both endogenous and exogenous DLISs in serum digoxin measurement can be eliminated by monitoring digoxin concentrations in the protein-free ultrafiltrates.

Bidirectional (positive/negative) interference of spironolactone, canrenone, and potassium canrenoate on serum digoxin measurement: elimination of interference by measuring free digoxin or using a chemiluminescent assay for digoxin

Spironolactone and potassium canrenoate (aldosterone antagonist diuretics) are often used with digoxin in clinical practice. Spironolactone, potassium canrenoate, and their common metabolite canrenone cross-react with the fluorescence polarization immunoassay (FPIA) for digoxin, and can falsely elevate serum digoxin concentrations. Serum digoxin concentrations were falsely lowered when the microparticle enzyme immunoassay (MEIA) was used. Aliquots of drug-free serum were supplemented with therapeutic and above-therapeutic concentrations of spironolactone, canrenone, and potassium canrenoate, and apparent digoxin activities were measured. We observed digoxin-like activities in the FPIA, but observed no activity with the MEIA or the chemiluminescent assay (CLIA). However, when serum digoxin pools prepared from patients receiving digoxin were supplemented with these compounds, we observed suppression of total digoxin levels with the MEIA. In contrast, no interference was observed in the presence of these compounds when CLIA was used for digoxin measurement. These compounds are strongly protein-bound, and no apparent digoxin activity was observed in the protein-free ultrafiltrate when drug-free sera were spiked with high levels of these compounds. Taking advantage of strong protein binding of these compounds and weak protein binding of digoxin (25%), interference of spironolactone, canrenone, and potassium canrenoate in FPIA and MEIA digoxin assays can be mostly eliminated by monitoring free digoxin concentration. Another approach to avoid this interference is to use the CLIA digoxin assay.

Digoxin and its related endogenous factors

The digitalis drugs are plant-derived cardenolide compounds used medicinally for several hundred years. These drugs elicit inotropic and chronotropic effects on the heart, but they also affect many other tissues. The mechanism of action involves inhibition of the ion-transport activity of a membrane-associated protein called Na, K-ATPase (sodium pump). Present theory holds that the sodium pump is the principal molecular receptor for the digitalis drugs. Recent evidence indicates the presence of naturally occurring digitalis-like compounds in mammals. It is believed these compounds, collectively known as either digitalis-like (DLF) or ouabain-like (OLF) factors, may be endogenous hormones regulating the biological activity of the sodium pump and its isoforms. The presence of deglycosylated and other congeners of one specific DLF, the digoxin-like immunoreactive factor (DLIF), has very recently been described in humans. Digoxin as a drug is the most widely prescribed digitalis in the U.S., and its measurement in serum has established a model for present-day therapeutic drug monitoring (TDM). Historically, the accurate measurement of digoxin in blood has been difficult. This article focuses on the present understanding of the clinical use of digoxin, factors that affect the accuracy of measuring digoxin, the principle of measuring metabolically active species of digoxin, and the effects of DLIF and other interfering substances in digoxin immunoassay.

Predictive performance study of two digoxin assays in subjects with various degrees of renal function

This prospective study was conducted to compare the predictive performance of fluorescence polarization immunoassay (FPIA, Abbott TDx Digoxin II) and radioimmunoassay (RIA, Kallestad Labs) with combined low-pressure liquid chromatography/RIA (LPLC/RIA) digoxin assay in measuring 15-17 serum digoxin concentrations (SDC) obtained after a single 10 microg/kg intravenous digoxin dose in patients with various degrees of renal function and at different SDC ranges. Eighteen men and women were stratified into 3 age- and gender-matched groups based upon renal function [N = 6 in each, group I (Cl(cr) < 10 mL/min), group II (Cl(cr) = 10-50 mL/min), and group III (Cl(cr) > 50 mL/min)]. Serum digoxin concentrations were measured at time zero; at 0.25, 0.5, 0.75, 1, 2, 3, 4, 6, 8, and 12 hours; and at 2, 3, 4, and 5-7 days after the digoxin dose, using the three different digoxin assays. TDx Digoxin II was unbiased [mean error -0.09 (95% CI -0.19, 0.01)] and RIA biased [mean error -0.29 (95% CI -0.36, -0.21)] to over-predict SDC by 14.2%. In group I patients, the analysis revealed a bias to over-predict SDC by 6.0% for TDx Digoxin II [mean error -0.16 (95% CI -0.29, -0.07)] and an unbiased performance by RIA. In groups II and III, both TDx Digoxin II and RIA showed biased performance, the mean magnitude of bias was low (< 20%). For intermediate SDC range (> 0.5 ng/mL and < or = 3.0 ng/mL), TDx Digoxin II was unbiased in predicting SDC, whereas RIA was biased to under-predict SDC [mean error 0.13 (95% CI 0.10, 0.16)] by 9.9%. The magnitude of bias observed in all cases was less than 20%. Both assays, TDx Digoxin II and RIA, imprecisely measured SDC for all samples combined, different groups and SDC ranges. In all time-paired samples, TDx Digoxin II (FPIA) performed better than the RIA. In conclusion, the magnitude of bias observed with either assay at different groups and SDC ranges was not likely to be clinically relevant. Therefore, either assay may be used to measure SDC in clinical practice.

