Radioimmunoassay for digoxin. Technic and clinical application

Nucleic Acid Amplification Techniques in Immunoassay: An Integrated Approach with Hybrid Performance

An immunoassay is mostly employed for the direct detection of food contaminants, and a molecular assay for targeting nucleic acids employs amplification techniques for distinguishing genes. The integration of an immunoassay with nucleic acid amplification techniques inherits the direct and rapid performance of an immunoassay and the ultrasensitive merit of a molecular assay. Enthusiastic attention has been attracted in recent years on the utilization of isothermal amplification techniques in an immunoassay, as well as the employment of a lateral flow immunoassay in a molecular assay. Thus, this Review discussed these kinds of approaches from two categories: immuno-nucleic acid amplification (I-NAA) and nucleic acid amplification-immunoassay (NAA-I). The advantages, drawbacks, and future developments were discussed for a comprehensive understanding.

Serum digoxin levels and mortality in 5,100 patients

A retrospective study of 5,100 patients on digoxin, with a four-week follow up after digoxin levels were measured, was done to determine the mortality rate. A significant increase in mortality was correlated with an increasing serum digoxin level, up to 50% at a level of 6.0 ng/mL and more. Clinical toxicity was suspected in only 0.25% of all patients on digoxin, although almost 10% had levels above the therapeutic range. Deliberate digoxin overdoses were fatal in 50% of cases. This study shows a correlation between increasing digoxin levels and increasing mortality rates. We recommend the use of serum digoxin measurements to identify those asymptomatic patients with elevated levels. The physician should seriously consider the indications for initiating or continuing digoxin treatment in any patient because of an increased mortality in patients with levels of more than 1.0 ng/mL.

A collection of therapeutic, toxic and fatal blood drug concentrations in man

In order to assess the significance of drug concentrations measured in clinical and toxicological investigations, it is essential that good collections of data are readily available. As a guide to interpreting findings, the present work provides a compilation of therapeutic, toxic and fatal blood concentration ranges of 298 drugs of interest to clinical pharmacologists, clinical toxicologists, and forensic toxicologists. Wherever possible, ranges are expressed concisely in terms of the maximum blood concentrations which account for 10, 50 and 90% of the data collected. They provide easy access to the most reliable information which relates the blood drug concentration to the biological response it produces. Where appropriate, the different toxic effects of a drug and/or the different degrees of severity of toxic symptoms associated with different drug levels are clearly defined. The original sources of all data used are provided to allow the analyst to obtain further analytical, pharmacokinetic and toxicological information should this be necessary. Those factors (e.g. age, capacity for drug metabolism, drug interactions, etc) which can modify the relationship between a drug concentration and the response it produces are briefly discussed.

The serum digoxin test and digoxin toxicity: a Bayesian approach to decision making

The clinician may often be uncertain about the presence of digoxin toxicity. This uncertainty is particularly important when the clinician must make initial therapeutic decisions about continuing or discontinuing digoxin. We describe a method that helps to clarify the role of the serum digoxin test in decreasing the uncertainty surrounding the diagnosis and treatment of toxicity. The relation between the test and toxicity was first determined in our patient population. An approach to the interpretation of the test based on the likelihood ratio was then developed by combining our data with selected data from the literature. The relation between the pretest risk of toxicity (the estimated risk of toxicity in the population under investigation before the test result is known) and the predictive value of the test was established. This relation was also used to analyze the importance of the degree of elevation of the test. The appropriate threshold probability for institution of treatment of toxicity was then determined by an interview technique. The test was able to make the patient's probability of toxicity cross the threshold probability for treatment of toxicity for an intermediate range of pretest risk. Our analysis suggests that the serum digoxin test may have a critical effect on therapeutic decisions and can be best considered as contributing to the spectrum of risk.

