Digoxin-like Immunoreactive Substance Research Articles

Determining the Risk of Elevated Digoxin Concentrations Following Loading Dose in Patients With Acute and Chronic Kidney Disease.

The optimal loading dose of digoxin in patients with reduced kidney function is unknown. Tertiary references recommend reduced loading doses; however, these recommendations are based on immunoassays that are falsely elevated by the presence of digoxin-like immunoreactive substances, a problem that is minimized in modern assays. To determine whether chronic kidney disease (CKD) or acute kidney injury (AKI) is associated with supratherapeutic digoxin concentrations after a digoxin loading dose. A retrospective analysis on patients who received an intravenous loading dose of digoxin with a digoxin concentration collected 6 to 24 hours after the end of the dose. Patients were stratified into 3 groups: AKI, CKD, and non-AKI/CKD (NKI) based on glomerular filtration rate and serum creatinine. The primary outcome was frequency of supratherapeutic digoxin concentrations (>2 ng/mL) and secondary outcomes included frequency of adverse events. A total of 146 digoxin concentrations were included (AKI = 59, CKD = 16, NKI = 71). Frequencies of supratherapeutic concentrations were similar between groups (AKI: 10.2%, CKD: 18.8%, NKI: 11.3%; P = 0.61). Pre-planned logistic regression demonstrated no significant relationship between kidney function group and the development of a supratherapeutic concentration (AKI: odds ratio [OR]: 1.3, 95% confidence interval [CI]: 0.4-4.5; CKD: OR 4.3, 95% CI: 0.7-23). This is the first study in routine clinical practice evaluating the relationship between kidney function and digoxin peak concentrations that differentiates AKI from CKD. We did not find a relationship between kidney function and peak concentrations; however, the group with CKD was underpowered.

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

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

Interference of enzalutamide on digoxin immunoassay: A case report

Androgen sensitive prostate cancer is a malignancy that may be treated with hormonal therapy. Enzalutamide, an androgen receptor antagonist, is indicated in patients with metastatic castration-resistant prostate cancer. The drug inhibits translocation of activated androgen receptors into the cell nucleus and prevents DNA binding, thereby inducing cell death and tumour regression. Digoxin is a widely used cardiac glycoside to treat heart failure and reduce ventricular rate in atrial fibrillation. Therapeutic drug monitoring is essential due to its narrow therapeutic index. However, digoxin assays can be affected by many substances including digoxin-like immunoreactive substances (DLIS), medications such as spironolactone, canrenoate, various Chinese medicines and anti-digoxin antibody. We present a case of falsely elevated digoxin levels due to enzalutamide interaction with the chemiluminescent microparticle immunoassay.

Role of Digoxin-Like Immunoreactive Substance in the Pathogenesis of Transient Tachypnea of Newborn

Background. Transient tachypnea of newborn (TTN) is usually observed in term or near-term infants. It constitutes an important part of the respiratory distress cases observed in the neonatal intensive care unit (NICU). Aim. This paper examines the effects of digoxin-like immunoreactive substance (DLIS) on fluid and ion balance, hemodynamic and echocardiographic parameters of neonates with TTN. Methods. Plasma DLIS, Na+, K+, urea, creatinine, serum and urine osmolarity, urine FeNa+, 24-hour urine output, echocardiographic investigation and mean blood pressure, and clinical parameters of disease severity were recorded in TTN group and compared with control on the 1st and 7th days of their lives. Results. Plasma DLIS levels were statistically higher in TTN group (0.66 ± 0.37 ng/mL) compared to control group (0.24 ± 0.20 ng/mL) both on the 1st day (P < 0.01) and the 7th day (P < 0.05). For TTN group, significant correlation was found between plasma DLIS levels and maximum respiratory rate, duration of tachypnea, and length of hospitalization on the 1st day. Plasma DLIS levels were correlated negatively with serum osmolarity levels. Plasma DLIS levels were positively correlated with urine output, urinary FeNa+ levels, cardiac output, left ventricles end diastolic diameters, and right ventricles end diastolic diameters. Conclusions. Increased DLIS levels were correlated with disease severity in cases with TTN. This increase may be a primary or secondary event in the disease progress. It may help reduce the fluid overload due to already disturbed cardiac functions in patients by increasing urine output and natriuresis; however it may also contribute to disease pathogenesis, by inhibiting alveolar Na+-K+-ATPase which further decreases fetal alveolar fluid resorption.