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.

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

Measurement of unbound digoxin in presence of Fab fragments may be useful in management of overdoses. The analysis can be performed on serum directly or on ultrafiltrate of serum. The architecture of the immunoassay may influence the validity of results obtained using these two approaches. We tested this hypothesis by preparing serum mixtures containing various concentrations of digoxin and Digibind and analyzed them by the immunoassays before and after ultrafiltration. Four samples collected from Digibind-treated patients were also analyzed before and after ultrafiltration. The slopes and the y-intercepts of the measured versus the expected values for serum and its ultrafiltrate overlapped for the MEIA digoxin assay. For other three immunoassays tested (ACS:180, Stratus, and On-Line), either the slope or the intercept for measured versus the expected results for serum were significantly different (P < 0.05) than those for ultrafiltrate. Following addition of digoxin and Digibind, differences in results for serum analyzed directly or after ultrafiltration were < 0.50 ng/ml. Comparable samples from digoxin-overdosed patients treated with Digibind had differences of > 1.0 ng/ml. Previous claims reporting direct analysis of digoxin in presence of antidote but not having used patient samples for validation should be revisited. To date, analysis of serum ultrafiltrate by an immunoassay proven not to have matrix bias remains the most accurate approach in measuring unbound digoxin in presence of antidote.

Efficient synthesis of 3-aminodigoxigenin and 3-aminodigitoxigenin probes

Oxidation of digoxigenin and digitoxigenin to the 3-ketones followed by reductive amination produced a mixture of amine epimers. The inability to separate the epimeric mixtures of chemiluminescent digoxigenin probes derived by conjugation to the acridinium label prompted us to develop an HPLC method to separate the amines. Labeling of the pure amines resulted in good yields of the isomerically pure probes.

Analytic performance of two automated nonpretreatment digoxin immunoassays

The analytic performance of two automated nonpretreatment digoxin methods, AxSYM Digoxin II and Vitros digoxin immunoassays, was assessed. Both assays had analytic sensitivities of less than 0.2 microg/L, were linear from digoxin concentrations of 0.5 to 4.0 microg/L, and showed acceptable precision, with a maximum total coefficient of variation (CV) of 8.9% and 6.4% for the AxSYM and Vitros, respectively. Comparison of the two methods using samples from patients receiving digoxin gave the following relationship: Vitros = 0.91 x AxSYM + 0.23 (r = 0.97, Sy,x = 0.12). Digoxinlike immunoreactive factor (DLIF) crossreactivity was examined in specimens from patients who had hepatic disease, renal insufficiency, had undergone cardiac surgery, and in neonatal cord blood samples. Minimal crossreactivity was observed for most samples and the average crossreactivity for each group of samples was comparable for the two methods. The recovery of digoxin added to samples from each group of DLIF was similar, except for that from cord blood samples, for which recovery was significantly lower with the AxSYM method. Titration of a digoxin-spiked serum pool with digoxin-immune Fab showed a similar decrease in the measured digoxin concentration for both methods. Overall, the analytic performance characteristics of these two methods were comparable.

