Digitoxinlike and digoxinlike immunoreactivitis in sera of patients with uremia and liver disease as measured by fluorescence polarization immunoassays: poor correlation between digitoxinlike and digoxinlike immunoreactivities.

Digoxinlike immunoreactive substances (DLIS) cross-react with antidigoxin antibodies and falsely elevante total digoxin levels. Cross-reactivity of DLIS with various immunoassays for digoxin has been extensively studied in the past. The digitoxin molecule differs from digoxin by having one extra hydroxyl group in the aglycone ring. Therefore, DLIS may also falsely elevate total digitoxin concentrations. However, in the past, limited studies have shown the presence of digitoxinlike immunoreactive substances (DTLIS) in cord blood and sera of neonates. We compared DLIS and DTLIS concentrations in sera of patients with uremia, liver disease, and hypoalbuminemia. We found measurable DLIS concentrations in 11 of 45 patients by the fluorescence polarization immunoassay (FPIA) for digoxin (range, 0.20-0.89 ng/ml digoxin equivalent). These patients did not receive any digoxin or digitoxin. Using the FPIA, we also found elevated DTLIS concentrations in 10 of these 45 patients (range, 2.88-21.24 ng/ml digitoxin equivalent). Given the narrow therapeutic range of digitoxin (15-30 ng/ml), this cross-reactivity is significant. Elevated concentrations of DTLIS in sera falsely elevated the measured concentrations of digitoxin (positive interference) when known amounts of digitoxin were added to such sera. Interestingly, we found a poor correlation between DLIS and DTLIS concentrations in sera of patients with liver disease and uremia (r = 0.58), with some patients having no measurable DLIS activity but measurable DTLIS activity and vice versa. Digoxin showed only 4-8% cross-reactivity against digitoxin antibody with a wide range of concentration. Some proposed DLIS compounds (nonesterified fatty acids, cholic acid, lysophospholipid, and DHEA-sulfate) did not show any cross-reactivity with the digitoxin assay at a concentrations much higher than the physiologic range. We conclude that DTLIS activity is present in patients with liver disease and uremia, and the correlation between DLIS activity and DTLIS activity is poor. Moreover, some proposed DLIS do not explain the DTLIS activity detected in serum.