A standard approach to compiling clinical pharmacokinetic data

A standard format for a Clinical Pharmacokinetic Summary is proposed. It consists of a heading, tables, notes, and references for each drug reviewed. The table presents a unified and logical set of clinically useful population pharmacokinetic parameters. They concern four major areas: absorption, distribution, elimination, and the relationship of concentration to effect. Within each major group, parameters dealing with extents and rates of processes are given. Each such parameter is really two: a population mea value (for example, average volume of distribution) and the standard deviation of individual values about this mean. The first value allows individual predictions of dosage or drug level to be made; the second allows computation of the likely proximity of subsequently observed quantities to those predictions. The table presents single consensus values for each population parameter, rather than a list of values. A procedure for computing these consensus values, and for revising them in the light of new data, or reinterpreted old data, is given. Examples of Summaries are given. The method appears applicable to a variety of drugs. We suggest our approach as a standard one for preparing Clinical Pharmacokinetic Summaries, and urge our colleagues to consider it for that purpose.

[Plasma concentrations of digoxin in patients under intensive care conditions and in patients undergoing anesthesia and operation (author' transl)]

Pharmacokinetic behavior of digoxin or beta-acetyldigoxin was examined in 66 patients (27 patiets under intensive care conditions, partially with controlled breathing, 22 patients undergoing extirpation of the uterus and 17 patients treated with radium or chemotherapeutics; 19 males and 47 females) by determining plasma concentrations of digoxin (PDC). After intravenous and oral application with a maintenance dose of 0.20--0.50 mg/day blood was taken daily during a 2 to 3 week period, resulting in 510 determinations. 24 hours after the first application of 0.50 mg digoxin i.v. the mean values of PDC amounted to 0.62 +/- 0.08 ng/ml. After 0.40 or 0.25 mg digoxin per day i.v. therapeutical concentrations could be observed at the third vs fifth day. An equilibrium of PDC was reached on the 6th day after starting digitalization using maintenance doses. Intravenous application of 0.25, 0.40 or 0.50 mg digoxin per day resulted in a mean steady state of 0.68 +/- 0.37, 0.86 +/- 0.33 or 1.27 +/- 0.49 ng/ml PDC, respectively. The results were significantly different (p less than 0.01--0.001). Serial measurements indicated a great variation of PDC. In patients without renal failure the intraindividual variation of the plasma concentrations was maximal 37.4% referring to the mean steady state, interindividual 37.1% and the evaluation of the inter- and intraindividual differences amounted to 54.1%. After oral administration of digoxin (maintenance dose: 0.50 mg/day) or beta-acetyldigoxin (maintenance doses: 0.20--0.40 mg/day) differences in PDC of 38.3% and 29.7% were obtained. Body weight, age and serum creatinine concentration were partly responsible for the variance of PDC. Multiple linear regression between stead state PDC and dose, age, body weight and serum creatinine concentration revealed 62.1% of the variance of the PDC after intravenous administration of digoxin. After oral administration of beta-acetyldigoxin 39.9% were obtained. Thus, 40% of the variance were caused by differences in distribution and elimination of digoxin after i.v. application. After oral application additional 20% of the variance could be attributed to resorption and possible disturbances.

[Radioimmunological estimation of digoxin in urine (author's transl)]

When separating free and antibody bound digoxin, dextrancoated charcoal suspensions in protein-free urine dilutions develop an improve sieve effect which improves with aging of the suspensions. On addition of albumin to the dilution medium (buffer), the adsorptive capacities become stabilized. In urine dilutions less than of 1:50, false positive digoxin concentrations were measured. Determinations of 0-5 ng digoxin/ml in the diluted urine (1:100) and in the buffer solution show identical results. The recovery of added digoxin (1 ng and 2 ng/ml) is 100%; the lower detection limit is 3.6 mug/l urine. Between-assay variation for means of duplicates shows in the main measuring range (1.5-2.8 ng digoxin/ml) a variation coefficient less than of 4.5%; for a commercial test this value is distinctly higher. Dihydrodigoxin is measured at 3% only. Application of this method for measuring cumulative glycoside elimination of 5 healthy persons (0.5 mg digoxin daily intravenously during 2 weeks) results in a minimal interindividual variation of 5.1%. In a test preparation with a bioavailability of 94.6% the variation is 5.2%.

