Résumé : We investigated the possible origin of the spuriously high results observed with the Abbott TDx Immunoassay in the 1991 Belgian external quality assessment scheme for digoxin. The present work ascribes this discrepancy to a matrix effect induced by the addition of merthiolate as preservative to the control sainples. It consequently Stresses the importance of avoiding the use of this compound for preparing such samples. © 1994, Walter de Gruyter. All rights reserved.