Concordance between thyroglobulin antibody assays

Background: Antithyroglobulin antibodies are a prevalent cause of interference in serum thyroglobulin immunoassays. Current guidelines recommend that antithyroglobulin antibodies should be measured concurrently with thyroglobulin when monitoring thyroid cancer patients post-thyroidectomy. However, the concordance between different antithyroglobulin assays has been questioned despite the availability of an international thyroglobulin antibody Reference Preparation. Methods: Four antithyroglobulin assays currently in use in UK laboratories (Siemens Immulite (R), Brahms GmbH, PerkinElmer AutoDELFIA and Siemens ADVIA Centaur (R)) were compared in a cohort of 145 thyroid cancer patients. Results: Using reference data provided by the kit manufacturer, concordance between the assays was 74%. Adjusting the cut-offs to maximize agreement increased concordance to 90%. Recovery of exogenous thyroglobulin using the Brahms Tg-plus immunoradiometric assay was neither a specific nor a sensitive test for the presence of a positive antibody result by any assay. Conclusions: Despite the availability of an international reference preparation, current antithyroglobulin assays show unacceptable variance.