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

False positive result of human chorionic gonadotropin caused by human anti-mouse antibodies

Immunochemical reactions are fast, can be automated, and generally do not require pretreatment of biological material. Based on these advantages, they are widely used. On the other hand, they are susceptible to analytical interference that can lead to inaccurate results. These factors include the presence of anti-mouse antibodies, causing false positive (or sometimes false negative) results. Although the anti-mouse antibodies over many decades have been repeatedly identified to be the causative source but due to the rarity of such encounters they remain insufficiently considered. Here we show a case, a 45 year-old female who was mis-diagnosed with pregnancy due to falsely elevated human chorionic gonadotropin (hCG) due to anti-mouse antibodies. This led to the patient undergoing two ultrasound examinations and laparoscopy before the hCG was repeated on alternative assays which showed negative results, preventing the patient from methotrexate treatment. Here we describe the details of the case, outline the assay principal, supporting the finding from literature and outlining a process on how to identify such interferences in timely manner.

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.

Transplacental Therapeutic Drug Monitoring in Pregnant Women with Fetal Tachyarrhythmia Using HPLC-MS/MS

Fetal arrhythmia develops in 0.1-5% of pregnancies and may cause fetal heart failure and fetal hydrops, thus increasing fetal, neonatal, and infant mortality. The timely initiation of transplacental antiarrhythmic therapy (ART) promotes the conversion of fetal tachycardia to sinus rhythm and the regression of the concomitant non-immune fetal hydrops. The optimal treatment regimen search for the fetus with tachyarrhythmia is still of high value. Polymorphisms of these genes determines the individual features of the drug pharmacokinetics. The aim of this study was to study the pharmacokinetics of transplacental anti-arrhythmic drugs in the fetal therapy of arrhythmias using HPLC-MS/MS, as well as to assess the effect of the multidrug-resistance gene ABCB1 3435C > T polymorphism on the efficacy and maternal/fetal complications of digoxin treatment. The predisposition to a decrease in the bioavailability of the digoxin in patients with a homozygous variant of the CC polymorphism showed a probable association with the development of ART side effects. A pronounced decrease in heart rate in women with the 3435TT allele of the ABCB1 gene was found. The homozygous TT variant in the fetus showed a probable association with an earlier response to ART and rhythm disruptions on the digoxin dosage reduction. high-performance liquid chromatography with tandem mass spectrometry (HPLC-MS/MS) methods for digoxin and sotalol therapeutic drug monitoring in blood plasma, amniotic fluid, and urine were developed. The digoxin and sotalol concentrations were determined in the plasma blood, urine, and amniotic fluid of 30 pregnant women at four time points (from the beginning of the transplacental antiarrhythmic therapy to delivery) and the plasma cord blood of 30 newborns. A high degree of correlation between the level of digoxin and sotalol in maternal and cord blood was found. The ratio of digoxin and sotalol in cord blood to maternal blood was 0.35 (0.27 and 0.46) and 1.0 (0.97 and 1.07), accordingly. The digoxin concentration in the blood of the fetus at the moment of the first rhythm recovery episode, 0.58 (0.46, 0.8) ng/mL, was below the therapeutic interval. This confirms the almost complete transplacental transfer of sotalol and the significant limitation in the case of digoxin. Previously, ABCB1/P-glycoprotein had been shown to limit fetal exposure to drugs. Further studies (including multicenter ones) to clarify the genetic features of the transplacental pharmacokinetics of antiarrhythmic drugs are needed.

Development of a new ultra‐high‐performance liquid chromatography–tandem mass spectrometry method for the determination of digoxin and digitoxin in plasma: Comparison with a clinical immunoassay

Cardiac glycosides digoxin and digitoxin are used in therapy for the treatment of congestive heart failure. Moreover, these compounds can be responsible for intoxication cases caused by fortuitous ingestion of leaves of Digitalis. Due to the narrow therapeutic range of these drugs, therapeutic drug monitoring is recommended in the clinical practice. In this context, immunoassays‐based methods are generally employed but digoxin‐ and digitoxin‐like compounds can interfere with the analysis. The aim of this study was to develop and validate an original UPLC–MS/MS method for the determination of digoxin and digitoxin in plasma. The method shows adequate sensitivity and selectivity with acceptable matrix effects and very good linearity, accuracy, precision, and recovery. A simple liquid–liquid extraction procedure was used for sample clean‐up. The method was applied for the analysis of n = 220 plasma samples collected in two different clinical chemistry laboratories and previously tested by the same immunoassay. The statistical comparison showed a relevant negative bias of the UPLC–MS/MS method versus the immunoassay. These results are consistent with an immunoassay overestimation of digoxin plasmatic levels due to cross‐reaction events with endogenous digoxin‐like substances.

