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Efficacy of a new blocker against anti-ruthenium antibody interference in the Elecsys free triiodothyronine assay

Following the introduction of a new formulation of the Elecsys free triiodothyronine (fT3) assay from Roche Diagnostics (Meylan, France) in our laboratory in November 2003, we experienced an increased frequency of high fT3 results not associated with low thyrotropin (TSH) values (1). The Elecsys fT3 assay is a competitive immunoassay involving a specific anti-T3 sheep monoclonal antibody labeled with a ruthenium complex, T3-biotin, and streptavidin-coated microparticles. The occurrence of a high fT3 result without a low TSH level is rare. Apart from the statistical 2.5% of euthyroid patient results found slightly above the upper fT3 limit, this association can be found in sera from patients treated with T3 or triiodothyroacetic acid (2, 3) (these treatments are not frequently used in our patients) or in sera containing an interfering substance: heterophilic antibodies, anti-T3 antibodies (4), anti-ruthenium antibodies (involved in the Elecsys assay) or interfering drugs such as non-steroidal antiinflammatory drugs (5). We report here the results obtained for 15 sera collected between May 2004 and March 2006. In these samples, high Elecsys fT3 results could not be confirmed with the Ria-gnost fT3 radioimmunoassay from CIS biointernational (Gyf-surYvette, France). Run in a two-step format, this assay is free from anti-T3 antibody interference (4). Riagnost fT3 results were within the reference interval (Table 1). The procedures were in accordance with the Helsinki Declaration of 1975 and the subsequent 1996 amendments. No drug known to possibly interfere in fT3 assays identified from clinical notes, interference from hete-