Interference from digitoxin-like immunoreactive factors reduced in a new monoclonal chemiluminescent digitoxin assay

Endogenous digoxin-like immunoreactive factors (DLIF) can interfere with some digoxin immunoassays. We looked for similar interference, called digitoxin-like immunoreactive factors (DTLIF) in two digitoxin immunoassays: A new chemiluminescent assay (CLIA), processed on the automated random access immunoassay system ACS:180, and a fluorescent polarization assay (FPIA), processed on the semiautomated TDx batch analyzer. One hundred thirty-seven samples of sera were tested from nondigitalized pregnant women, patients with liver or kidney diseases, and cord blood. The CLIA digitoxin assay uses a murine monoclonal antibody and requires no sample pretreatment; the FPIA digitoxin assay uses a polyclonal rabbit antibody and requires sample precipitation. Both assays have a similar dynamic range and sensitivity and give comparable results with commercial controls and external quality control survey samples. Although the CLIA detected no digitoxin in any sample tested, the FPIA showed apparent digitoxin concentrations of more than 2.0 ng/ml for 100% and 44% among cord blood and liver disease specimens, respectively. The highest DTLIF concentration was found in serum from a patient with liver disease (18.1 ng/ml). When spiked with 32 ng/ml digitoxin, six of the samples containing DTLIF generated FPIA digitoxin values of 6% to 27.5% more than the expected digitoxin levels. Two specimens with no detectable DTLIF activity were run as controls, and when spiked with digitoxin, showed target digitoxin concentrations in the FPIA. The CLIA recovered near the target digitoxin values (32 ng/ml) in all spiked samples. It was concluded that the polyclonal FPIA digitoxin assay may give discordant digitoxin concentrations in some patient groups because of interference from digitoxin-like immunoreactive factors. The CLIA digitoxin assay is not affected by DTLIF interference.

Therapeutic drug monitoring: antiarrhythmic drugs

Antiarrhythmic agents are traditionally classified according to Vaughan Williams into four classes of action. Class I antiarrhythmic agents include most of the drugs traditionally thought of as antiarrhythmics, and have as a common action, blockade of the fast-inward sodium channel on myocardium. These agents have a very significant toxicity, and while they are being used less, therapeutic drug monitoring (TDM) does significantly increase the safety with which they can be administered. Class II agents are antisympathetic drugs, particularly the beta-adrenoceptor blockers. These are generally safe agents which do not normally require TDM. Class III antiarrhythmic agents include sotalol and amiodarone. TDM can be useful in the case of amiodarone to monitor compliance and toxicity but is generally of little value for sotalol. Class IV antiarrhythmic drugs are the calcium channel blockers verapamil and diltiazem. These are normally monitored by haemodynamic effects, rather than using TDM. Other agents which do not fall neatly into the Vaughan Williams classification include digoxin and perhexiline. TDM is very useful for monitoring the administration (and particularly the safety) of both of these agents.

Impairment of absorption of digoxin by acarbose

This study was conducted to determine the effect of acarbose on serum concentrations of digoxin in healthy male volunteers. A randomized crossover design with three phases was used. In phase 1 participants received 0.5 mg of digoxin alone. In phase 2 they received 0.5 mg of digoxin 0.5 hours after a 200-mg dose of acarbose. In phase 3 they received 100 mg of acarbose 0.5 hours before each meal three times daily for 3 days. On the fourth day, they received 0.5 mg of digoxin 0.5 hours after a 100-mg dose of acarbose. Area under the concentration-time curve (AUC0-48) and mean maximum concentration (Cmax) were significantly lower and tmax significantly increased in phases 2 and 3 compared with phase 1. These results indicate that the absorption of digoxin is reduced by administration of acarbose, and that one of the major mechanisms of this interaction may be due to the pharmacodynamics of acarbose.

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.

Effects of digoxinlike immunoreactive substances and digoxin FAB antibodies on the new digoxin microparticle enzyme immunoassay

Digoxin-like immunoreactive substance (DLIS) is known to interfere with fluorescence polarization immunoassay (FPIA) (Digoxin II, Abbott Laboratories) and falsely elevates the total digoxin concentrations. Digoxin FAB antibody (Digibind) is also known to affect digoxin results by FPIA assay. The authors studied the effects of DLIS and Digibind on a new microparticle enzyme immunoassay (MEIA) for digoxin recently introduced by Abbott Laboratories, compared with the standard FPIA method and chemiluminescence assay (ACS-digoxin, Ciba-Corning). They studied 30 volume-expanded patients (term pregnancy, liver and renal disease) for the presence of DLIS. None of these patients received digoxin. They observed measurable DLIS concentrations in 12 of 30 patients by the FPIA assay and in only 1 patient by both MEIA and ACS assays. The concentration of DLIS in that patient was 0.31 ng/ml of digoxin equivalent by the MEIA assay, 0.36 ng/ml by the ACS assay, and 1.15 ng/ml by the FPIA assay. When they supplemented serum containing digoxin with low to high concentrations of digibind (0.5, 1.0, 2.0 and 4.0, 10, and 20 micrograms/ml), and measured digoxin concentrations by FPIA, MEIA, and ACS assays, they observed lower than expected values of total digoxin. However, when they supplemented serum containing no digoxin with high concentration of digibind (5.0, 10.0 and 20.0 micrograms/ml) and supplemented protein-free ultrafiltrates with digoxin, they observed expected digoxin concentrations in the ultrafiltrates by all three assays, indicating that the ultrafiltrates are essentially free of digibind.