&lt;span class="caption"&gt;TECHNICAL REPORT:&lt;/span&gt; Investigation of Possible Interference by Digoxin-Like Immunoreactive Substances on the Architect� &lt;i&gt;i&lt;/i&gt;Digoxin CMIA in Serum Samples from Pregnant Women, and Patients with Liver Disease, Renal Insufficiency, Critical Illness, and Kidney and Liver Transplant

Although the possible interference of digoxin-like immunoreactive substances (DLIS) on the Architect iDigoxin chemiluminiscent microparticle immunoassay (CMIA) has been emphasized by the manufacturer, a specific study about this subject has still not been carried out. Apparent serum digoxin concentrations were determined using the Architect iDigoxin CMIA from Abbott Laboratories in digoxin-free pregnant women (n = 50), and patients with liver disease (n = 50), renal insufficiency (n = 50), kidney (n = 25) or liver (n = 25) transplant, and critical illness (n = 50). In all of the patients included in this study, apparent serum digoxin concentrations were lower than the correspondent quantification limit (< 0.30 microg/L). The Architect iDigoxin CMIA assay would be relatively free from endogenous DLIS positive interferences.

In vitro selection and characterization of deoxyribonucleic acid aptamers for digoxin

The low therapeutic index of digoxin necessitates careful monitoring of its serum levels. Most of digoxin immunoassays suffer from interferences with digoxin-like immunoreactive substances. Since aptamers have been shown to be highly specific for their targets, the aim of this study was to develop DNA aptamers for this widely used cardiac glycoside. Digoxin was coated onto the surface of streptavidin magnetic beads. DNA aptamers against digoxin were designed using Systematic Evolution of Ligands by Exponential enrichment method (SELEX) by 11 iterative rounds of incubation of digoxin-coated streptavidin magnetic beads with synthetic DNA library, DNA elution, electrophoresis and PCR amplification. The PCR product was cloned and sequenced. Binding affinity was determined using digoxin–BSA conjugate, coated onto ELISA plate. Inhibitory effect of anti-digoxin aptamer was conducted using isolated guinea-pig atrium. Three aptamers (D1, D2 and D3) were identified. Binding studies of fluorescein-labeled truncated (without primer binding region) D1 and D2 and full length D1 anti-digoxin aptamers were performed and their corresponding dissociation constants values were 8.2×10−9, 44.0×10−9 and 17.8×10−9M, respectively. This is comparable to what other workers have obtained for interaction of monoclonal antibodies raised against digoxin. There was little difference in binding affinity between full length and truncated anti-digoxin D1 aptamer. D1 anti-digoxin aptamer also inhibited the effects of digoxin on the isolated guinea-pig atrium. D1 anti-digoxin aptamer distinguished between digoxin and ouabain in both tissue study and binding experiments. Our finding indicated that D1 anti-digoxin aptamer can selectively bind to digoxin. Further studies might show its suitability for use in digoxin assays and as a therapeutic agent in life-threatening digoxin toxicity.

Erythrocyte Na +–Li + counter-transport activity and digoxin-like substances in insulin dependent diabetic women with preexisting preeclampsia

Aim of the study To determine whether there is pathogenetic link between red cells sodium–lithium counter-transport activity and digoxin-like immunoreactive substances (DLIS) in plasma of insulin-dependent diabetic (IDDM) and non-diabetic women with preexisting preeclampsia (PE). Subjects and methods We studied Na +/Li + CT activity in red cells and plasma levels of DLIS in 11 IDDM women with preexisting PE (Group 1), 13 IDDM without preexisting PE (Group 2) 23 non-diabetic women with preexisting PE (Group 3) and 12 non-diabetic women with normal pregnancy (Group 4) at least 4 months after delivery. Results Na +/Li + CT activity was higher in Group 1 compared to Group 2 (mean ± SEM 0.316 ± 0.05 vs 0.190 ± 0.02 mmol/LRBC/hr p < 0.05) and in Group 3 compared to Group 4 (0.365 ± 0.004 vs 0.168 ± 0.01 mmol/LRBC/hr, p < 0.01). Plasma levels of DLIS were higher in Group 3 compared to Group 4 (0.727 ± 0.189 vs 0.295 ± 0.066 ng/ml; p < 0.05); there was no statistically significant difference between the two diabetic groups. In Groups 1 and 3, Na +/Li + CT activity was correlated to the plasma levels of DLIS ( r = 0.927; p < 0.001 and r = 0.485; p < 0.05 respectively). Conclusion Increased Na +/Li + CT activity and increased plasma levels of DLIS may contribute to PE in IDDM and non-diabetic women.

