Suspected DLIS interference in the dimension DGNA digoxin assay method and the clinical application of the revised digoxin target range.

The authors present a case of an elderly female patient with heart failure and renal dysfunction treated with digoxin, where 2 commercial immunoassay methods (DRI, Microgenics, and DGNA, Dade Behring) showed a clinically very significant discrepancy on the same plasma sample, viz. 0.5 and 2.3 nmol/L, respectively. The sample was also referred to a third external laboratory that returned a result of 0.9 nmol/L using mFPIA (AxSYM, Abbott). Subsequent ultrafiltration (30,000 Dalton) on the sample essentially eliminated the difference, suggesting an interference from a large molecular weight compound(s), potentially the well-described digoxin-like immunoreactive substance(s) (DLIS). Although further study is required to verify that the DLIS implicated was indeed the interfering species, it does again highlight the importance of careful method selection in the clinical therapeutic drug monitoring laboratory to ensure that such well-established potential problems do not result in inappropriate dosage reduction with consequent lack of adequate drug exposure and serious clinical sequelae.