Effect of a Chinese medicine "kyushin" on serum digoxin concentration measurement in dogs

The effect of the Chinese medicine kyushin administered orally on serum digoxin measurement was studied in an animal model. Administration of 6 pills of kyushin caused a significant increase in the serum concentration of a digoxin-like immunoreactive substance (DLIS). The DLIS concentration increased to a peak value of 1.14 ng/ml at 11/2 hours and declined to 0.5 ng/ml after 24 hours. A one-week administration of 10 pills of kyushin a day produced a steady-state DLIS concentration ranging from 0.91 to 1.07 ng/ml. Among the 7 different kyushin ingredients, toad venom (ch'an-su in Chinese) was the only drug that produced DLIS in the dogs' serum. The similarity in structures of digoxin and toad venom most likely plays an important role in producing a cross-reactivity of DLIS with immunoassay antibody to digoxin. Although these findings require confirmation in human beings, they suggest that this nonprescription drug may seriously compromise the accuracy and interpretation of digoxin concentration measurements.