Enzyme-multiplied immunoassay (EMIT 2000) digoxin assay compared with fluorescence polarization immunoassay and amerlex 125I-radioimmunoassay at two Australian centers

Management of digoxin therapy using pharmacokinetics in a patient undergoing continuous venovenous hemofiltration

PURPOSE

The management of digoxin therapy using pharmacokinetics in a patient undergoing continuous venovenous hemofiltration (CVVH) is reported.

SUMMARY

A 46-year-old African-American woman with New York Heart Association class IV, American College of Cardiology- American Heart Association stage D heart failure arrived from an outside facility with complaints of dyspnea after minimal exertion, orthopnea, and lower-extremity edema. A transthoracic echocardiogram revealed an estimated left ventricular ejection fraction of 15%. The patient subsequently required left ventricular assist device placement on hospital day 5 as a potential bridge to transplantation. A total digoxin loading dose of 500 μg i.v. (8.2 μg/kg) was given in two divided doses six hours apart. The next morning, the serum digoxin concentration was 1.9 ng/mL, and the patient was started on a maintenance digoxin dosage of 125 μg i.v. daily. On postoperative day (POD) 20, the patient developed acute kidney injury, and CVVH was initiated. The sieving coefficient (Sc), transmembrane clearance (CLtm), digoxin concentration in ultrafiltration fluid (Cuf), and need for supplemental digoxin were determined to account for CVVH- associated digoxin loss. After 14 days of CVVH, the patient's clinical condition improved, and CVVH was transitioned to intermittent hemodialysis. On POD 66, the patient was discharged to an extended-care facility without adverse reactions related to digoxin therapy.

CONCLUSION

Analysis of serum digoxin concentration and digoxin Cuf values suggested that digoxin was cleared by CVVH, allowed calculation of Sc and CLtm values, and facilitated determination of digoxin requirements in a critically ill patient requiring CVVH.

[Evaluation of Viva-E drug testing system]

BACKGROUND

The importance and usefulness of therapeutic drug monitoring (TDM) have been emphasized, and analysis of drugs has been increased in clinical laboratories. We evaluated the analytical performance and clinical usefulness of a recently introduced enzyme multiplied immunoassay instrument, Viva-E Drug Testing System (Dade Behring Inc., USA).

METHODS

Using patients' samples and quality control material, we evaluated the analytical performance of Viva-E for a total of 11 drugs (cyclosporine, tacrolimus, mycophenolic acid, valproic acid, digoxin, theophylline, carbamazepine, phenytoin, phenobarbital, vancomycin, and gentamicin) with respect to linearity, precision, and correlations with other methods according to CLSI guidelines. Cobas Integra 800 (Roche Diagnostics, Switzerland) and API 4000 LC-MS/MS System (Applied Biosystems, USA) were used to make a comparison. In addition, we analyzed analysis time.

RESULTS

Viva-E showed a good linearity (r2 >or= 0.97) for all items. Within-run CVs were within 5% and total CVs were within 10% for all drugs except for tacrolimus and digoxin at low concentrations. The system correlated well with the other methods (r=0.9283-0.9778). The time required for reporting the first sample was 11 min and the analysis time was 1.1 min.

CONCLUSIONS

Since Viva-E showed a good analytical performance required for TDM in its linearity, precision, and accuracy with its wide drug menus including cyclosporine, tacrolimus, and mycophenolic acid, stat and random accessing functions, and the consolidation to a single workstation, it could be very useful in the clinical laboratory for various needs.

Diagnosis and management of cerebral salt wasting (CSW) in children: the role of atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP)

OBJECTIVE

The aim of this study is to report our experience with diagnosis and management of cerebral salt wasting (CSW) in children and to evaluate the role of atrial natriuretic peptide/brain natriuretic peptide (ANP/BNP) in pediatric patients.

