Digoxin-likeImmunoreactivityEliminatedfrom Serum by CentrifugalUltrafiltrationbefore

Digoxin determined in the Abbott “TDx ” by fluorescence polarization immunoassay by the manufacturer’s recom-mended method involving precipitation of protein with 5-sulfosalicylic acid (SSA) is subject to interference from endogenous compounds having digoxin-like immunoreactiv-ity. Guided by the work of Graves et al. (Clin Chem 1986;32:1506-9), we eliminated interference caused by di-goxin-like immunoreactivity by substituting ultrafiltration for precipitation with SSA to remove protein. Using the manufac-turer’s method, we quantified digoxin in serum from 53 patients in three clinically defined groups who were receiving no digoxin, finding apparent digoxin in excess of the 200 ng/L detection limit in 24 % of the 17 pregnant women, 59 % of the 17 renal-dialysis patients, and all of 19 neonatal cord-blood samplesexamined. No measurabledigoxinimmunoreactivit