Falsely elevated serum digitoxin concentrations due to cross-reactivity of water-extractable digitoxin-like immunoreactivity of Chinese medicine Chan SU: elimination of interference by use of a chemiluminescent assay.

Chinese medicines are available without prescription in health food stores. One such Chinese preparation, Chan SU, is used as a cardiotonic agent. Digoxin-like immunoreactivity of Chan SU has been reported in the past. In this report we demonstrated significant digitoxin-like immunoreactivity of Chan SU. For example, when a 20-microl aliquot of an aqueous extract of Chan SU (2 mg/ml) was added to drug-free serum, the observed digitoxin-like immunoreactivity was 51.40 ng/ml by the fluorescence polarization assay. In contrast, a new chemiluminescent assay for digitoxin did not show any immunoreactivity. When very small amount of aqueous extract of Chan SU was added into serum containing digitoxin, the observed digitoxin concentrations were falsely elevated when measured by the fluorescence polarization immunoassay (FPIA), but did not change significantly when measured by the chemiluminescent immunoassay (CLIA). Significant digitoxin-like immunoreactivity was also observed (FPIA) in mice after feeding with Chan SU. Because bufalin, cinobufotalin and cinobufagin are major components of Chan SU, digitoxin-like immunoreactivity of these purified compounds was also studied. Bufalin was identified as the major digitoxin-like immunoreactive compound responsible for most of the interference in serum digitoxin measurement using the FPIA.