MATERIALS AND METHODS

We present nine children suffering from prevalent cerebral disease--seven of whom underwent anesthesia and surgical procedures--with features of CSW, seen within a 22-month period. The symptoms, patient characteristics (including hormone status), monitoring, treatment protocol, and outcome are described.

RESULTS

Natriuresis (urine Na+ concentrations 131 to >250 mmol/l) and polyuria (5.5+/-1.5 ml/kg/h) with increased Na+ turnover (maximum Na+ loss: median 1.50 mmol Na+/kg/h, range 0.47 to >3.50) vanished within 2 weeks in 6/9 patients (increase in serum Na+ from 127+/-2 mmol/l to 136+/-1). K+ excretion was also high (maximum K+ loss: median 0.18 mmol K+/kg/h, range 0.09-0.53). ANP/BNP as suspected causes of salt wasting were elevated only in 1/6 and 2/7 patients, respectively. Plasma renin activities and aldosterone levels were either suppressed or in the low normal range.

CONCLUSION

Natriuresis and polyuria are the main diagnostic criteria for CSW. The fluid balance in CSW is negative, in contrast to a positive fluid balance in SIADH. The length of the disease is self-limited and generally ceases within 2 weeks, while Na+, K+, and fluid turnover should be monitored carefully. Only a minority of our children showed elevated ANP/BNP levels. A dose/effect relationship for natriuretic peptide levels and increased Na+ turnover could not be established.

Properties of Novel Anti-digoxin Antisera in Radioimmunoassay Using Homologous and Site Heterologous Tritium-Labeled Antigens Involving a [3H]-Leucine Moiety

The specificities of antisera against digoxin C-3' or C-3'' hemisuccinate-bovine serum albumin (BSA) conjugate were assessed by cross-reactivity studies with digoxin metabolites by radioimmunoassay (RIA) using the homologous and the site heterologous tritium-labeled antigens. One of the tracers used was digoxin 3'-hemisuccinyl-[3H]-leucine; the other was digoxin 3''-hemisuccinyl-[3H]-leucine, which had been prepared from digoxin 3''-hemisuccinate. When the tracer with [3H]-leucine at the C-3' position was used, antisera (I-1, I-3) elicited by digoxin 3'-hemisuccinate-BSA conjugate showed the following cross-reactivity: digoxigenin bisdigitoxoside (0.34%, 76%), digoxigenin monodigitoxoside (0.11%, 65%), digoxigenin (0.02%, 26%) and dihydrodigoxin (9.4%, 1.2%). However, when using the homologous antigen, antiserum (I-1) was highly specific against the digitoxose chain. When the site heterologous antigen, digoxin 3''-hemisuccinyl-[3H]-leucine was combined, this antiserum showed high cross-reactivity to digoxin degradation products. This digoxin RIA using antiserum (I-1) with the homologous antigen measures unmetabolized digoxin. On the other hand, the RIA system using antiserum (I-3) with the homologous antigen had cross-reactivity with the metabolites in accordance with their relative cardio-activities, so this system would be useful in therapeutic drug monitoring of 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.

Development of radioimmunoassay for measurement of serum digoxin in digitalized patients using novel anti-digoxin antiserum

There is an antiserum elicited by digoxin 3'-hemisuccinate-bovine serum albumin (BSA) conjugate possessing high specificity for digoxin. Our study focused on development of RIA using this novel antiserum for measurement of digoxin in serum from digitalized patients. The property of the new antiserum was investigated by RIA with digoxin 3'-hemisuccinyl-[3H]leucine. The separation of bound and free fractions was performed using a dextran-coated charcoal suspension. The new antiserum bound approximately 50% of digoxin 3'-hemisuccinyl-[3H]leucine with a final dilution of 1:30000. The intra- and inter-assay coefficients of variation were <9% in the range of 0.52-4.17 ng/ml. The mean digoxin concentration in serum samples (n=35) from digitalized patients was estimated to be 0.68 ng/ml, which was lower than its measurement of digoxin with the commercial antidigoxin BSA serum and monoclonal anti-digoxin. It is apparent that the RIA described here has sufficient precision. The RIA system was available for the measurement of digoxin in serum from digitalized patients.