The serum digitalis concentration--does it diagnose digitalis toxicity?

We propose that an investigation of the serum digitalis concentration as a test for digitalis toxicity should (1) study patients with similar toxic manifestations, (2) obtain control concentrations from nontoxic patients with symptoms suggesting toxicity, (3) define criteria for toxicity and nontoxicity, (4) select representative patients, (5) describe the study population and (6) analyze how much diagnostic information the serum digitalis concentration provides that cannot be inferred from other observations. To determine if available evidence validates the digitalis concentration as a test for toxicity 27 reports were reviewed. No investigation employed symptomatic controls. Of five studies most consistent with points 1-5 only three demonstrated higher mean serum digitalis concentrations in toxic patients. Whether knowledge of the digitalis concentrations was diagnostically more useful than knowledge of the digitalis dosage, renal function, serum potassium concentration and cardiac status was not determined in any study. The usefulness of the serum digitalis concentration as a test for digitalis toxicity is therefore not established.

[Bioavailability of beta-acetyldigoxin and digoxin (author's transl)]

Five beta-acetyldigoxin and two digoxin preparations were given orally to eleven healthy volunteers. In a single dose crossover study bioavailability of the oral preparations was compared to an intravenous injection of digoxin as a standard for complete bioavailability. A mean bioavailability of 82% (sx = 1.8) was found for beta-acetyldigoxin tablets (Novodigal¿), PETN/beta-acetyldigoxin tablets (Nitro-Novodigal¿), oxyfedrin/beta-acetyldigoxin tablets (ildamen¿-Novodigal¿) and beta-acetyldigoxin liquid. Analysis of variance shows no differences in absorption for the five tested beta-acetyldigoxin preparations. Beta-Acetyldigoxin administered orally in an alcoholic solution is nearly completely absorbed (94%) whereas a corresponding digoxin solution is available to a significantly lesser content (79.2%). Comparable differences in absorption were found for beta-acetyldigoxin tablets (81%) and digoxin tablets (Digacin¿) (63.5%).

The measurement of urinary digoxin and dihydrodigoxin by radioimmunoassay and by mass spectroscopy

A radioimmunoassay for urinary digoxin is described which includes an initial solvent extraction to remove factors in urine which cause non-specific interference in the assay. The recoveries obtained using different solvents are compared and the non-specific factors influencing the assay investigated further. These effects were overcome by the use of a small urine volume (10 mul) in a direct, unextracted, urine assay and the results obtained correlated closely with those from the assay using prior extraction (r=0.99). No false positive results were obtained with unextracted urine samples from hospitalised patients not receiving digoxin. The specificity was also determined with regard to the natural steroids, spironolactone and the metabolites of digoxin including dihydrodigoxin. The metabolite dihydrodigoxin, with a saturated lactone ring, was not detected whereas the mono-, and bis-digitoxo-sides and digoxigenin metabolites did cross react in the assay. It was not possible to separate dihydrodigoxin and digoxin by thin-layer chromatography or solvent extraction due to their similar structures, however, mass spectroscopy was successful in this respect and was employed to obtain the ratio of dihydrodigoxin to digoxin in extracted urine samples. Levels of urinary digoxin excreted by patients maintained on different oral doses of the drug were measured. The percentage excreted in the urine as digoxin correlated closely with the oral dose (r = 0.96) but was found to be lower than that reported in most previous studies. Mass spectroscopy measurements showed that an average of 16.4% (range 12.2-19.7%) of the total oral dose was excreted as dihydrodigoxin in the urine of nine patients investigated.

Postmortem assay of digoxin by radioimmunoassay

Analysis of postmortem blood samples from patients previously on maintenance digoxin therapy suggests that the results are of value in assessing the degree of digitalization at the time of death. Control cases gave results within the normal therapeutic range wheras of six cases in which digoxin was suspected of being implicated in the death five had 'serum' digoxin levels above the therapeutic range. Differences in digoxin concentration were noted in blood collected from three sites in the body, and it is suggested that postmortem blood should be collected from the leg veins if assessment of antermortem digitalization is to be made.