Toad Poisoning: Clinical Characteristics and Outcomes

Objective

This study was performed to describe the clinical characteristics and outcomes of patients with toad poisoning in Thailand.

Methods

We carried out a retrospective study of patients with toad poisoning from the Ramathibodi Poison Center Toxic Exposure Surveillance System during a 5-year period (2012–2016).

Results

We studied 36 patients poisoned by toad toxin. The median age was 31 years. Most patients were male (66.7%) and had ingested toad meat (50%). The most common presentation was gastrointestinal (GI) symptoms with a median onset of 2 h after ingestion. Twelve patients presented with bradycardia; seven presented with shock and one with cardiac arrest. In the initial EKGs of all patients, the most common abnormality was sinus bradycardia.Two patients developed cardiac arrest early during management in the emergency room (within 15 minutes after ER arrival or within 4.5 h after ingestion). During admission, one patient developed sinus bradycardia, and two developed bradyarrhythmia; however, all three were stable. No tachyarrhythmias such as ventricular tachycardia were detected in any patient. Some patients (11.1%) presented with hyperkalemia. Serum digoxin was detected in five of seven patients tested, ranging from 0.43 to >8 ng/mL. Most patients (75%) were admitted to the hospital; the median duration of hospitalization was 2 d (range 0.5–5 d). The overall mortality rate was 8.3%, and all three patients that died ate toad meat and/or eggs and developed cardiac arrest. All patients received supportive with/without symptomatic care including GI decontamination, inotropic drugs, cardiac pacing, and management of hyperkalemia. One patient received intravenous calcium for hyperkalemia but did not develop dysrhythmia after calcium administration. One patient received digoxin-specific antibody fragments (DsFab), after which he clinically improved and was discharged.

Conclusion

Toad poisoning commonly caused GI symptoms and bradycardia. However, in severe cases, death occurred. Tachyarrhythmia was not observed. Supportive, symptomatic care might be the main therapies for this poisoning, especially if DsFab is not available.

The Botanical Glycoside Oleandrin Inhibits Human T-cell Leukemia Virus Type-1 Infectivity and Env-Dependent Virological Synapse Formation

At present, there are no antiretroviral drugs that inhibit incorporation of the envelope glycoprotein into newly-synthesized virus particles. The botanical glycoside, oleandrin, derived from extracts of Nerium oleander, has previously been shown to reduce the levels of the gp120 envelope glycoprotein on human immunodeficiency virus type-1 (HIV-1) particles and inhibit HIV-1 infectivity in vitro. We therefore tested whether oleandrin or an extract from N. oleander could also inhibit the infectivity of the human T-cell leukemia virus type-1 (HTLV-1): A related enveloped retrovirus and emerging tropical infectious agent. The treatment of HTLV-1+ lymphoma T-cells with either oleandrin or a N. oleander extract did not significantly inhibit viral replication or the release of p19^Gag-containing particles into the culture supernatants. However, the collected virus particles from treated cells exhibited reduced infectivity on primary human peripheral blood mononuclear cells (huPBMCs). Unlike HIV-1, extracellular HTLV-1 particles are poorly infectious and viral transmission typically occurs via direct intercellular interactions across a virological synapse. We therefore investigated whether oleandrin or a N. oleander extract could inhibit virus transmission from a GFP-expressing HTLV-1+ lymphoma T-cell-line to huPBMCs in co-culture assays. These results demonstrated that both oleandrin and the crude phytoextract inhibited the formation of virological synapses and the transmission of HTLV-1 in vitro. Importantly, these findings suggest oleandrin may have broad antiviral activity against enveloped viruses by reducing the incorporation of the envelope glycoprotein into mature particles, a stage of the infection cycle not targeted by modern HAART.

Bufotoxin poisoning that showed the sign of acute digitalis overdose in the patient of Kyushin® intoxication

Introduction:

Kyushin® is a widely used herbal medicine in East Asia for heart failure. Toad venom in present in Kyushin®, which has positive inotropic effect as digitoxin.

