Effect of Chinese medicines Chan Su and Danshen on EMIT 2000 and Randox digoxin immunoassays: wide variation in digoxin-like immunoreactivity and magnitude of interference in digoxin measurement by different brands of the same product.

Chan Su is a Chinese medicine prepared from the skin gland of a Chinese toad and is used in treating arrhythmia and other heart diseases. Danshen is prepared from the Chinese medicinal plant and is used for various cardiovascular diseases including angina pectoris. The authors studied the potential interference of such medicines with the widely used EMIT 2000 (Dade Behring; Deerpark, IL) digoxin assay and the recently marketed Randox digoxin assay (Randox Laboratories Ltd, Antrim, United Kingdom) (both run on the Bayer ADVIA 1650 analyzer) (Bayer Diagnostics, Tarrytown, NY) and compared their results with an FPIA (Abbott Laboratories) and a chemiluminescent immunoassay (CLIA; Bayer Diagnostics) for digoxin. Aliquots of drug-free serum were supplemented with 1 microL ethyl acetate extract of Danshen or aqueous extract of Chan Su, and apparent digoxin concentrations were measured by all four digoxin immunoassays (FPIA, EMIT, Randox, CLIA). The authors also supplemented aliquots of several different serum pools prepared from patients taking digoxin with very small amounts of Chan Su or Danshen extract and compared digoxin values with the control digoxin values (serum pool containing no Chinese medicine). The authors observed no interference of Danshen in either EMIT, Randox, or CLIA assay but observed an interference with the FPIA assay. On the other hand, the authors observed high interference of Chan Su in the FPIA assay but moderate interference with the EMIT 2000 and Randox digoxin assays. CLIA assay was again free from any interference. The authors also observed a wide variation in digoxin-like immunoreactivity and magnitude of interference in digoxin immunoassay in different brands of Chan Su and Danshen, indicating poor quality control in manufacturing of these Chinese medicines. Taking advantage of the high protein binding of digoxin-like immunoreactive components of Chan Su, the authors further demonstrated that interference of Chan Su in EMIT 2000 and Randox assays can be mostly eliminated by monitoring free digoxin.