Comparison of microparticle enzyme and fluorescence polarization immunoassays in pediatric patients not receiving digoxin

Previously, investigators have measured endogenous digoxin-like immunoreactive substances (DLTS) in pediatric patients. Digoxin-like immunoreactive substances may cross-react with digoxin assays to produce false-positive digoxin concentrations; hence, the validity of digoxin concentrations in pediatric patients is questionable. The authors compared the presence and magnitude of apparent DLIS using the microparticle enzyme immunoassay (MEIA) AxSYM Digoxin II and the fluorescence polarization immunoassay (FPIA) TDx Digoxin II, in the serum of 80 pediatric patients who were hospitalized with normal serum creatinine but had not been administered digoxin. Patients ranged in age from 1 day to 16 years (mean age, 4.96 +/- 5.17 years). Serum creatinine and total bilirubin were 0.5 +/- 0.18 mg/dl and 1.3 +/- 0.17 mg/dl, respectively. Forty-eight percent of MEIA samples and 79% of FPIA samples had measurable DLIS values. Digoxin-like immunoreactive substance concentrations for the MEIA (0.07 +/- 0.09 ng/ml) and FPIA assays (0.1 +/- 0.1 ng/ml) were statistically different (p = 0.01); however, no sample had a DLIS value >0.38 ng/ml. A poor correlation was noted between patient age, serum creatinine, total bilirubin, and DLIS concentration. The MEIA and FPIA assays effectively minimized DLIS crossreactivity making both technologies clinically acceptable for serum digoxin measurement in pediatric patients with normal serum creatinine and total bilirubin.