Digitalis toxicity: epidemiology and clinical use of serum concentration measurements

Despite continuing advances in understanding of the basic pharmacology of the cardiac glycosides, digitalis intoxication remains a common clinical problem. Physician education programs and increasing use of serum or plasma concentration data have, however, been shown to be capable of substantially reducing the incidence of digitalis toxicity. Methodologic progress and availability of commercial radioimmunoassay kits have placed measurement of clinically relevant serum or plasma cardiac glycoside concentrations within the capability of most well equipped clinical laboratories. Extensive experience with serum digitalis levels now provides a basis for ongoing examination of the role of these measurements in clinical practice. Results of studies to date demonstrate that mean serum digoxin and digitoxin levels are significantly higher in patients with electrocardiographic evidence of toxicity compared with patients without such evidence. It must be emphasized, however, that because of overlap in serum digitalis levels between these two groups, sole dependence on these levels for established of a diagnosis of digitalis toxicity is not warranted. Multiple factors influence individual responses to cardiac glycosides, and serum concentration data must be interpreted in the over-all clinical context. Type and extent of underlying heart disease are important determinants of the clinical response to any given dose or concentration of cardiac glycoside. Knowledge of the serum digitalis concentration is likely to be helpful in the setting of suspected digitalis intoxication in the absence of an adequate history, or in the presence of fluctuating renal function, overt or suspected malabsorption, or uncertain bioavailability. More generally, such measurements may prove useful whenever an unanticipated response to digitalis is encountered, whether it be suspected toxicity or the absence of an expected therapeutic effect.

Amikacin: a rapid and sensitive radioimmunoassay

A rapid, sensitive, and specific radioimmunoassay has been developed for the measurement of amikacin, a new semisynthetic aminoglycoside antibiotic. Antisera were produced by immunizing rabbits with amikacin conjugated to porcine thyroglobulin. Sera were screened for amikacin antibodies with (3)H-labeled amikacin. Free and bound radioactivity were separated by the second antibody method. As little as 5 ng of the antibiotic can be measured in serum, cerebrospinal fluid, and urine. The standard curve is unaffected by pH (5-11), ionic strength (0.01 M to 0.3 M), MgCl(2) or CaCl(2) (1 to 8 mM/liter), or urea (10-300 mg/100 ml). The antibodies raised to amikacin are not able to distinguish between kanamycin and amikacin. The cross reactivity of all other antibiotics tested was <0.1%. Co-incubation of first and second antibody allows for a 30-min incubation period, making the assay useful for stat determinations. The standard curve is linear on a logit-log plot, allowing for rapid computer analysis of the standard curve, interpolation of sample values, and quality-control monitoring.

Serum digoxin levels using an 125I-labelled antigen: Validation of method and observations on cardiac patients

1. Determinations of serum digoxin levels utilizing commercially available kits with an 125I-labelled antigen were precise and not materially different from results obtained with a 3H-labelled antigen. 2. In order to approximate the steady state level, serum digoxin levels should be drawn either before or at least six hours following the administration of an oral tablet. 3. Concomitantly given thiazide diuretics did not interfere with the absorption of a tablet of digoxin. 4. In the digitalized patient, slow alterations in serum levels after oral administration appeared well correlated with, at least, the negative chronotropic effects of the drug. 5. Maximal exercise testing, a maneuver often applied to cardiac patients, does not significantly alter the serum digoxin level.