Analytical performance of a new chemiluminescent phenytoin (ACS:180) assay

The authors, as a beta testing site, evaluated the ACS:180-phenytoin chemiluminescent assay (Ciba-Corning Diagnostics Corp., Medfield, MA, U.S.A.) by comparing its performance with a widely used fluorescence assay for phenytoin (Abbott Laboratories, Abbott Park, IL, U.S.A.). The ACS:180-phenytoin assays were run on a ACS-180 analyzer and fluorescence polarization assays on a TDx analyzer. The within-run precision for ACS-phenytoin assay was determined using controls obtained from Ciba-Corning. The CVs were 2.9% for low control (mean = 5.5, SD = 0.16 microgram/ml, n = 10), 2.8% for the medium control (mean = 13.4, SD = 0.37 microgram/ml, n = 10), and 2.7% for the high control (mean = 24.6, SD = 0.66 microgram/ml, n = 10). The corresponding between run precisions were 4.1% for the low control (mean = 5.4, SD = 0.22 mg/ml, n = 10), 3.1% for the medium control (mean = 13.8, SD = 0.43 mg/ml, n = 10), and 2.9% (mean = 24.5, SD = 0.70 mg/ml, n = 10) for the high control. The assay was linear from 0.5 to 40 micrograms/ml of serum phenytoin concentrations with a detection limit of 0.24 microgram/ml. The recoveries were 93-97% for concentrations of phenytoin of 5-30 micrograms/ml. They also compared 111 serum specimens collected from patients receiving phenytoin. The concentrations of phenytoin ranged from none detected to 32.4 micrograms/ml. Using fluorescence polarization assay as x-axis (reference method) and ACS:180-phenytoin assay as y-axis, they obtained the following regression line: y = 1.0x - 0.26, r = 0.993. They conclude that the ACS-phenytoin assay has a good precision and that the results correlate well with the fluorescence polarization assay.

Comparison of four digoxin immunoassays with respect to interference from digoxin-like immunoreactive factors

OBJECTIVES

Comparison of a new monoclonal digoxin assay with three polyclonal digoxin assays for their cross-reactivity to digoxin-like immunoreactive factors (DLIF) and digoxin metabolites.

DESIGN AND METHODS

Sixty-six nondigitalized patient samples from 5 different groups: neonates, women in 3rd trimester pregnancy, and patients with liver or renal diseases, or undergoing organ transplants, and 139 samples from digoxin-treated patients of 4 categories (hospital sick, liver, renal, and outpatients) were compared in 4 different digoxin assays: (a) ACS Digoxin (ACS) developed for the automated chemiluminescent Ciba Corning ACS 180 system, (b) Baxter Stratus (Stratus, a fluoroimmunoassay), (c) Ciba-Corning Magic (Magic, a radioimmunoassay), and (d) an in-house radioimmunoassay (RIA). The ACS and RIA were also compared for their cross-reactivity to four principal digoxin metabolites.

RESULTS AND CONCLUSION

Among the nondigitalized specimens, no significant DLIF interference was found for all 4 assays among the pregnant women or liver and transplant patients. However, the neonates registered high DLIF interference with Magic and RIA, but none for ACS or Stratus. DLIF interference in renal samples was highest in the Magic assay and lowest in RIA. Among the specimens with digoxin, a higher number of discrepant samples were found from the sick patients than from outpatients. In 75% of such discrepant samples, the ACS result was less than other assay results, suggesting DLIF as the probable cause. The two assays differed most in their cross-reactivity to the deglycated metabolites, digoxigenin and its mono-digitoxoside.