Reply to the letter to the editor by Johannes Ring and Rudi Valenta on the article "Assessment of dextran antigenicity of intravenous iron products by an immunodiffusion assay"

in patients". We agree with Ring and Valenta that in vitro tests of possible antigens with monoclonal anti- bodies can generally neither assess the risk of anaphylaxis in an individual patient, nor can they predict the numerical risk of such anaphylaxis in the clinical setting. We did, however, find a correlation between our results and clinical findings with the tested intravenous iron preparations and thus concluded that "immunoassay data agree well with clinical observations and thus represent a possible approach for the evaluation of the risk of dextran-induced anaphylactic reaction (DIAR)". Iron sucrose (Venofer ® ) was introduced in the 1950s by Laboratorien Hausmann AG, the predecessor of Vifor (International) Inc., as the first dextran-free intravenous iron preparation in Europe. In connection with the registration of Venofer ® in the US, the reverse single radial immunodiffusion assay used in our study 1 was developed in 1998 by Dr. H. Hedin (Pharmacia & Upjohn AB, Uppsala, Sweden) for Vifor to fully exclude the presence of dextran, which might occur as an impurity of sucrose. Although no such test is required by the regulatory authorities, it is still used today as an additional safety measure in Vifor's routine quality control analyses. Over the years, we have tested with this assay not only various types of dextrans but also different intravenous iron dextran preparations, which all showed a positive result. In contrast, carbo- hydrates and intravenous iron products that do not contain dextran always gave a negative result. When the two new dextran-based iron preparations Feraheme ® and MonoFer ® came on the market, we tested them in this assay. Ferumoxytol (Feraheme ® , Rienso ® ) contains a carboxymethylated dextran 2 and was marketed as non-immunogenic, but caused an anaphylactic reaction after application to a patient with a known history of adverse reaction to iron dextran 3 . Iron isomaltoside 1000 (MonoFer ® ) contains reduced Dextran 1 as a ligand 4 , which does not react in the immunoassay and probably acts as a hapten like Dextran 1 5 . Thus, both intravenous iron preparations were expected to give a negative result. Surprisingly, we found that Feraheme ® and MonoFer ® gave a positive reaction in the immunodiffusion assay. Because we were running out of the antibody used for quality control, we recently outsourced the devel- opment of a new antidextran antibody and of an enzyme-linked immune-sorbent assay (ELISA) based on this new antibody to an independent, external laboratory (GenScript, Piscalaway, NJ, USA). The antibody was developed by immunisation of BALB/c mice with dextran 50'000-KLH conjugate. Monoclonal antibodies (mouse IgG1-isotype) were produced in hybridoma cells. Reverse single radial immunodiffusion assays with this new antidextran antibody confirmed the published results 1 , i.e. the positive reactions for Feraheme ® , MonoFer ® , and Dextran 5, as well as the negative result for the reduced Dextran 1 isolated from MonoFer ® . Moreover, ELISAs were performed by GenScript on blinded