Digoxin-like and digitoxin-like immunoreactive substances in elderly people. Impact on therapeutic drug monitoring of digoxin and digitoxin concentrations.

We compared digoxin-like (DLIS) and digitoxin-like (DTLIS) immunoreactive substance concentrations for 30 people older than 65 years with those for 25 people younger than 50. None received digoxin or had liver disease, uremia, or volume expansion. We found no DTLIS in any specimen, and only 1 specimen from an elderly person demonstrated a low DLIS concentration. In addition, for 22 non-volume expanded patients (8 younger than 50 years and 14 older than 65) receiving digoxin, the fluorescence polarization (FPIA) and the microparticle enzyme (MEIA) immunoassays revealed comparable serum digoxin concentrations, indicating an insufficient DLIS concentration to interfere with digoxin immunoassay results. Therefore, elderly people who are not volume expanded do not have elevated DLIS or DTLIS concentrations. Furthermore, for patients with liver disease or uremia (18 older than 65 years and 20 younger than 50), the DLIS and DTLIS concentrations were elevated. Finally, for 5 patients with liver disease who received digoxin, serum digoxin concentrations were lower by MEIA and higher by FPIA, indicating the patients had elevated DLIS levels that interfered with the assays. Elevated DLIS and DTLIS concentrations are associated with volume expansion and not age.