To the Editor.
—Regarding the article by Mooradian and Wynn in the April issue of the Archives,1 about discrepancies between the calculated digoxin clearance and the serum digoxin level in elderly patients, it should be no surprise that the serum digoxin levels by radioimmunoassay (RIA) were higher than expected. In the study by Erickson and Green,2 analysis of "normal human plasma supplemented by known amounts of digoxin... [by] RIA methods consistently yielded results >100%." This tendency of the RIA to produce falsely elevated levels may be related to the endogenous digoxinlike substance called endoxin,3 which has been shown to be present in patients with a variety of clinical states, including renal impairment4 and hypertension,5 as well as in normal individuals.6 Thus, the dismissal of estimates of digoxin clearance based on serum levels of digoxin by RIA (which, at least potentially, are falsely high) may
Reider C. Discrepancies Between the Calculated Digoxin Clearance and the Serum Digoxin Level in Elderly Patients. Arch Intern Med. 1987;147(10):1852-1853. doi:10.1001/archinte.1987.00370100166035