Ligand-Binding Assays: Old Challenges, New Solutions

BioanalysisVol. 3, No. 6 Special Focus: PK assays for biotherapeutics - EditorialLigand-binding assays: old challenges, new solutionsPhilip OldfieldPhilip Oldfield400 Bellerive, Hudson, Quebec, Canada, J0P 1H0. Search for more papers by this authorEmail the corresponding author at philip.oldfield@POldfieldBC.comPublished Online:18 Mar 2011https://doi.org/10.4155/bio.11.29AboutSectionsView ArticleView Full TextPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack CitationsPermissionsReprints ShareShare onFacebookTwitterLinkedInRedditEmail View articleBibliography1 YallowR, Berson S. Immunoassay of endogenous plasma insulin in man. J. Clin. Investig., 39,1157–1175 (1960).Crossref, Medline, Google Scholar2 Toseland P, Oldfield PR Lawson AM. Tentative identification of a digoxin-like immunoreactive substance. Therapeutic Drug Monitoring10(2),168–171 (1988).Crossref, Medline, CAS, Google Scholar3 Kohler G, Milstein C. Continuous cultures of fused cells secreting antibody of predefined specificity. Nature256,495 (1975).Crossref, Medline, CAS, Google Scholar4 Claassen M, van Zyl GU, Korsman SN et al. Pitfalls with rapid HIV antibody testing in HIV infected children in the Western Cape, SA. J. Clin. Virol.37,68–71 (2006).Crossref, Medline, CAS, Google Scholar5 Moreau E, Philippe J, Couvent S, Leroux-Roels G. Interference of soluble TNF-α receptors in immunological detection of TNF-α. Clin. Chem.42(9),1450–1453 (1996).Crossref, Medline, CAS, Google Scholar6 Harries M, Smith I. The development and clinical use of trastuzumab (Herceptin®). Endocrine-Related Cancer9,75–85 (2002).Crossref, Medline, CAS, Google Scholar101 Luminex Corporation. www.luminexcorp.comGoogle Scholar102 Meso Scale Discovery. www.mesoscale.com/CatalogSystemWeb/WebRootGoogle Scholar103 Gyros US Inc. www.gyros.com/en/home/index.htmlGoogle Scholar104 ANP Technologies Inc. www.anptinc.comGoogle Scholar105 Farfield Sensors Ltd. www.farfield-group.comGoogle Scholar106 ForteBio Inc. www.fortebio.comGoogle ScholarFiguresReferencesRelatedDetailsCited BySensitive glucagon quantification by immunochemical and LC–MS/MS methodsVeniamin N Lapko, Patrick S Miller, G Paul Brown, Rafiqul Islam, Sarah K Peters, Richard L Sukovaty, Peggy F Ruhn & Chris J Kafonek3 December 2013 | Bioanalysis, Vol. 5, No. 23 Vol. 3, No. 6 Follow us on social media for the latest updates Metrics Downloaded 657 times History Published online 18 March 2011 Published in print March 2011 Information© Future Science LtdFinancial & competing interests disclosureThe author has no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.No writing assistance was utilized in the production of this manuscript.PDF download

Discordant Results from “Real-World” Patient Samples Assayed for Digoxin

Digoxin has a narrow therapeutic index and is primarily renally eliminated. To optimize dosing of digoxin, therapeutic drug monitoring has been important since assays became available in the 1970s. Immunoassays are not specific, and cross-reactivity with endogenous and exogenous compounds has been reported for more than 20 years. Interassay concordance has not been investigated in recent years in "real-world" patient samples. To identify whether different digoxin immunoassays produce clinically different results in real-world situations, estimate the frequency of discordance, and determine whether an equation-based estimate compares well with digoxin immunoassays. Plasma samples were sent to 2 accredited laboratories simultaneously and the digoxin results were compared. Results of immunoassays conducted using the Cedia DRI Digoxin Assay and the DGNA Digoxin Assay were compared with an equation-based estimate of plasma digoxin concentration. Thirty-six digoxin samples were assayed; in 39% of these, digoxin concentrations were discordant and different dosage adjustments would have followed. The presence of digoxin-like immunoreactive substances may explain some of this discordance. The mean of the equation-based result was similar to the immunoassay results, but marked variability was evident. The DGNA assay produced higher results on 24 samples; 9 higher values occurred with the DRI method. Commercial digoxin immunoassays frequently produce clinically significant discordant results. The equation-based estimate does not appear to be an acceptable alternative to therapeutic drug monitoring. Immunoassay manufacturers should be required to improve assay performance by including real-world blood samples in development and clinicians should consider digoxin assay results warily.