A rapid, simple assay for digoxin

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

Contribution of quantitative assay technics to the understanding of the clinical pharmacology of digitalis

Despite recent advances in understanding of the pharmacokinetics and electrophysiologic effects of cardiac glycosides, digitalis toxicity remains distressingly common in clinical practice. Another substantial group of patients is markedly underdigitalized, failing to gain the full therapeutic benefits of optimal use of these drugs. Since cardiac digitalis toxicity is a dose-related phenomenon, and serum or plasma digoxin and digitoxin concentrations rise with increasing doses, at least a statistical correlation between circulating levels and clinical state might be expected. Increasing availability of serum or plasma digitalis concentration measurements thus offers the clinician a potential means of improving the patient's chances of benefiting from treatment with cardiac glycosides. Assay methods in current use include a double-isotope dilution derivative method (digitoxin), red cell 86 Rb-uptake inhibition (digitoxin and digoxin), Na + -K + ATPase inhibition (digitoxin), ATPase enzymatic displacement (digitoxin and digoxin), gas chromatography (digoxin), and radioimmunoassay (digitoxin, digoxin, and ouabain).

The rapidly expanding literature reporting clinical experience with these technics reflects general agreement that mean serum or plasma digoxini and digitoxin levels are significantly higher in patients with clinical evidence of toxicity compared with nontoxic patients. Nevertheless, multiple factors influence individual responses, and blood level data must be interpreted in the overall clinical context. Hypokalemia, hypercalcemia, hypomagnesemia, acid-base disturbances, hypoxemia, and hypothyroidism all tend to decrease tolerance to any given digitalis dose or blood level. Autonomic nervous system tone and other drugs concurrently received must also be considered. Advanced heart disease in general, and coronary artery disease in particular, appear to predispose patients to apparent digitalis toxicity at relatively lower serum or plasma levels.

Cardiac glycoside assay technics have also proven useful in various studies of the clinical pharmacology of digoxin, digitoxin, and ouabain. Handling of digoxin by patients on cardiopulmonary bypass has been assessed, and gastrointestinal absorption has been evaluated in normal subjects; poor and erratic absorption of the drug has been documented in patients with malabsorption syndromes. Potentially important drug-drug interactions of agents such as phenobarbital and phenylbutazone with digitoxin have been studied, as well as the effects of steroid-binding resins on digoxin and digitoxin metabolism. Studies of ouabain pharmacokineties by radioimmunoassay have demonstrated a plasma half-life of 21 hours, indicating that, as in the case of digoxin and digitoxin, half-life of serum or plasma concentration after establishment of blood-tissue equilibrium bears a close relationship to duration of clinical effect.

Ouabain-specific antibodies: immunochemical properties and reversal of Na + , K + -activated adenosine triphosphatase inhibition

Antibodies with high affinity and specificity for the cardiac glycoside ouabain were raised in rabbits. The antigen used was a conjugate of ouabain linked through its rhamnose moiety to terminal alpha-amino groups of poly D,L alanyl-human serum albumin. Ouabain-specific antibodies were present as early as 3 wk, and rose steadily in titer over the initial 20-33 wk of immunization. Levels as high as 6.5 mg specific immunoglobulin per ml antiserum were reached in one rabbit at the end of 45 wk. The average intrinsic association constants for ouabain were 1.3 x 10(9) M(-1) and 1.6 x 10(9) M(-1) in antisera studied in detail, and there was evidence of restricted heterogeneity of binding site affinities. A high degree of specificity was demonstrated. Significant cross-reactivity occurred only with other cardioactive steroid compounds such as acetyl strophanthidin, digoxin, and digitoxin, while endogenous steroids did not cross-react even when present in 1000-fold excess. A rapid and convenient radioimmunoassay procedure for plasma or urine ouabain concentrations was developed using these antibodies. Competition between ouabain-(3)H tracer and unlabeled ouabain for specific antibody binding sites allowed the measurement of ouabain concentrations as low as 0.1 ng/ml or less without need for extraction procedures. The high association constants observed in these studies permit antibody reversal of established myocardial effects of ouabain. Both blockade and reversal of ouabain inhibition of canine myocardial microsomal Na(+), K(+)-activated ATPase by antibody were documented, suggesting a possible mechanism for reversal of cellular effects.