Case presentation:

An 81-year-old, female patient presented with decreased mental status after overdose of Kyushin®. The first electrocardiogram showed junctional tachycardia with 142/min, suddenly dropped to 27/min and followed by ventricular fibrillation. After one cycle of CPR, spontaneous circulation returned but junctional bradycardia, tachycardia, and ventricular fibrillation appeared. After six times of defibrillation, spontaneous circulation returned, and mechanical ventilator and transcutaneous pacing were applied. Plasma toxicology test revealed digitoxin 66.90 ng/mL by cloned enzyme donor immunoassay and digoxin 0.76 ng/mL by kinetic interaction of microparticles in solution immunoassay. After 8 h from presentation, the patient’s mental status came to be alert, and then transcutaneous pacing was removed.

Discussion:

Cloned enzyme donor immunoassay has been reported to be highly cross-reactive with digoxin-like substances, which strongly supports bufotoxin to be responsible for arrhythmia of the patient.

Conclusion:

Poisoning of bufotoxin in Kyushin® can cause cardiac arrhythmia, even arrest, but without digoxin-specific Fab, conventional therapy could be successful.

Characterization of bilirubin interference in three commonly used digoxin assays

BACKGROUND

Due to the narrow therapeutic range of digoxin, determining serum/plasma digoxin concentrations is critical for assessing patients with congestive heart failure, atrial fibrillation, and certain types of arrhythmias. However, digoxin quantification by competitive immunoassays is susceptible to interferences that may alter the accuracy of its measurement in patient plasma. This study aimed to characterize the extent of bilirubin interference in three commonly used digoxin immunoassays.

METHODS

Digoxin concentrations were compared using the Beckman Coulter® Unicel DxI 800, the Vitros® 4600, and the Roche Cobas® 8000 in neat or digoxin-spiked icteric and non-icetric plasma samples. A mixing study was performed to demonstrate how digoxin quantification is affected by bilirubin. An equation was derived that predicts the response of the DxI 800, given known bilirubin and digoxin concentrations.

RESULTS

The DxI reported detectable concentrations of digoxin in high bilirubin samples with no added digoxin, while the Vitros® 4600 and Cobas® 8000 gave virtually undetectable results. Spiking digoxin into samples with elevated bilirubin concentrations resulted in a higher percent recovery for the DxI 800 when compared to the other two platforms. The mixing study also revealed an increase in the percent recovery in the DxI 800, while the Vitros® 4600 and Cobas® 8000 were comparable to the expected concentration of digoxin.

CONCLUSIONS

The DxI 800 is most prone to interference by bilirubin, while the Vitros® 4600 and Cobas® 8000 are relatively unaffected. Icteric samples should be interpreted with caution if digoxin quantification is needed, especially on the DxI 800 assay.

Convallatoxin, the active cardiac glycoside of lily of the valley, minimally affects the ADVIA Centaur digoxin assay

OBJECTIVE

Lily of the valley is a poisonous plant due to the presence of the cardiac glycoside convallatoxin which is known to interfere with serum digoxin measurement using the LOCI digoxin assay and other digoxin assays. We evaluated potential interference of convallatoxin as well as extract of lily of the valley with the ADVIA Centaur digoxin assay by comparing results obtained using the LOCI digoxin assay.

MATERIALS AND METHODS

Aliquots of a drug-free serum pool and a digoxin serum pool were supplemented with nanograms to 1 μg quantities of convallatoxin or 1.0 and 2.5 μL of lily of the valley extract per milliliter of serum followed by measurement of digoxin concentrations using the LOCI and ADVIA Centaur digoxin assays.

RESULTS

Apparent digoxin concentrations were minimal using the ADVIA Centaur digoxin assay when aliquots of drug-free serum were supplemented with convallatoxin or extract of lily of the valley but apparent digoxin levels were very high using the LOCI digoxin assay. Moreover, minimal interference in serum digoxin measurement using the ADVIA Centaur digoxin assay was observed when aliquots of serum digoxin pool were further supplemented with lily of the valley extract. As expected, the LOCI digoxin assay showed significant interference of convallatoxin in serum digoxin measurement.

CONCLUSIONS

Significant interference of convallatoxin in serum digoxin measurement using the LOCI digoxin assay could be minimized using the ADVIA Centaur digoxin assay.