Suspected DLIS Interference in the Dimension DGNA Digoxin Assay Method and the Clinical Application of the Revised Digoxin Target Range

The authors present a case of an elderly female patient with heart failure and renal dysfunction treated with digoxin, where 2 commercial immunoassay methods (DRI, Microgenics, and DGNA, Dade Behring) showed a clinically very significant discrepancy on the same plasma sample, viz. 0.5 and 2.3 nmol/L, respectively. The sample was also referred to a third external laboratory that returned a result of 0.9 nmol/L using mFPIA (AxSYM, Abbott). Subsequent ultrafiltration (30,000 Dalton) on the sample essentially eliminated the difference, suggesting an interference from a large molecular weight compound(s), potentially the well-described digoxin-like immunoreactive substance(s) (DLIS). Although further study is required to verify that the DLIS implicated was indeed the interfering species, it does again highlight the importance of careful method selection in the clinical therapeutic drug monitoring laboratory to ensure that such well-established potential problems do not result in inappropriate dosage reduction with consequent lack of adequate drug exposure and serious clinical sequelae.

Therapeutic Drug Monitoring of Digoxin

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.

Changing Characteristics of the TDx Digoxin II Assay in Detecting Bufadienolides in a Traditional Chinese Medicine

In 1992, apparent digoxin concentrations determined by the Abbott TDx II assay 5 hours after the ingestion of 10 pills of traditional Chinese medicine containing toad secretions (chan su) by 7 volunteers, yielded results that were equimolar to bufalin measured by 2 in-house bufalin radioimmunoassays (RIAs). Recently, a 17-year-old Chinese female unintentionally took 100 (instead of 10) of these pills for a sore throat but suffered no ill effects. The blood bufalin concentration at 3 hours by 1 of the 2 RIAs was 10.93 nmol/L, which was commensurate with the dose. However, the apparent digoxin measured by a TDx II assay produced in 2004 was only 3.08 nmol/L, which probably reflects the change in the specificity of the polyclonal digoxin antisera used in the assay over the years. In 1989, the TDx assay was commended for its ability to detect poisoning from plant and animal cardenolides, a property that seems to be waning and, thus, bad news for those wishing to use the assay to detect alternative cardenolides. But, on the other hand, it possibly eliminates the "specter" of digoxin-like immunoreactive substance (DLIS) that has afflicted some digoxin assays, which can only be good news.

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.

Digoxin-Like Immunoreactive Substance in Nonoliguric Hyperkalemia of the Premature Infant

Nonoliguric hyperkalemia of premature infants probably results from a transient inhibition of membrane-bound Na⁺/K⁺-ATPase during the first 24 h after birth. We hypothesized that the endogenous digitalis-like activity of the serum of premature infants, which inhibits the Na⁺/K⁺-ATPase, triggered hyperkalemia. Serum concentrations of potassium ([K⁺]) and of the digoxin-like immunoreactive substance ([DLIS]) were measured during the first 24 h after birth in 60 infants including 30 infants <30 gestational weeks. Contrary to our hypothesis, there was a negative linear correlation between [DLIS] at birth and [K⁺] 24 h after birth (r² = 0.24, p < 0.002). 24 h after birth there was no correlation between [DLIS] and [K⁺]. Thus, a major role of DLIS in nonoliguric hyperkalemia could not be established.

Effect of Asian and Siberian Ginseng on Serum Digoxin Measurement by Five Digoxin Immunoassays

