Radioimmunoassay Measurement of Serum Cardiac Glycoside Levels: Using Pharmacologic Principles to Solve Crossreactivity Problems

Laboratories performing analyses for serum cardiac glycosides are sometimes faced with the problem of distinguishing between digoxin and digitoxin in a specimen. The antibodies to the cardiac glycosides supplied with radioimmunoassay kits for these drugs have some measurable degree of cross reactivity. Therapeutic levels of digitoxin are approximately ten times greater than those of digoxin, and the half-lives of these drugs in serum differ by a factor of four. These facts have been combined into a series of rules which allow the technologist to distinguish between digoxin and digitoxin in a sample and provide a level of the drug that has been corrected for crossreactivity.