Heterogeneous and homogeneous immunoassays for drug analysis

Immunoassays are very useful techniques to perform screening and semi-quantitative analysis of hundreds of different xenobiotics. Small sample volumes are required and pretreatment is usually unnecessary (e.g., homogeneous immunoassays). Fully automated and high-throughput systems are available, which help physicians to take timely decisions. However, immunoassays do suffer from interference from both endogenous and exogenous factors that limit their application in quantitative analysis. These assays use different labels (e.g., colorimetric, fluorescent, chemiluminescent or electrochemiluminescent) and different methods for generating and measuring signals, but the basic principles are usually similar. This review outlines the practical aspects of immunoassays in bioanalysis and describes their application in clinical chemistry for xenobiotic analysis, namely medicines and drugs of abuse.

Rapid detection of the active cardiac glycoside convallatoxin of lily of the valley using LOCI digoxin assay

OBJECTIVES

To explore the luminescent oxygen channeling technology-based digoxin immunoassay (LOCI digoxin assay) for rapid detection of lily of the valley extract and convallatoxin. The potential in vitro binding of convallatoxin with Digibind was also evaluated.

METHODS

Aliquots of a drug-free serum pool and a digoxin serum pool were supplemented with lily of the valley extract or convallatoxin, and then apparent digoxin concentrations were measured using the LOCI digoxin assay. Mice were administered lily of the valley extract or 50 μg of convallatoxin, and digoxin concentrations in serum specimens were measured 1 and 2 hours after gavage. Aliquots of a serum pool supplemented with convallatoxin or lily of the valley extract were further supplemented with various concentrations of Digibind and free apparent digoxin concentrations were measured.

RESULTS

Apparent digoxin concentrations were observed when aliquots of a drug-free serum pool were supplemented with convallatoxin or lily of the valley extract, and also with convallatoxin or herbal extract. Bidirectional interference of convallatoxin and lily of the valley extract with serum digoxin measurement using the LOCI assay was also observed. Digibind was capable of binding convallatoxin in vitro.

CONCLUSIONS

LOCI digoxin assay can be used for rapid detection of convallatoxin, and Digibind can bind convallatoxin in vitro.

Clinically insignificant negative interferences of spironolactone, potassium canrenoate, and their common metabolite canrenone in new dimension vista LOCI digoxin immunoassay

Spironolactone, a potassium-sparing diuretic metabolized to canrenone is often used with digoxin to treat various conditions including congestive heart failure. Potassium canrenoate is a similar drug, which is also metabolized to canrenone. Due to reported both positive and negative interference of spironolactone, potassium canrenoate, and their common metabolite canrenone with digoxin immunoassays, we investigated potential interference of these compounds with the new homogenous sequential chemiluminescent assay for digoxin based on the luminescent oxygen channeling technology (LOCI digoxin) for application on the Dimension and Vista platform. When aliquots of a drug-free serum pool were supplemented with various amounts of spironolactone, potassium canrenoate, or canrenone and apparent digoxin values were measured using Dimension Vista LOCI digoxin assay, we observed no detected value except when aliquots were supplemented with very high amounts of potassium canrenoate or canrenone. However, we observed that apparent digoxin concentrations were very low. When aliquots of a serum digoxin pool (prepared by pooling specimens from patients receiving digoxin), were further supplemented with various amounts of spironolactone, potassium canrenoate, or canrenone and serum digoxin concentrations were remeasured using the LOCIdigoxin assay, only statistically significant falsely lower digoxin values (negative interference) were observed in specimens containing very high amounts of canrenone or potassium canrenoate. However, such small bias may not have any clinical significance. We conclude that new Dimension Vista LOCI digoxin assay is virtually free from interferences of spironolactone, potassium canrenoate, and their common metabolite canrenone.

Digitalis toxicity: a fading but crucial complication to recognize

Digoxin usage has decreased in the treatment of congestive heart failure and atrial fibrillation as a result of its inferiority to beta-adrenergic inhibitors and agents that interfere with the deleterious effects of the activated renin-angiotensin-aldosterone system. As a result of reduction of usage and dosage, glycoside toxicity has become an uncommon occurrence but may be overlooked when it does occur. Older age, female sex, low lean body mass, and renal insufficiency contribute to higher serum levels and enhanced risk for toxicity. Arrhythmias suggesting digoxin toxicity led to its recognition in the case presented here.