Asian and Siberian ginsengs contain glycosides with structural similarities to digoxin. We studied potential interference of ginseng in 5 digoxin immunoassays in 3 Asian (2 liquid extracts, 1 capsule) and 3 Siberian ginseng preparations (1 liquid extract, 2 capsules). With the fluorescence polarization immunoassay (FPIA), we observed apparent digoxin activity in 1 Asian liquid preparation and in the liquid extract and 1 capsule form of Siberian ginseng. In mice fed ginseng, we observed digoxin activities in the serum (Asian, 0.48–0.68 ng/mL [0.6–0.9 nmol/L]; Siberian, 0.20–0.47 ng/mL [0.3–0.6 nmol/L]), indicating that such interferences also occur in vivo. Serum pools prepared from samples from patients receiving digoxin and then supplemented with Asian or Siberian ginseng showed falsely increased digoxin values using the FPIA (eg, for Asian ginseng, 1.54 ng/mL [2.0 nmol/L] vs control value, 1.10 ng/mL [1.4 nmol/L]) and falsely decreased values using the microparticle enzyme immunoassay (MEIA; 0.73 ng/mL [0.9 nmol/L] vs control value, 1.04 ng/mL [1.3 nmol/L]). Digoxin-like immunoreactive substances (DLISs) showed synergistic effects with ginsengs in interfering with the FPIA and MEIA for digoxin. No interference was observed with 3 other digoxin assays, even in the presence of elevated DLISs.

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.

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.

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.

Plasma and cerebrospinal fluid endogenous digoxin-like immunoreactivity in patients with aneurysmal subarachnoid haemorrhage.

Recent evidence indicates the presence of naturally occurring digitalis-like compounds in mammals, collectively known as either digitalis-like (DLF) or ouabain-like (OLF) factors, presumed to be endogenous hormones regulating the biological activity of the NA+/ K(+)-ATPase and its isoforms. This substance has been postulated to enhance renal tubular sodium excretion and to increase peripheral vascular resistance. Digoxin-like immunoreactive substance (DLIS) was observed in plasma of some patients with spontaneous subarachnoid haemorrhage (SSAH). Accumulating evidence suggests the central nervous system as a site of synthesis, but also as a site of hypertensinogenic action of endogenous cardioglycosides. The present study intends to establish the ratio of the DLIS in plasma to that in cerebrospinal fluid (CSF) in patients with SSAH and to investigate possible connection of this substance with development of arterial vasospasm. A prospective analysis of DLIS levels was performed on plasma and CSF samples obtained in 40 patients who had suffered a recent SSAH. DLIS levels were determined by the fluorescence polarisation immuno-assay method immediately after the admission to the Ward, and again seven days later. The comparison of CSF and plasma DLIS levels did not show statistically significant differences between the results--neither for the first (Z = 0.530; P = 0.591) nor for the seventh day after the disease onset (Z = 0.448; P = 0.654). Three possible hypothetical explanations of these results are offered: a) substance determined by digoxin immuno-assay has no essential likeness to digoxin; b) loss of the haemato-encephalic barrier integrity enabling free substance exchange between plasma and central nervous system; c) digoxin-like substance production within the central nervous system. Further, comparison of DLIS plasma levels (7th day from onset of SSAH) with angiography results showed that patients with multiple vasospasm had essentially higher plasma DLIS levels compared to patients with no vasospasms (Z = 2.59; P = 0.0097). The amount of extravasated blood, assessed on the basis of cranial CT scanning, was also connected with higher plasma DLIS levels (X2 = 3.29; P = 0.0305). The enhanced arterial narrowing which occurs in SSAH may be in part mediated by increased digitalis-like factor activity.

Postmortem digoxin-like immunoreactive substances (DLIS) in patients not treated with digoxin.

Endogenous digoxin-like immunoreactive substances (DLIS) cross-react in immunoassays of digoxin. The postmortem rise in digoxin levels in patients treated with the drug may be due to its redistribution. It is unclear what is the contribution of DLIS to this increase and whether DLIS are present postmortem in patients not treated with digoxin. The objectives of this study were to determine whether DLIS are present after death in patients not treated with digoxin, whether a postmortem increase in DLIS is detectable and whether sampling site can affect DLIS concentrations. DLIS (measured as digoxin, TDx Abott) were determined in blood samples drawn antemortem from ICU patients; postmortem samples from femoral artery and cardiac chambers were taken at least 12 h after the death of these same patients. DLIS concentrations > or = 0.2 ng/ml were measured in 44 and 40% of patients antemortem and postmortem (femoral), respectively. No difference was found in DLIS levels between antemortem and postmortem femoral and cardiac samples. Age, ICU stay and postmortem sampling time did not affect the postmortem increase in DLIS. None of the levels was in the toxic range. DLIS may be present after death and their concentration does not increase postmortem. The interpretation of postmortem digoxin concentrations that fall in the therapeutic range should be done cautiously; such measurable levels do not necessarily indicate misuse or malicious intent even in patients who had not been treated with the drug.