Digoxin Assays: Frequent, Substantial, and Potentially Dangerous Interference by Spironolactone, Canrenone, and Other Steroids

Background: A case of digoxin toxicity resulted from falsely low values with the MEIA II assay for digoxin (AxSYM®; Abbott). The low results were caused by negative interference from canrenone and spironolactone, the latter of which has recently been advocated for the treatment of severe heart failure. Analytical interference from spironolactone has been reported, but little information is available for this effect with newer digoxin assays.

Methods: We examined nine assays (AxSYM, IMx®, TDx®, Emit®, Dimension®, aca®, TinaQuant®, Elecsys®, and Vitros®) for interference by spironolactone, canrenone, and three metabolites. Additionally, all routine digoxin measurements (AxSYM) over a period of 16.5 months (n = 3089) were monitored for interference.

Results: Suppression of the expected values by canrenone (3125 μg/L) was observed for the AxSYM (42% of expected value), IMx (51%), and Dimension (78%) assays. A positive bias was observed for the aca (0.7 μg/L), the TDx (0.62 μg/L), and the Elecsys (&gt;0.58 μg/L). Twenty-five of 669 routinely monitored patients had falsely low results. Nineteen of these had potentially toxic concentrations of digoxin (Emit; &gt;2.0 μg/L), although the AxSYM assay indicated therapeutic or less severe toxic concentrations (Δmax = 7.1 μg/L). Except for two unresolved cases, this was attributable to spironolactone, canrenone, hydrocortisone, or prednisolone. Standard doses of spironolactone (up to 50 mg/day) in patients with heart failure displayed inhibition &lt;11%.

Conclusions: The frequency and magnitude of the false-negative results particularly compromise the use of both microparticle enzyme immunoassays. Not only may toxic concentrations remain unidentified, but intoxication could occur should dosage be increased because of falsely low results. With 11 million digoxin tests/year ordered in the US, conceivably many patients could be adversely affected.

Improved sensitivity of digoxin assay by modification of the EMIT 2000 method

A modified EMIT 2000 digoxin assay was developed on the Cobas Mira plus analyzer for the determination of very low plasma concentrations of the drug. The major modifications were a higher plasma volume withdrawn during the analysis step and calibration curves constructed in the range 0-2 ng/ml using calibrators made up with biological matrix. Assays were controlled with an internal, four-level quality control (targets: 0.15; 0.60; 1.70; 2.70 ng/mL). The within-day and day-to-day mean observed values +/- SD (n = 10) of these quality controls were 0.14 +/- 0.02 and 0.15 +/- 0.02 ng/mL; 0.57 +/- 0.01 and 0.64 +/- 0.03 ng/mL; 1.55 +/- 0.06 and 1.62 +/- 0.04 ng/mL, 2.82 +/- 0.09 and 2.82 +/- 0.12 ng/mL, respectively. The detection and the quantification limits were 0.02 and 0.08 ng/mL, respectively. No significant difference was observed between digoxin plasma concentrations measured by the original and the modified EMIT 2000 digoxin assay in 25 plasma specimens, ranging from 0.4 to 3.0 ng/mL, from patients receiving the drug. This modified digoxin EMIT 2000 assay was subsequently used to study digoxin pharmacokinetics after each of 18 healthy volunteers was administered a single 0.5 mg oral dose. The pharmacokinetic parameters found in this study were in accordance with the literature in healthy subjects, using radioimmunoassay (RIA) for digoxin plasma concentration determinations. In conclusion, the lower limit of quantification of this modified EMIT 2000 digoxin assay is similar to that of RIA and can serve as a valuable screen for digoxin pharmacokinetic interactions studies.