Capillary electrophoretic enzyme immunoassay for digoxin in human serum

The combined use of capillary electrophoretic (CE) separation and homogeneous enzyme immunoassay for analyzing drugs in hemolyzed, lipemic, or icteric serum samples was investigated. An FDA-approved EMIT assay kit for digoxin in human serum was used. After the enzyme immunoassay, the enzymatic reaction product (NADH) and remaining substrate (NAD+), together with internal standard (p-nitrophenol, NP), were electrokinetically injected into a polyacrylamide-coated electrophoresis capillary and separated under applied potential. Detection was made by monitoring the UV adsorption at wavelength of 260 nm. The digoxin level in human serum was determined by comparing the peak area ratio of NADH and NP to the ratios established by the known digoxin standards. In this study, the factors that influence the CE separation were also investigated. Under the optimum conditions, NADH, NAD+, and NP were separated at electric field strength of 438 V/cm in the coated capillary (100 microns x 57 cm) with 200 mM Tris-borate buffer (pH 7.9) containing 0.2% hydroxypropyl methylcellulose. CE analyses of serum samples spiked with NADH standards at concentrations of 100 and 400 microM resulted in detection variabilities of less than 2% and analytical recoveries of 98-102%. Both an internal calibration plot for NADH and a dose-response curve for digoxin in serum were constructed. Calibrator serum, patients' sera with hemolyzed, lipemic, and icteric interference factors, and other pigmented blood components (e.g., serum albumin, bilirubin, hemoglobin, uric acid, coproporphyrin, melanin, protoporphyrin IX, and uroprophyrin) demonstrated no interference in this method. The authors believed that this method is useful for analyzing digoxin in hemolyzed, lipemic, and icteric blood samples that are known to create problems in conventional EMIT assays and may be applicable to other EMIT-based assays for monitoring drugs in complex biological matrices.