[A non-fatal Nerium oleander self-poisoning: case report and discussion]

Nerium oleander is potentially lethal plants after ingestion. We report a case of poisoning by these plants. Our patient complained of nausea, vomiting, and diarrhoea. He had bradycardia during first twelve hours. He was discharge after 3 days. All parts of these plants are toxic and contain a variety of cardiac glycosides including oleandrin. In most cases, clinical management of poisoning by N. oleander involves administration of activated charcoal and supportive care. Digoxin specific Fab fragments are an effective treatment.

Abbott ARCHITECT clinical chemistry and immunoassay systems: digoxin assays are free of interferences from spironolactone, potassium canrenoate, and their common metabolite canrenone

Spironolactone, which is metabolized to canrenone, is often used in combination with digoxin. Potassium canrenoate is a similar drug that is also metabolized to canrenone. As a result of reported interference of spironolactone, potassium canrenoate, and their common metabolite canrenone with digoxin immunoassays, we investigated potential interference of these compounds with two relatively new digoxin assays for application on ARCHITECT clinical chemistry platforms (cDig, particle-enhanced turbidimetric inhibition immunoassay) and ARCHITECT immunoassay platforms (iDig, chemiluminescent microparticle immunoassay), both from Abbott Diagnostics. When aliquots of drug-free serum pool were supplemented with various amounts of spironolactone, potassium canrenoate, and canrenone, no apparent digoxin concentration was observed using cDig assay on ARCHITECT c4000, c8000, and c16000 or iDig assay on i1000SR and i2000SR analyzers. In addition, we observed no false increase in serum digoxin value when aliquots of a digoxin pool were further supplemented with various amounts of spironolactone, potassium canrenoate, or canrenone. We conclude that both the cDig and iDig assays on the ARCHITECT analyzers are free from interferences by spironolactone, potassium canrenoate, and canrenone.

Effect of spironolactone, potassium canrenoate, and their common metabolite canrenone on Dimension Vista Digoxin Assay

Spironolactone, a potassium sparing diuretic metabolized to canrenone, is often used with digoxin to treat various conditions including congestive heart failure. Potassium canrenoate is a similar drug that is also metabolized to canrenone. Due to reported interference of spironolactone, potassium canrenoate, and their common metabolite canrenone with digoxin immunoassays, we investigated potential interference of these compounds with Dimension Vista Digoxin immunoassay using Flex reagent cartridge. Aliquots of a drug-free serum pool were supplemented with various amounts of spironolactone, potassium canrenoate, or canrenone and apparent digoxin values were measured using Dimension Vista digoxin assay, we observed none-detected value except when aliquots were supplemented with higher amounts of spironolactone or canrenone. Similarly, when aliquots of a serum digoxin pool (prepared by pooling specimens from patients receiving digoxin) where further supplemented with various amounts of spironolactone, potassium canrenoate, or canrenone, we observed moderately falsely elevated digoxin values only in specimens containing higher amounts of spironolactone or canrenone. We conclude that spironolactone and canrenone but not potassium canrenoate may cause modest interference with Dimension Vista digoxin assay but such interferences may not be clinically significant except with very high amounts of canrenone.

Free and total digoxin in serum during treatment of acute digoxin poisoning with Fab fragments: case study

A woman ingested 10 mg of methyldigoxin in a suicide attempt and presented 19 hours after ingestion with clinical signs of glycoside intoxication. Her serum level of digoxin was 7.4 ng/mL, and antidotal therapy with Fab antibody fragments was started. The manufacturer's recommended dosing scheme was modified, with 80 mg Fab administered intravenously within 15 minutes followed by a continuous infusion at 30 mg/h. Total serum concentration of digoxin increased markedly within minutes after Fab therapy was started, while the level of free digoxin immediately decreased into the nontoxic range without recrudescent toxic effects of digoxin. The cumulative amounts of free and bound digoxin that were excreted in urine within 30 hours after ingestion were 900 microg and 1600 microg, respectively. Half-life of bound digoxin in urine was 9.9 hours; mean rate of clearance of bound digoxin in the urine was 7.0 mL/min. On the basis of these kinetic data, a smaller initial bolus dose of Fab followed by a continuous infusion may be a more tailored, cost-effective, and relatively safe therapy for patients who have overdosed on cardiac glycosides.

Glomerular filtration rate estimation using the Cockcroft-Gault and modification of diet in renal disease formulas for digoxin dose adjustment in patients with heart failure

OBJECTIVE

The aim of this study was to compare the estimated glomerular filtration rate (GFR) using the Cockcroft-Gault and the 4-, 5-, and 6-variable Modification of Diet in Renal Disease (MDRD) formulas for digoxin dose adjustment.

METHODS

Steady-state serum digoxin concentrations were determined in 100 patients with heart failure and normal to severely impaired renal function. Total clearance (CL) and predicted average concentrations of digoxin were calculated using general pharmacokinetic principles.

RESULTS

The mean+/-SEM (median) estimated GFR values were 48.9+/-2.8 (46.5) mL/min/1.73 m(2) using the Cockcroft-Gault formula, 61.4+/-3.6 (56.4) mL/min/1.73 m(2) using the MDRD4 formula, 56.8+/-3.3 (52.1) mL/min/1.73 m(2) using the MDRD5 formula, and 53.3+/-3.0 (48.7) mL/min/1.73 m(2) using the MDRD6 formula, with high correlation coefficients between the estimates (r > or = 0.928, P < 0.001). Significant correlations were found between the digoxin total CL and estimated GFR by the Cockcroft-Gault (r = 0.649, P < 0.001), MDRD4 (r = 0.634, P <0.001), MDRD5 (r = 0.635, P < 0.001), and MDRD6 (r = 0.652, P < 0.001) formulas. A significant negative correlation of the digoxin total CL/GFR ratio with estimated GFR was obtained (r = -0.356, P < 0.001), with a high variability for this ratio for GFR lower than 60 mL/min. Analogous correlation coefficients were found between the obtained and predicted digoxin concentrations calculated using the estimated GFR by the Cockcroft-Gault (r = 0.628, P < 0.001), MDRD4 (r = 0.642, P < 0.001), MDRD5 (r = 0.650, P < 0.001), and MDRD6 (r = 0.665, P < 0.001) formulas, with a wide dispersion between the values in all cases.

CONCLUSION

For GFR lower than 60 mL/min, the high interindividual variation of the digoxin total CL found among patients with similar renal function is an important limiting factor in the prediction of digoxin dosage regimens.

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.

Measuring the unbound concentration fails to resolve analytic interferences in digoxin immunoassays

Therapeutic drug monitoring of digoxin is well established in the clinical management of cardiac patients treated with the drug. Recently, target concentrations have been revised in patients with congestive heart failure to 0.5 to 0.8 microg/L, challenging the sensitivity limits of most immunoassays. These widely used methods are often criticized, particularly on specificity grounds resulting from interference from exogenous and endogenous sources. One solution to remove higher molecular weight interference has been to ultrafilter plasma samples before assaying. The present study included 261 patient digoxin samples and compared two commercial ultrafiltration devices (Centrifree and Micrcon) that share the same separation membrane (YM-30). The results showed widely discordant apparent unbound digoxin concentrations in the ultrafiltrate from these devices with a Deming regression line of Centrifree = 1.31 x Micrcon + 0.042 (95% confidence interval for slope of 1.237 to 1.391) and apparent unbound fractions ranging from 15% to 610% of the unfiltered plasma digoxin concentrations, suggesting that ultrafiltration did not resolve such interference issues. The concept of measuring lower unbound digoxin concentrations as a result of lower therapeutic range for total (bound plus unbound) digoxin will also render most immunoassays insensitive and inappropriately calibrated. There is a strong imperative to review digoxin monitoring practices in the light of current clinical imperatives for both specificity and sensitivity reasons.

Determination of Digoxin in Human Serum Using Stable Isotope Dilution Liquid Chromatography/Electrospray Ionization-Tandem Mass Spectrometry

A method for the determination of digoxin in human serum using a liquid chromatography/electrospray ionization-tandem mass spectrometry (LC/ESI-MS/MS) technique is reported. Digoxin and the internal standard, [21,21,22-(2)H(3)]digoxin, were extracted from 250 mul of human serum using a solid phase extraction cartridge (Oasis HLB) and analyzed by LC/ESI-MS/MS in the selected reaction monitoring mode. The intra- and inter-assay reproducibility and accuracy were satisfactory within the quantification range of 0.20-3.20 ng/ml. The concentrations of digoxin in the serum samples obtained from digitalized patients (n=19) were in the range of 0.25-2.84 ng/ml, which were compared to those obtained by radioimmunoassay.